The Mysterious UI Hang Which Resolved Itself After 20s

Warning: This post includes ETW, Windbg, Kernel and Process memory dumps. If you don´t want to deep dive into the Windows Internals you should stop reading now.

One strange issue was a UI hang. Normally these are easy to solve because something is doing CPU intensive things on the UI thread, the UI thread is stuck because of a blocking call to wait for something to happen (e.g. to read a 2 GB large file),  or a deadlock has occurred. But this case was different. The UI was stuck but sometimes it did recover after 20s. The UI looked like this while it was not responding:


With Windbg we can examine where the UI thread is stuck from a live process or a memory dump. For managed code we need to load sos.dll as usual.


From the screenshot above we find that the managed stack is calling WaitMessage

0:000> !ClrStack
OS Thread Id: 0x2dbc (0)
Child SP       IP Call Site
0053ec9c 761a2a9c [InlinedCallFrame: 0053ec9c] System.Windows.Forms.UnsafeNativeMethods.WaitMessage()
0053ec98 58a4d1ea System.Windows.Forms.Application+ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr, Int32, Int32)
0053ed24 58a4cbee System.Windows.Forms.Application+ThreadContext.RunMessageLoopInner(Int32, System.Windows.Forms.ApplicationContext)
0053ed74 58a4ca60 System.Windows.Forms.Application+ThreadContext.RunMessageLoop(Int32, System.Windows.Forms.ApplicationContext)
0053eda0 58a35d59 System.Windows.Forms.Application.Run(System.Windows.Forms.Form)
UIHang.Program.Main() [D:\Source\FocusHang\UIHang\Program.cs @ 19]

which is a perfectly legal call stack and is by no way an indication of a hung UI thread with the id 0x2dbc. The deadlock check command for unmanaged locks !locks yielded no results and the managed counterpart !SyncBlk also showed nothing.

Dumping the other threads can be done in Windbg but when many threads are involved the Parallel Stacks window of Visual Studio is much better:


The other threads also look normal. By the way how would a not normal stack look like? If something has got stuck I simply check out the stacks with the longest stacktrace because these are usually which are actually doing more than waiting for things to happen. But as you can see from the picture above there are no long stacks involved.

The current dump shows nothing. What can we do? Get more dumps! These showed hangs happening in

  • user32.dll!PeekMessage
  • user32.dll!SetFocus
  • user32.dll!ShowWindow

but nowhere was a reason visible why they were hanging. The window manager of Windows inside the kernel is the win32k subsystem. If something is stuck at a deeper level then it is happening inside the kernel and user mode stacks are useless. Procdump (my favorite memory dump creation tool) can give you a peek inside the kernel by dumping not only the user mode part of the call stack but also the kernel stacks (this works on Windows 10 only as far as I know). If you have looked carefully at the Windbg output you will notice that the memory dump was performed with the -mk option (see Comment: in the Windbg window) which creates a second dump file besides the user mode dump

D:\UIHang>procdump -mk -ma UIHang.exe

ProcDump v9.0 – Sysinternals process dump utility
Copyright (C) 2009-2017 Mark Russinovich and Andrew Richards
Sysinternals –

[16:56:31] Dump 1 initiated: D:\UIHang\UIHang.exe_180218_165631.dmp
[16:56:31] Dump 1 writing: Estimated dump file size is 177 MB.
[16:56:31] Dump 1 complete: 177 MB written in 0.3 seconds
[16:56:31] Dump 1 kernel: D:\UIHang\UIHang.exe_180218_165631.Kernel.dmp
[16:56:32] Dump count reached.

When you open the xxxx.Kernel.dmp file you can navigate to the user mode thread 0x2dbc from our stuck UI thread to see where the call stack continues in the kernel:


Sometimes you can learn something new by looking at the kernel side. In this case the Kernel waits for a new window messages in NtUserWaitMessage but it is still not clear why this call never wakes up. In that case it makes sense to examine the contents of the window message queue. Unfortunately that can only be done by MS support because the whole windowing stuff is not exposed in Windbg or any published Windbg extension that I am aware of. When sending data to someone else we should get as much evidence as possible. My current favorite data collection for such types of issues are

  • ETW Sample Profiling with 8kHz sample rate and Context Switch Tracing
  • Memory Dump of frozen process
  • Kernel Memory Dump

Full Kernel Memory dumps are a pain because the are huge. If you are on Windows 10 or Server 2016 there is the option to take a kernel memory dump of only the active memory ( which is great because this excludes the file system cache which usually many GB in size. To force the creation of a kernel dump which excludes the file system cache you can create a reg file with the contents below:


Windows Registry Editor Version 5.00




and import the reg file. After that you need to reboot the machine. When you now press

Left Ctrl + Scroll Lock + Scroll Lock

you get a sad face which contains as bug check code MANUALLY INITIATED CRASH which is just what we want. You can use the .reg file also on Windows 7 machines where you get a full memory dump because the FilterPages registry key is ignored on older Windows versions.


On my 16 GB machine I now get a small 2,2 GB dump file.


If you want to transfer large files you should compress the data as much as possible. 7z archives are in my experience about 20% smaller than regular .zip files at the expense of ca. x5 times longer compression times. You can use multithreaded compression with the LZMA2 switch which splits the data into blocks which can be compressed by multiple threads. If you resort back to LZMA then then the speedup is much less. If you are doing this on a server machine where users start working after you have taken a memory dump you should perhaps stick to the .zip format to compress on a single core and stay nicely in the background.

7z a -m0=LZMA2 c:\temp\Kernel.7z MEMORY.DMP

If you use LZMA2 or LZMA (with many different files it will also be highly parallel like LZMA within one file) 7z will use all cores it can get. The compressed file is then a 577MB file which can be much easier sent around. A quick look by Microsoft support revealed the the message queue on our main UI thread is stuck to wait for window messages on another thread with the thread id 1880. Lets check the dump file for that thread with e.g. Windbg

0:011> ~~[1880]s
eax=00000000 ebx=00000002 ecx=00000000 edx=00000000 esi=00000000 edi=0000050c
eip=7769e7ac esp=0866f7a8 ebp=0866f818 iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000202
7769e7ac c20c00          ret     0Ch
0:011> k
 # ChildEBP RetAddr  
00 0866f7a4 7642ebf9 ntdll!NtWaitForSingleObject+0xc
01 0866f818 70e0d5bd KERNELBASE!WaitForSingleObjectEx+0x99
02 0866f87c 70e0d80e clr!CLRSemaphore::Wait+0xc0
03 0866f8b8 70e0d8a8 clr!ThreadpoolMgr::UnfairSemaphore::Wait+0x132
04 0866f924 70d6edf1 clr!ThreadpoolMgr::WorkerThreadStart+0x389
05 0866fa44 76568654 clr!Thread::intermediateThreadProc+0x55
06 0866fa58 77694a77 kernel32!BaseThreadInitThunk+0x24
07 0866faa0 77694a47 ntdll!__RtlUserThreadStart+0x2f
08 0866fab0 00000000 ntdll!_RtlUserThreadStart+0x1b

Hm. That is a .NET Threadool thread which waits for more work to come. There is no indication what did happen before on that thread. But since we have recorded ETW tracing we know thanks to high frequency sample profiling data with 8KHz a lot of the history of that thread. The default sampling rate of 1kHz is not sufficient when you are searching for cheap method calls which can have far reaching consequences. In this case we are not looking for a performance issue but we want to know which methods this thread has executed before it was waiting for more work.

Lets check out what thread 0x1880=6272 was doing in WPA:


It was executing a TPL task on that thread were the suspiciously named ctor of HangForm was called. But what is even more important that the thread was destroyed exactly 20s after it has executed the ctor of the form where Windows destroys the window of our hung from on thread exit (see Selection Duration of 20.011s). After that the UI was responsive again. At least that is the observation. But  that still does not explain why the main UI thread was stuck even if you create a window on another thread you can create multiple UI threads within an application without issues as long as you do not mix them up. WinForms is very vigilant in this respect and will always throw an InvalidOperationException of the form

InvalidOperationException: “Crossthread operation not valid: Control ‘<name>’ accessed from a thread other than the thread it was created on.

if you try bad things like that.

The only way I know how to connect window message pumps from different threads is to call AttachThreadInput. When we search the profiling data for that method we find this:

 |    |    |    |    |- UIHang.exe!UIHang.HangForm::StartUIOnOtherThread 0x0

 |    |    |    |    |    |- UIHang.exe!UIHang.HangForm::.ctor 0x0

 |    |    |    |    |    |-!System.Windows.Forms.Control.Show()

 |    |    |    |    |    |- UIHang.exe!dynamicClass::IL_STUB_PInvoke 0x0

 |    |    |    |    |    |    |- user32.dll!SetParentStub

 |    |    |    |    |    |    |    win32u.dll!NtUserSetParent

 |    |    |    |    |    |    |    ntdll.dll!LdrInitializeThunk

 |    |    |    |    |    |    |    ntdll.dll!LdrpInitialize

 |    |    |    |    |    |    |    ntdll.dll!_LdrpInitialize

 |    |    |    |    |    |    |    wow64.dll!Wow64LdrpInitialize

 |    |    |    |    |    |    |    wow64.dll!RunCpuSimulation

 |    |    |    |    |    |    |    wow64cpu.dll!Thunk0Arg

 |    |    |    |    |    |    |    wow64cpu.dll!CpupSyscallStub

 |    |    |    |    |    |    |    ntoskrnl.exe!KiSystemServiceCopyEnd

 |    |    |    |    |    |    |    win32kfull.sys!NtUserSetParent

 |    |    |    |    |    |    |    win32kfull.sys!xxxSetParentWorker

 |    |    |    |    |    |    |    |- win32kfull.sys!xxxShowWindowEx

 |    |    |    |    |    |    |    |- win32kfull.sys!zzzAttachThreadInput

The window was created on another thread but it did not attach the thread input queue directly. Instead it was calling user32.dll!SetParent which did in the kernel in the win32k subsystem attach the window input queues by calling zzzAttachThreadInput. That is all happening on our non message pumping TPL Task thread which is the missing ingredient to finally understand why our main UI thread was blocked due to a programming error on a seemingly unrelated thread. We have from the zzzAttachThreadInput method even with 8kHz sampling rate only one stack trace where one still needs a bit of luck to see the root cause so nicely with ETW data.

More documentation about that Win32 behavior would be great. These details seem to be discussed on the Old New Thing Blog (Sharing an input queue takes what used to be asynchronous and makes it synchronous, like focus changes). A few more hints are shown at P41  According to that you will attach thread input queues implicitly if you

  • Set a parent window (user32.dll!SetParent)
  • Set an Owner window (user32.dll!SetWindowLongPtr(win32window, GWLP_HWNDPARENT, formhandle)
  • Or install journal hooks  (user32.dll!SetWindowsHookEx with an JournalRecordProc)

Win32K ETW Tracing?

You can also enable tracing for the Win32K subsystem to track the window focus events by adding this ETW provider to your xperf command line


But if the window message pump is stuck the results of this ETW provider and the WPA Window In Focus chart can be misleading.


Window message queue issues are notoriously hard to debug because most relevant data is only available during live debugging while you still can query window states with e.g. Spy++. But if you only have a memory dump you will have a hard time to figure out what went wrong. A kernel dump would give you all information but since no public information is present how you can examine the contents a window message queue this must be left as an exercise for Microsoft support. If someone knows how to get e.g. thread affinity from a user mode memory dump from an HWND please leave a note below. Once again memory dumps and ETW tracing have helped to find the actual root cause. The memory dump helps to find stuck threads and strange data points. ETW helps you to find how you did get into that state once you know from the dump file where you need to look further.

This time I have learned that .NET Threadpool threads seem to be shut down 20s after they had no real work to do and that Windows will destroy window handles once the creating thread terminates. That can unblock a previously stuck UI as a side effect. If you want to play with the UIHang application you can find it here:!AhcFq7XO98yJgrklCE9_p4RuHoG0Mg


Be Careful Where You Put GC.SuppressFinalize

I had an interesting issue to debug which resulted in a race condition where the finalizer was being called while the object was still in use. If you know how .NET works this should ring some alarm bells since this should never happen. The finalizer is expected to run only when no one has a reference to the finalizable object anymore.

A simple reproducer is below. It creates 50K finalizable objects. Each object allocates 500 bytes of unmanaged memory which is released either by calling dispose on it on a dedicated thread or the finalizer thread will kill the rest during application shutdown.

using System;
using System.Linq;
using System.Runtime.InteropServices;
using System.Threading;
using System.Threading.Tasks;

class Program
    static void Main(string[] args)
        // create 50K events
        var events = Enumerable.Range(1, 50 * 1000)
                                .Select(x => new Event())

        ManualResetEvent startEvent = new ManualResetEvent(false);

        Task.Factory.StartNew(() =>
            startEvent.WaitOne();  // wait for event
            foreach (var ev in events) // dispose events

        startEvent.Set(); // start disposing events
        Thread.Sleep(1);  // wait a bit and then exit

public class Event : IDisposable
    internal IntPtr hGlobal;  // allocate some unmanaged memory

    public Event()
        hGlobal = Marshal.AllocHGlobal(500);

    ~Event()  // finalizer 

    public void Dispose()
        if( hGlobal !=  IntPtr.Zero) // check if memory is gone
            Marshal.FreeHGlobal(hGlobal); // free it
            GC.SuppressFinalize(this); // Prevent finalizer from running it again
            hGlobal = IntPtr.Zero;

Looks good to you? Let it run:


Ups that should not happen. When trying to run the application under the VS debugger everything works on my machine™. No matter how hard I try it will never crash under the debugger. But if I start it without debugging it will crash every time.

Debug The Problem

When the application crashes without the debugger on a machine where VS is installed you will get a nice dialog


where you can click Debug. Then I choose to debug managed and unmanaged debugging


Because part of the issue has to do with the .NET Runtime we need managed and unmanaged debugging. It is therefore wise to enable Native and Managed debugging.


If you do not manually select both debugging engine VS will default to unmanaged debugging only where we will miss our managed stack frames which is not particularly helpful:


With the correct debugging engine we find that while the finalizer was called a heap corruption was reported:


While another thread is also disposing events


So what is the problem here? Could it be that the finalizer is disposing the same instance on which our TPL thread is still working? A concurrent double free sounds likely but by only using Visual Studio we cannot prove it. If a finalizer is called while we the object is still alive we would have found have a pretty serious GC bug. On the other hand if that would be the case many people would have complained.

Gather More Evidence

To analyze the crash with other tools it is good to save a memory dump from the crashing aplication. You can do this pretty easily with

D:\Source\FinalizerFun\bin\Release>procdump -ma -e -x . FinalizerFunNetFull.exe

ProcDump v9.0 – Sysinternals process dump utility
Copyright (C) 2009-2017 Mark Russinovich and Andrew Richards
Sysinternals –

[21:13:50] Exception: 04242420
[21:13:52] Exception: 80000003.BREAKPOINT
[21:13:52] Exception: C0000374
[21:13:52] Unhandled: C0000374
[21:13:52] Dump 1 initiated: .\FinalizerFunNetFull.exe_180204_211352.dmp
[21:13:52] Dump 1 writing: Estimated dump file size is 83 MB.
[21:13:52] Dump 1 complete: 83 MB written in 0.1 seconds
[21:13:52] Dump count reached.

procdump is a command line tool to take memory dumps in many ways. This time we take a full memory dump -ma for unhandled exceptions -e where we execute a process -x and put the dump to the current directory . followed by the executable and optional command line arguments for the executable. The most difficult part is that I always forget that the first parameter after -x is not the executable and its arguments but the dump folder. If you try to capture a dump on first chance exceptions before it becomes unhandled you normally use -e 1 but for reasons not known to me this did never trigger the creation of a dump file. If all fails you can still take a memory dump while the “… has stopped working” dialog is shown with procdump for a given pid like “procdump -ma pid”.

You can open the memory dump with Visual Studio without problem by dragging and dropping the .dmp file from the explorer into VS


Press Debug with Mixed to see managed and unmanaged code. Many people shy away from memory dumps. But if you dare to open them the debugging experience is the same as it would be for a live process which is stuck at a breakpoint. The only difference is that you cannot continue execution. VS will show your source code and the crashing thread just like it would happen during a live debugging session:


VS has great memory dump support (since ca. VS2012/2013 around).  If you have an automated build system it is possible to get full source code debugging  for your released application. The feature is called Source Server support. For TFS builds it is a simple config switch of your build. With git things are more involved If you have Source Indexed builds you definitely want to enable Source Server support for the debugger to get live and memory dump debugging without the need to download the source files.  In Debug –  Options


check all items below Enable source server support. Unfortunately VS 2017 has broken Source Server Support which is tracked here:

It is working on VS 2013, 2015 or 2017 15.6 (still beta). As workaround you can copy srcsrv.dll from an earlier VS edition to the VS2017 one to get source server support back again.

No Not Windbg!

We have reached a dead end with Visual Studio. It is time to admit that the nice GUI based tools although powerful are not always the most helpful ones when you want to completely understand an issue. First we need to download Windbg for which MS has put up a page This will point you to the Windows SDK page


from where you can download the Windows SDK installer. If the installer wont start you have a newer version of the Win 10 SDK already installed. In that case you can download the latest SDK installer from

When you start the installer you need to press next a few times to get to the list of features you want to install. Check Debugging Tools for Windows and press Install.


Now you will find the 32 bit version of Windbg in

“C:\Program Files (x86)\Windows Kits\10\Debuggers\x86\windbg.exe”

and the 64 bit version at

“C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\windbg.exe”

Start the correct Windbg version (x86,x64) and load the crash dump file.


Since it is a managed process we need to load to a managed debugging extensions named sos.dll. This is done with the Windbg command

.loadby sos clr

If you get a failure of the form

0:005> .loadby sos clr
The call to LoadLibrary(C:\Windows\Microsoft.NET\Framework\v4.0.30319\sos) failed, Win32 error 0n193
    “%1 is not a valid Win32 application.
Please check your debugger configuration and/or network access.

then you did load the wrong Windbg because you did open a 32 bit dump with the 64 bit version of Windbg. Things get easier with the upcoming new Windbg which is in Beta where is only one debugger which will load the right debugger.
The cryptic command tells the debugger to load the sos.dll from the same directory where the .NET Runtime dll clr.dll is located. If you wish you can also fully qualify the name like

.load C:\Windows\Microsoft.NET\Framework\v4.0.30319\sos.dll

The 64 bit framework dll is located at

.load C:\Windows\Microsoft.NET\Framework64\v4.0.30319\sos.dll

If you dare to use ” for the path then you need to adhere to the C style escape rules where you need to use \\ to get a \. If you analyze a memory dump on another machine with a different .NET Framework version installed some of the SOS commands might not work or sos.dll refuses to be loaded. In that case you can check out my OneDrive folder!AhcFq7XO98yJgoMwuPd7LNioVKAp_A which contains a pretty up to date list of nearly all .NET Framework sos dlls. You need to extend the symbol path to the downloaded sos dlls (.sympath+ c:\mscordackwksDownloadDir) and then load it via the full path. Things become easier in the future if Windbg automatically loads the right sos.dll from the symbol server which seems now to be in place.

We have a managed debugging extension loaded. Now what? First we test if the extension works by executing the !Threads command

0:000> !Threads
c0000005 Exception in C:\Windows\Microsoft.NET\Framework\v4.0.30319\sos.Threads debugger extension.
      PC: 0b13b8e3  VA: 00000000  R/W: 0  Parameter: ed04c8b4
0:000> !Threads
ThreadCount:      4
UnstartedThread:  0
BackgroundThread: 4
PendingThread:    0
DeadThread:       0
Hosted Runtime:   no
       ID OSID ThreadOBJ    State GC Mode     GC Alloc Context  Domain   Count Apt Exception
   0    1 2594 02959160   2022220 Preemptive  046E6CF8:00000000 02952d00 0     MTA 
   5    2 1a08 02966f30     2b220 Preemptive  046F3CDC:00000000 02952d00 0     MTA (Finalizer) System.BadImageFormatException 046ebff4
   9    3 4300 06d3f690   3021220 Preemptive  046E829C:00000000 02952d00 0     MTA (Threadpool Worker) 
  11    4 2cec 06d41e78   1029220 Preemptive  046EA1E4:00000000 02952d00 0     MTA (Threadpool Worker) 

For some reason the first time I execute the command I get an exception. But it works the second time. This is happening to me since years on many different machines. I have no idea what the bug is but it should be fixed someday. We know that we have 4 threads and one thread did throw a BadImageFormatException. Lets examine that thread. The first column are the Windbg thread numbers given by Windbg to switch easier between threads. The command to switch to a specific thread 5 where our exception lives is


Then we can execute the sos command to dump the managed thread stack with

0:005> !ClrStack
OS Thread Id: 0x1a08 (5)
Child SP       IP Call Site
0676f888 7748ed3c [HelperMethodFrame: 0676f888] System.Runtime.InteropServices.Marshal.ThrowExceptionForHRInternal(Int32, IntPtr)
0676f8fc 70d0065e System.Runtime.InteropServices.Marshal.FreeHGlobal(IntPtr) [f:\dd\ndp\clr\src\BCL\system\runtime\interopservices\marshal.cs @ 1211]
0676f908 0291116a Event.Dispose() [D:\Source\vc17\FinalizerFuncNetFull\FinalizerFuncNetFull\Program.cs @ 51]
0676f914 029111a9 Event.Finalize() [D:\Source\vc17\FinalizerFuncNetFull\FinalizerFuncNetFull\Program.cs @ 43]
0676fb10 714e63f2 [DebuggerU2MCatchHandlerFrame: 0676fb10] 

We know that thread number 5 is the finalizer thread and we see that it is indeed calling into Event.Finalize where our exception happens. So far we did not get more information than we could get from the much easier to use Visual Studio debugger. Now lets check on which event object the finalizer was called. For that we can use a heuristic command named !dso which is the short form of Dump Stack Objects.

0:005> !dso
OS Thread Id: 0x1a08 (5)
ESP/REG  Object   Name
0676F5A8 046ebff4 System.BadImageFormatException
0676F698 046ebff4 System.BadImageFormatException
0676F6AC 046ebff4 System.BadImageFormatException
0676F6D0 046ebff4 System.BadImageFormatException
0676F6FC 046ebff4 System.BadImageFormatException
0676F710 046ebff4 System.BadImageFormatException
0676F718 046ebff4 System.BadImageFormatException
0676F71C 046ebff4 System.BadImageFormatException
0676F7BC 046ebff4 System.BadImageFormatException
0676F7FC 046ebff4 System.BadImageFormatException
0676F8FC 046507c0 Event
0676F958 046507c0 Event
0676F98C 046507c0 Event
0676F998 046507c0 Event
0676F9A8 046507c0 Event
0676F9B0 046507c0 Event
0676F9C0 046507c0 Event

The command is rather dumb and dumps the same object reference several times where it was located as pointer on the thread stack. There is actually a much better extension out there for that which is called netext ( To “install” the extension you can copy it to the Windbg default extension folder which allows you to load the dll with no directory qualifier on my machine to

  • C:\Program Files (x86)\Windows Kits\10\Debuggers\x86\winext
  • C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\winext

Now we can load it

0:000> .load netext
netext version Feb  6 2018
License and usage can be seen here: !whelp license
Check Latest version: !wupdate
For help, type !whelp (or in WinDBG run: ‘.browse !whelp’)
Questions and Feedback:
Copyright (c) 2014-2015 Rodney Viana (
Type: !windex -tree or ~*e!wstack to get started

0:005> !wstack

Listing objects from: 0676b000 to 06770000 from thread: 5 [1a08]

046ebff4 701d13c4   0  0         92 System.BadImageFormatException
046507c0 028b6260   0  0         12 Event

2 unique object(s) found in 104 bytes

to get a much less cluttered output. This extension is pure gold because it allows you to write LINQ style debugger queries to e.g. dump all object instances which derive from a common base class. It has extended support for WCF connections, sockets and APS.NET specific things.

From the dump we know that the event 046507c0 did cause an exception in the unmanaged heap. Was someone else working with this object? Visual Studio is of no help here but we can use the !GCRoot command to find out who references this object from somewhere else:

0:005> !GCRoot 046507c0
Thread 1a08:
    0676f908 0291116a Event.Dispose() [D:\Source\vc17\FinalizerFuncNetFull\FinalizerFuncNetFull\Program.cs @ 51]
            ->  046507c0 Event

Thread 4300:
    08edf790 0291116a Event.Dispose() [D:\Source\vc17\FinalizerFuncNetFull\FinalizerFuncNetFull\Program.cs @ 51]
            ->  046507c0 Event

    08edf79c 02911108 Program+<>c__DisplayClass0_0.<Main>b__1() [D:\Source\vc17\FinalizerFuncNetFull\FinalizerFuncNetFull\Program.cs @ 23]
        ebp+2c: 08edf7ac
            ->  046324b4 System.Collections.Generic.List`1[[Event, FinalizerFuncNetFull]]
            ->  05655530 Event[]
            ->  046507c0 Event

    08edf79c 02911108 Program+<>c__DisplayClass0_0.<Main>b__1() [D:\Source\vc17\FinalizerFuncNetFull\FinalizerFuncNetFull\Program.cs @ 23]
        ebp+28: 08edf7b0
            ->  046507c0 Event

Found 4 unique roots (run '!GCRoot -all' to see all roots).

The finalizer thread 1a08 was expected but what is thread 4300 doing with our object? Lets switch to that thread. We can use either the thread number of the OS thread id with the even more cryptic command

0:005> ~~[4300]s
eax=00000000 ebx=00000001 ecx=00000000 edx=00000000 esi=00000001 edi=00000001
eip=7748ed3c esp=08edf2b8 ebp=08edf448 iopl=0         nv up ei pl nz na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000206
7748ed3c c21400          ret     14h
0:009> !ClrStack
OS Thread Id: 0x4300 (9)
Child SP       IP Call Site
08edf754 7748ed3c [InlinedCallFrame: 08edf754] 
08edf750 7013bb80 DomainNeutralILStubClass.IL_STUB_PInvoke(IntPtr)
08edf754 7010d8b9 [InlinedCallFrame: 08edf754] Microsoft.Win32.Win32Native.LocalFree(IntPtr)
08edf784 7010d8b9 System.Runtime.InteropServices.Marshal.FreeHGlobal(IntPtr) [f:\dd\ndp\clr\src\BCL\system\runtime\interopservices\marshal.cs @ 1212]
08edf790 0291116a Event.Dispose() [D:\Source\vc17\FinalizerFuncNetFull\FinalizerFuncNetFull\Program.cs @ 51]
08edf79c 02911108 Program+c__DisplayClass0_0.b__1() [D:\Source\vc17\FinalizerFuncNetFull\FinalizerFuncNetFull\Program.cs @ 23]
08edf7e0 70097328 System.Threading.Tasks.Task.InnerInvoke() [f:\dd\ndp\clr\src\BCL\system\threading\Tasks\Task.cs @ 2884]
08edf7ec 70096ed0 System.Threading.Tasks.Task.Execute() [f:\dd\ndp\clr\src\BCL\system\threading\Tasks\Task.cs @ 2498]
08edf810 700972fa System.Threading.Tasks.Task.ExecutionContextCallback(System.Object) [f:\dd\ndp\clr\src\BCL\system\threading\Tasks\Task.cs @ 2861]
08edf814 7010bcd5 System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean) [f:\dd\ndp\clr\src\BCL\system\threading\executioncontext.cs @ 954]
08edf880 7010bbe6 System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean) [f:\dd\ndp\clr\src\BCL\system\threading\executioncontext.cs @ 902]
08edf894 70097178 System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef) [f:\dd\ndp\clr\src\BCL\system\threading\Tasks\Task.cs @ 2827]
08edf8f8 7009704d System.Threading.Tasks.Task.ExecuteEntry(Boolean) [f:\dd\ndp\clr\src\BCL\system\threading\Tasks\Task.cs @ 2767]
08edf908 70096fcc System.Threading.Tasks.Task.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem() [f:\dd\ndp\clr\src\BCL\system\threading\Tasks\Task.cs @ 2704]
08edf90c 700e87f2 System.Threading.ThreadPoolWorkQueue.Dispatch() [f:\dd\ndp\clr\src\BCL\system\threading\threadpool.cs @ 820]
08edf95c 700e865a System.Threading._ThreadPoolWaitCallback.PerformWaitCallback() [f:\dd\ndp\clr\src\BCL\system\threading\threadpool.cs @ 1161]
08edfb80 7143eb16 [DebuggerU2MCatchHandlerFrame: 08edfb80] 

Ahh that is our TPL thread which is also freeing the object. The call stack shows that we have caught it in action while it was still calling Marshal.FreeHGlobal the finalizer did finalize it right away! That is pretty serious since that must not happen. To see the full picture we need a mixed mode stack with no hidden stack frames like Visual Studio is showing us. For mixed mode stacks there is another Windbg extension best suited. It is called sosex (

0:009> .load sosex
This dump has no SOSEX heap index.
The heap index makes searching for references and roots much faster.
To create a heap index, run !bhi
0:009> !mk
Thread 9:
        SP       IP
00:U 08edf2b8 7748ed3c ntdll!NtWaitForMultipleObjects+0xc
01:U 08edf2bc 753f1293 KERNELBASE!WaitForMultipleObjectsEx+0x103
02:U 08edf450 714dff96 clr!WaitForMultipleObjectsEx_SO_TOLERANT+0x3c
03:U 08edf4a0 714dfcd8 clr!Thread::DoAppropriateWaitWorker+0x237
04:U 08edf52c 714dfdc9 clr!Thread::DoAppropriateWait+0x64
05:U 08edf598 714dff3c clr!CLREventBase::WaitEx+0x128
06:U 08edf5e4 71560152 clr!CLREventBase::Wait+0x1a
07:U 08edf5fc 714fe9dc clr!WaitForEndOfShutdown_OneIteration+0x81
08:U 08edf670 714fea29 clr!WaitForEndOfShutdown+0x1b
09:U 08edf67c 714fcd76 clr!Thread::RareDisablePreemptiveGC+0x52f
0a:U 08edf6c8 714e8374 clr!JIT_RareDisableHelper+0x24
0b:M 08edf74c 7013bb95 DomainNeutralILStubClass.IL_STUB_PInvoke(IntPtr)
0c:M 08edf750 7013bb80 DomainNeutralILStubClass.IL_STUB_PInvoke(IntPtr)
0d:M 08edf784 7010d8b9 System.Runtime.InteropServices.Marshal.FreeHGlobal(IntPtr)(+0xe IL,+0x19 Native) [f:\dd\ndp\clr\src\BCL\system\runtime\interopservices\marshal.cs @ 1212,17]
0e:M 08edf790 0291116a Event.Dispose()(+0x1d IL,+0x12 Native) [D:\Source\vc17\FinalizerFuncNetFull\FinalizerFuncNetFull\Program.cs @ 51,13]
0f:M 08edf79c 02911108 Program+<>c__DisplayClass0_0.<Main>b__1()(+0x21 IL,+0x70 Native) [D:\Source\vc17\FinalizerFuncNetFull\FinalizerFuncNetFull\Program.cs @ 23,17]

There we see that the thread did call into unmanaged code to free the heap memory but the CLR will not let it run managed code anymore because we are shutting down the process. There are some gotchas related to managed application shutdown where I did write a long time (12 years now) ago here: Most things are still the same. The key takeaway is that when managed application shutdown is initiated the .NET Runtime ensures that

  • managed code calling into unmanaged code never returns (clr!WaitForEndOfShutdown)
  • All managed threads are suspended except for the finalizer thread

0:009> ~
#  0  Id: aa0.2594 Suspend: 1 Teb: 002d2000 Unfrozen
   1  Id: aa0.18e0 Suspend: 1 Teb: 002d5000 Unfrozen
   2  Id: aa0.3ac4 Suspend: 1 Teb: 002d8000 Unfrozen
   3  Id: aa0.30c0 Suspend: 1 Teb: 002db000 Unfrozen
   4  Id: aa0.1d34 Suspend: 1 Teb: 002de000 Unfrozen
  5  Id: aa0.1a08 Suspend: 0 Teb: 002e1000 Unfrozen
   6  Id: aa0.2954 Suspend: 1 Teb: 002e4000 Unfrozen
   7  Id: aa0.3cf4 Suspend: 1 Teb: 002e7000 Unfrozen
   8  Id: aa0.3d2c Suspend: 1 Teb: 002ea000 Unfrozen
.  9  Id: aa0.4300 Suspend: 1 Teb: 002ed000 Unfrozen
  10  Id: aa0.4224 Suspend: 1 Teb: 002f0000 Unfrozen
  11  Id: aa0.2cec Suspend: 1 Teb: 002f3000 Unfrozen

  • All finalizeable objects are declared as finalizable
  • Only the finalizer thread is allowed to run to finalize all now considered garbage objects

The problem with that approach is that there is an inherent race condition where a not yet completed Dispose call is calling into unmanaged code then the finalizer will try to call the unmanaged cleanup call a second time.

Is This A Problem?

Well lets check who in the .NET Framework calls GC.SuppressFinalize


There are quite a few classes in the Base Class Library which implement finalizers this way. GC.SuppressFinalize is always called last which is a time bomb waiting to crash on you at the worst possible time and killing e.g. your UI while you are closing everything. Lets try an experiment by changing our code to create Brushes instead of events:

// create 50K events
var events = Enumerable.Range(1, 50 * 1000)
                        .Select(x => new SolidBrush(Color.AliceBlue))

When I let it run I get a nice AccessViolationException which some of us certainly have sporadically seen but have been left back wondering why that exception did happen to them:


To be fair. Not all classed listed above are susceptible to that race condition. Some classes already check if a shutdown is running and do in that case nothing:

   if (!NclUtilities.HasShutdownStarted)

The Fix

There are several ways to get around that. The easiest is to move the GC.SuppressFinalize call before the Dispose call which will prevent the finalizer from running during shutdown if a Dispose call is already executing. If an exception escapes from the Dispose call it will not be tried by the finalizer a second time which sounds like a good deal for most resources.

public void Dispose()
    if( hGlobal !=  IntPtr.Zero) // check if memory is gone
        GC.SuppressFinalize(this); // Prevent finalizer from running it again
        Marshal.FreeHGlobal(hGlobal); // free it            
        hGlobal = IntPtr.Zero;

Another way is to check if a shutdown or an AppDomain unload is happening right now:

    if (!Environment.HasShutdownStarted && !AppDomain.CurrentDomain.IsFinalizingForUnload())

.NET Core on the other hand does not suffer from that issue because a .NET Core application has no final finalizer call which prevents that race condition entirely. Now go and check your finalizers to make your application correctly shutting down.

Update 1

As requested by Steve I present a fixed safe version:

In 2018 you should not write a finalizer at all. The basic Dispose(bool bDisposing) pattern is from a time where we had no SafeHandles. Today I would write my Event class entirely without a finalizer but the unmanaged resource/s are self contained by finalizable Safehandles. A typical wrapper would look like the one below which owns the memory pointer:

sealed class SafeNativeMemoryHandle : SafeHandleZeroOrMinusOneIsInvalid
    public SafeNativeMemoryHandle(int size):base(true)

    protected override bool ReleaseHandle()
        if (this.handle != IntPtr.Zero)
            this.handle = IntPtr.Zero;
            return true;
        return false;

With that infrastructure in place we can improve the event class to the much easier version which will never leak any memory although it contains no finalizer at all:

/// <summary>
/// SafeEvent class needs no finalizer because unmanaged resources
/// are managed by the SafeNativeMemoryHandle which is the only class which needs a finalizer.
/// </summary>
public class SafeEvent : IDisposable
    internal SafeNativeMemoryHandle hGlobal;  // allocate some unmanaged memory

    public SafeEvent()
        hGlobal = new SafeNativeMemoryHandle(500);

    public void Dispose()
        hGlobal = null;

You can also create event hierarchies by making the Dispose method virtual without fear to leak any handles from its derived classes. Each class which contains unmanaged resources should contain its own self cleaning members and you are done as long as there are no dependencies between them.

Crashing VS2017 15.5.2

I was just playing with the new readonly struct features of C# 7.2. To check if things got faster I first declared a new struct

    readonly struct FastPoint
        public int X { get; set; }
        public int Y { get; set; }

But Visual Studio will then complain

Program.cs(12,20,12,21): error CS8341: Auto-implemented instance properties in readonly structs must be readonly.
Program.cs(13,20,13,21): error CS8341: Auto-implemented instance properties in readonly structs must be readonly.

Ok. No problem lets make the setter private. But the error stays. Perhaps I need to add some modifier to the get property. Lets try readonly

    public int X { readonly get; private set; }

This results in

error CS0106: The modifier ‘readonly’ is not valid for this item

Ok. Now I am desperate. Lets try ref readonly.  Hah something happens:


But not for the better. VS eats up all the memory and if you try to compile it will transfer the leak into PerfWatson2.exe as well.


Ok that was not it. The final solution was to remove the setter completely. Interestingly you can still set the property although it has no declared setter.

    readonly struct FastPoint
        public int X { get; }
        public int Y { get; }

        public FastPoint(int x, int y)
            X = x;
            Y = y;

This seems to be a C# 6 feature I was until now not aware of. Problem solved. But wait what was the ever increasing memory of the compiler?

From the call stacks we can deduce quite a bit


Roslyn is parsing a property declaration and has found ref which is a valid token. Now some memory is allocated for the token but later is treated as wrong token. That in itself would not be too bad but it seems that the parser seems to rewind and then tries parsing the same wrong property declaration again which results in infinite memory consumption. I have reported the issue here

which will hopefully be fixed. The error message is ok in hindsight but it did confuse me the first time. If you want to play with the newest C# features you need to open the build properties tab, press Advanced and then you can select the e.g. C# latest minor version to always use the latest C# version.


Lets hope you are not hitting new memory leaks as fast as I did.

The Case Of NGen.exe Needing 50 GB Of Memory

This is an old bug which seems to be in the .NET Framework since a long time but since it is highly sporadic it was not found until now. I have got reports that on some machines NGen.exe did use all of the computers memory which did lead in Task Manager to this pattern:


The biggest process on that machine was always Ngen.exe and everything was very slow. This tells me that NGen did not recover from time to time from its high memory consumption but that it did allocate like crazy until the machine had no physical memory anymore. When an application uses all memory the OS will page out all memory to the hard disk when no physical memory is left. After writing many GB of data to the hard disk NGen can continue to allocate more memory until no physical memory is left and the OS will write all memory to the page file again. This will continue until Ngen.exe finally hits the commit limit which is the sum of Physical Memory + Page File Size which results in an Out Of Memory error. Only then the process will terminate.

When NGen did go crazy like this MS support suggested to delete the registry key Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727\NGenService\Roots and its decendants. That fixed the issue but it remained mysterious why this was happening and what exactly did break. When you delete the registry key NGen will build up its Ngen root dll cache automatically.  Inside the registry keys was no obvious garbage data visible and the issue remained mysterious. But finally I have got my hands on a machine where the issue was still present which allowed me to take more evidence data.

What Do We Know?

  • ngen install somedll.dll or ngen createpdb causes NGen.exe to consume many GB of memory
  • NGen breaks due to corrupted registry keys
  • After deleting the registry key below \HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727\NGenService\Roots NGen will build up the registry structure by itself which “fixes” the issue

What data should we get? Part of it is based on experience and the other part is more psychological. If you hand over a bug report to someone else you should anticipate that he/she is not familiar with your favorite debugging tool (e.g. Windbg). Filing a bug report with random findings is easy. Filing a bug report which enables the support personnel  to get down to the real root cause is much harder. When I have a nice reproducible bug which I can repeat easily as often as I want I tend to get all data I can get. When file/registry issues are involved I would get some or all of the things below.

Full Scale Data Capturing

  • Capture a procmon trace which will show all accessed registry keys and files
    • That is easy to do and provides a general understanding which registry keys are accessed
  • Dump the affected files/registry keys
    • E.g. export the registry hive \HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727\NGenService from regedit so someone can take a look at the accessed data
    • That is much easier than to crawl though a memory dump to reconstruct the accessed registry/file contents
  • Capture several memory dumps while the problem builds up
    • procdump is a great tool to take several dumps in a row once a specific condition has been reached
  • Capture ETW Traces to get the full picture
    • It potentially shows the evolution of the problem.  But this depends highly on the skill set of the support engineer if that data is useful to them
  • Capture a Time Travel Trace
    • This gives much detail but it can be time consuming to analyze a multi GB trace. If you or the support engineer do not know exactly where to look you will only produce a multi GB random data file which is not used at all

Getting the right data which somebody else can work with is tricky since you do not know with which tools someone else is most comfortable with. Normally you will start with the easiest data capturing method and based on the analysis of the data you did get you will need to decide what else is missing to get the full picture. That usually will involve capturing more data with a different tool.  But if you capture everything from procmon tracing up to a time travel trace the chances are good that you can reduce the time until the investigation leads to somewhere from weeks down to minutes. Lets start with the easy data gathering approach first:

Getting A Procmon Trace

Procmon is a SysInternals Utility which can record all process starts along with all file and registry accesses. When you start it you can add a filter for the process of interest. In our case it is ngen.exe.


Since Procmon records system all events the resulting memory needed by this tool can become quite large. To prevent that is is most of the time better to discard all not interesting events from memory by checking the checkbox of File – Drop Filtered Events


That is important if you need to wait for an incident to run for hours. You should always get the latest version of procmon since from time to time some memory leaks or other things are fixed which could hinder a successful data collection. The gathered data can then be saved as PML file which can be read into the tool again on a different machine. To save the data you can choose a nice file name which describes the problem. Naming is hard but be precise what the trace actually contains. In a few weeks even you do not know what that file was for.


Under the hood Procmon uses ETW to gather the data. What does that mean? For every file/registry access and process start/dll load you will get a full call stack if you click on the event properties which can tell you already as much as a memory dump:


In our NGen case we find that NGen deserialized the native image roots from a registry list. That is a very powerful capability but you need to set the symbol server and the path to dbghelp.dll from a Windbg installation (x64 usually) to get valid call stacks. There is one caveat: Procmon cannot decode stack traces from managed code which makes this otherwise great tool severely limited mainly for unmanaged stack trace analysis.

Dump The Registry Keys

Who said that data collection is difficult? When we look at the NGen registry keys which were accessed we find a large list of all NGenned dlls “cached” in the registry. This is done for performance reasons. The .NET Framework had always a soft spot for the Registry. NGen does it and the GAC also. That is much faster to read than to traverse over 4000 directories only for the 64 bit NGenned dlls.


Since the corruption is data dependent we can simply export the whole NGenService tree into a text file which can hopefully help to diagnose the data corruption.


The resulting text file was over 200 MB in size. It is unlikely that you will find the root cause by looking at a 200 MB text file line by line. We need more clues where we need to look at.

Can We Already Solve?

Pattern identification is a very important skill you need to develop if you want to analyze an issue. One if not the most powerful analysis method is differential analysis. Usually you have a good case and a bad case which you can compare and see where the behavior starts to diverge. But it also works in the other way to find common patterns. The presence or the absence of a deviation can both be a useful hint. It is a good idea to capture the data not only once but several times to be able to find stable patterns in the data.

By looking at the procmon registry trace we can first filter only for the querying of registry contents of the Roots node


There we find that the last accessed registry key is always the same one. It is a binary registry key named ImageList. But wait. The call stack of that event is not particularly enlightening but it is a strong hint that either that was the last registry key it did read and then one of the previously read registry keys did contain invalid data or that this registry key is the one which is corrupted. Lets check the contents of the ImageList value:


Hm. Not sure if that is the problem. Lets get more data.

Capture Memory Dumps

There are many ways to capture memory dumps but the most flexible tool besides DebugDiag is procdump. It is a simple command line tool which can trigger the creation of a memory dump in very sophisticated ways. In our case it is straightforward. We want to start NGen and then take 3 dumps with 1s in between because the memory leaks is growing very fast.

C:\Windows\assembly\NativeImages_v4.0.30319_64\System\0c9bec7e4e969db233900a4588c91656>procdump -s 1 -n 3 -ma -x c:\temp ngen.exe createpdb c:\temp

ProcDump v9.0 – Sysinternals process dump utility
Copyright (C) 2009-2017 Mark Russinovich and Andrew Richards
Sysinternals –

Process:               ngen.exe (14168)
CPU threshold:         n/a
Performance counter:   n/a
Commit threshold:      n/a
Threshold seconds:     1
Hung window check:     Disabled
Log debug strings:     Disabled
Exception monitor:     Disabled
Exception filter:      [Includes]
Terminate monitor:     Disabled
Cloning type:          Disabled
Concurrent limit:      n/a
Avoid outage:          n/a
Number of dumps:       3
Dump folder:           c:\temp\
Dump filename/mask:    PROCESSNAME_YYMMDD_HHMMSS
Queue to WER:          Disabled
Kill after dump:       Disabled

Press Ctrl-C to end monitoring without terminating the process.

Microsoft (R) CLR Native Image Generator – Version 4.7.2556.0
Copyright (c) Microsoft Corporation.  All rights reserved.
[23:16:42] Timed:
[23:16:42] Dump 1 initiated: c:\temp\ngen.exe_171212_231642.dmp
[23:16:42] Dump 1 writing: Estimated dump file size is 1418 MB.
[23:16:44] Dump 1 complete: 1419 MB written in 2.4 seconds
[23:16:46] Timed:
[23:16:46] Dump 2 initiated: c:\temp\ngen.exe_171212_231646.dmp
[23:16:47] Dump 2 writing: Estimated dump file size is 4144 MB.
[23:17:42] Dump 2 complete: 4145 MB written in 55.8 seconds
[23:17:44] Timed:
[23:17:44] Dump 3 initiated: c:\temp\ngen.exe_171212_231744.dmp

The command line parts are -s 1 to wait one second between each dump, -n 3 to take three dumps before it exits, -ma to take a full memory dump, -x expects as first argument the dump folder and all other arguments are the executable and its command line arguments. If you look at the command line parameters you will find a lot more. The output a little frightening at first but there is a secret switch (procdump -? -e) which will print a lot of useful examples how procdump is meant to be used. Actually this switch is not secret but nearly no one will read the large command line help until the end which is the reason I spell it out explicitly.

We can load the dump file into the new Windbg which will us give automatically a nice call stack window where NGen was just allocating memory:


That information should be sufficient for any support guy to drill down to the root cause. To make sense of the call stack you need local variables which are not part of the public symbols of MS. For us outsiders that is as far as we can analyze the problem. Really? Lets have a look at the method names. NGen deserializes a Root Array of native image roots from the registry. While it is deserializing a specific root object it deserializes something with a method BinaryDeSerializeLogicalImageList. That sounds familiar to the binary registry node ImageList from our registry dump. When we only could know the registry key it was just deserializing. This involves a little bit poking the in the dark. I would expect that the stack between DeSerialize and BinaryDeSerializeLogicalImageList contains hopefully somewhere the registry key name.

With the k command we get the call stack and the current stack pointers

0:000> k
 # Child-SP          RetAddr           Call Site
00 000000c9`5f2fdcc0 00007ffe`327a8912 ntdll!RtlpLowFragHeapAllocFromContext+0x2a
01 000000c9`5f2fdda0 00007ffe`05eebde6 ntdll!RtlpAllocateHeapInternal+0xf2
02 000000c9`5f2fde60 00007ffe`05eec700 mscorsvc!operator new+0x30
03 000000c9`5f2fde90 00007ffe`05eed445 mscorsvc!ArrayOfPointers::CreateAndAppendNode+0x2c
04 000000c9`5f2fded0 00007ffe`05eed7f1 mscorsvc!Configuration::BinaryDeSerializeLogicalImageList+0xcd
05 000000c9`5f2fe060 00007ffe`05eeffb0 mscorsvc!Configuration::DeSerialize+0x206
06 000000c9`5f2fe300 00007ffe`05ee81b2 mscorsvc!Root::DeSerialize+0x379
07 000000c9`5f2fe630 00007ffe`05eecd98 mscorsvc!RootList::DeSerializeRoot+0x9c
08 000000c9`5f2fe690 00007ffe`05f0b69c mscorsvc!RootList::GetRootArray+0x1a6
09 000000c9`5f2fe960 00007ffe`05f0bb79 mscorsvc!CCorSvcMgr::GetLogicalImageForRootedNI+0xd4
0a 000000c9`5f2fec60 00007ff6`aef17dd7 mscorsvc!CCorSvcMgr::CreatePdb2+0x229
0b 000000c9`5f2ff1c0 00007ff6`aef11f32 ngen!NGenParser::ProcessNewCommandLineOptionsHelper+0x99d
0c 000000c9`5f2ff5d0 00007ff6`aef11d54 ngen!IsNewCommandLine+0x196
0d 000000c9`5f2ff730 00007ff6`aef1276a ngen!trymain+0x19c
0e 000000c9`5f2ffd90 00007ff6`aef126f8 ngen!wmain+0x4e
0f 000000c9`5f2ffe20 00007ffe`30221fe4 ngen!BaseHolder,&Delete,2>,0,&CompareDefault,2>::~BaseHolder,&Delete,2>,0,&CompareDefault,2>+0x2a6
10 000000c9`5f2ffe50 00007ffe`327eef91 kernel32!BaseThreadInitThunk+0x14
11 000000c9`5f2ffe80 00000000`00000000 ntdll!RtlUserThreadStart+0x21

The brute force method is to dump the stack from start to end with

0:000> db c9`5f2fde90  c9`5f2ff1c0

000000c9`5f2fe710  18 e7 2f 5f c9 00 00 00-43 00 3a 00 2f 00 41 00  ../_....C.:./.A.
000000c9`5f2fe720  6e 00 79 00 4e 00 61 00-6d 00 65 00 57 00 69 00  n.y.N.a.m.e.W.i.
000000c9`5f2fe730  6c 00 6c 00 44 00 6f 00-2e 00 64 00 6c 00 6c 00  l.l.D.o...d.l.l.
000000c9`5f2fe740  00 00 74 00 75 00 62 00-73 00 2e 00 49 00 6e 00  ..t.u.b.s...I.n.
000000c9`5f2fe750  74 00 65 00 72 00 6f 00-70 00 2c 00 20 00 56 00  t.e.r.o.p.,. .V.
000000c9`5f2fe760  65 00 72 00 73 00 69 00-6f 00 6e 00 3d 00 31 00  e.r.s.i.o.n.=.1.
000000c9`5f2fe770  30 00 2e 00 30 00 2e 00-30 00 2e 00 30 00 2c 00  0...0...0...0.,.
000000c9`5f2fe780  20 00 43 00 75 00 6c 00-74 00 75 00 72 00 65 00   .C.u.l.t.u.r.e.
000000c9`5f2fe790  3d 00 4e 00 65 00 75 00-74 00 72 00 61 00 6c 00  =.N.e.u.t.r.a.l.
000000c9`5f2fe7a0  2c 00 20 00 50 00 75 00-62 00 6c 00 69 00 63 00  ,. .P.u.b.l.i.c.
000000c9`5f2fe7b0  4b 00 65 00 79 00 54 00-6f 00 6b 00 65 00 6e 00  K.e.y.T.o.k.e.n.
000000c9`5f2fe7c0  3d 00 33 00 31 00 62 00-66 00 33 00 38 00 35 00  =.3.1.b.f.3.8.5.
000000c9`5f2fe7d0  36 00 61 00 64 00 33 00-36 00 34 00 65 00 33 00  6.a.d.3.6.4.e.3.
000000c9`5f2fe7e0  35 00 2c 00 20 00 70 00-72 00 6f 00 63 00 65 00  5.,. .p.r.o.c.e.
000000c9`5f2fe7f0  73 00 73 00 6f 00 72 00-41 00 72 00 63 00 68 00  s.s.o.r.A.r.c.h.
000000c9`5f2fe800  69 00 74 00 65 00 63 00-74 00 75 00 72 00 65 00  i.t.e.c.t.u.r.e.
000000c9`5f2fe810  3d 00 61 00 6d 00 64 00-36 00 34 00 00 00 00 00  =.a.m.d.6.4.....

where we find the registry key which is currently being worked on:

0:000> du 000000c9`5f2fe718
000000c9`5f2fe718  "C:/AnyNameWillDo.dll"

It looks like the ImageList of this dll is corrupted which did cause NGen to go into an infinite loop. A deeper look at the surrounding registry keys from the registry export revealed that another registry key of the previous dll was also corrupted. This is really strange and I have no idea how NGen could manage to corrupt two unrelated registry keys RuntimeVersion (string) and ImageList  (binary).

Capture ETW Traces

Based on our previous investigations we should get data about memory allocation, CPU consumption and accessed registry keys which should give us a good understanding how the problem evolves over time. To capture ETW data you need normally to download and install the Windows Performance Toolkit which is part of the Windows SDK. But since Windows 10 the command line only tool named wpr.exe is part of Windows itself. That can be important if you are working on a machine which is locked down with e.g. Device Guard and you cannot install new software easily and you cannot execute not Authenticode signed binaries which rules many home grown data collection tools out. Normally I use ETWController ( which enables me to capture mouse and keyboard interactions along with screenshots which has proven to be invaluable many times. But on a locked down machine one needs to use the tools which you can start.

C:\WINDOWS\system32>wpr -start CPU -start Registry -start VirtualAllocation -start GeneralProfile

… Ngen …. 

C:\WINDOWS\system32>wpr -stop c:\temp\NgenGoneCrazy.etl

After loading the ETL file into WPA and a little working out the important metrics like CPU, Allocation and Registry accesses we get this one:


    We find that practically all CPU is spent in allocating memory while the method BinaryDeSerializeLogicalImageList  was executed. The VirtualAlloc graph shows a frightening allocation rate of 1,4 GB/s which is the most massive memory leak I have seen since a long time. The last graph shows that the huge allocation rate starts once the ImageList of the dll C:/AnyNameWillDo.dll was read. After that no more registry keys were read which is strong indicator that this registry key is the one knocking NGen out.

    After realizing that it was easy to come up with a minimal registry file which will bring NGen down


    Windows Registry Editor Version 5.00

    When you import that reg file and you execute

    ngen.exe createpdb c:\temp

    in the directory where the native image of is located NGen will explode. But beware that at least on Windows 10 my machine did freeze and never recover when all physical memory was allocated. It seems that older Windows editions (e.g. Server 2008 R2) deal with such rapidly allocating applications in a better way which lets you at least terminate the application once it has gotten all physical memory.


    After have drilled down that far it is pretty clear where NGen did break although it remains a mystery how the registry keys were corrupted. The case is currently being investigated at Microsoft which will hopefully result in a more robust NGen which will ignore bogus registry entries and delete them which should cause the recreation of the missing NGen root entries some time later. The data sent to MS are memory dumps, and ETL Trace, procmon trace and the reg file to corrupt the registry on a test machine.


    Troubleshooting is not magic although many people assume magic happening here. It is pretty straightforward to capture the relevant data with some experience. Analyzing the captured data is indeed a complex undertaking and requires a lot of experience. The goal of this blog post is to help other people dealing with failures to understand what data is needed and why. Capturing the data is much easier and faster than to analyze it. If you capture the right data you will make the work of others trying to help you a lot easier.

    I always tell people that it makes no sense to assign one guy of a team as troubleshooter and send him to a Windbg/ETW training. First of all if someone is assigned to a task he dislikes he will never be good at it. Second it needs a lot of practice and experience to be able to drill down this deep. If you are trying to analyze such an issue once every few months you will not have the necessary skills to use the tools correctly.  If you want to bring a member of your team at a good level to troubleshoot hard issues he/she must be willing to look at a memory dump nearly every day. If no one raises his hand for this task you can spare the money for the Windbg/ETW training. But it makes sense to bring all team members to a level where everyone understands what data is needed to let some specialized guys to look into the issues in an efficient manner by providing enough and the right data to successfully nail the root cause. As a positive side effect more people will get used to these tools and some guys will like to drill deeper. These are the ones you should send to a Windbg/ETW training.

    That’s all for today. Remember: Great tools are useless. Tools become great when people use them.

    Bringing The Hardware And Windows To Its Limits

    I like to experiment a lot which often leads to surprising results. Sometimes I compare performance engineering to quantum mechanics: In quantum mechanics a measurable value (observable) is not determined until the measurement is performed. I stretch quantum mechanics here and claim that you have bad performance if you never measure. Or to put it into another way:

    The probability to get a well performing system without quantitative repeatable regular performance measurements is zero.

    When you look deep enough you will find not so well known things. Memory allocation and access performance is such a topic which could span whole books because it is such a fundamental thing which most application developers are not aware of.

    What do I mean with that? Lets perform an experiment:

    1. Allocate 2000MB of memory.
    2. Measure the access performance of every 4096th byte of the allocated memory.
    3. Repeat the measurement to get consistent results a second time.

    Below is a small C++ application to do this:

    #include <chrono>
    class Stopwatch
            _Start = std::chrono::high_resolution_clock::now();
        void Start()
            _Start = std::chrono::high_resolution_clock::now();
        std::chrono::milliseconds Stop()
            _Stop = std::chrono::high_resolution_clock::now();
            return std::chrono::duration_cast<std::chrono::milliseconds>(_Stop - _Start);
        std::chrono::high_resolution_clock::time_point _Start;
        std::chrono::high_resolution_clock::time_point _Stop;
    #pragma optimize( "", off )
    void Touch(void *p, size_t N)
        char *pB = (char *)p;
        char tmp;
        for (size_t i = 0; i < N; i += 4096)
            tmp = pB[i];
    #pragma optimize("", on)
    void main()
        const int NBytes = 2 * 1000 * 1024 * 1024; // 2000 MB of memory
        char *bytes = new char[NBytes];
        Stopwatch sw;
        Touch(bytes, NBytes );  // touch every 4096th byte
        auto ms = sw.Stop();
        printf("Did touch %d bytes in %lld ms\n", NBytes, ms.count());
        ms = sw.Stop();
        printf("Did touch 2 %d bytes in %lld ms\n", NBytes, ms.count());

    When we execute it the numbers look promising

    Did touch 2097152000 bytes in 13 ms
    Did touch 2 2097152000 bytes in 0 ms

    13ms for the first access time and 0ms the second time. This is pretty good even for a debug build. For completeness lets execute the same thing as Release build because everyone tells you that you should never ever trust performance values from debug builds.

    Did touch 2097152000 bytes in 377 ms
    Did touch 2 2097152000 bytes in 0 ms

    Second time still looks good but what has happened to the first time access performance? The release build has become 30 times slower! How can this be? Lets step though it with a debugger and check the memory consumption in Task Manager after the allocation but before we have touched the memory.





    Well that is interesting. Both versions have committed 2000 MB of memory but the debug version has it in its working set already. The release build consumes basically zero physical memory. Just in case you need quick recap what commit size and working set means:

    • Commit size is the amount of memory you did allocate with new, malloc, calloc, GlobalAlloc, …
    • Working Set is the physical memory the operating system has assigned to your process (real ram chip usage). The working set can be smaller because the operating system can page out data of your process to make room for other also memory hungry applications.

    Ok so this means the OS did page out my data for the release build? Well no not in this case. There is a wrinkle to it. All operating systems try to be as lazy as possible to move the costs of memory allocation and usage at the latest time possible.

    The Operating System View Of Memory Allocation And Access

    When an allocation happens the OS first needs to check if the process has enough address space left for the allocation. This is especially true for x86 processes which can allocate only 4 GB of memory where we can run out of free addresses where to put our allocation because the memory is fragmented like below. We could allocate the memory but we have no address space hole big enough to satisfy the reservation request.


    I have never seen that happen on x64 processes but it is a pretty common issue on x86 processes. This is called reserving memory in Windows lingo. Apart from checking if enough address space is available in the process nothing happens which is therefore a super fast operation.

    The second stage is to commit memory. The new[], malloc functions will usually reserve and commit the memory in one go with a call to VirtualAlloc on Windows. This time the OS needs to do some further checks.

    • Check if allocation size > Commit Limit
      • The Commit Limit is the maximum memory all applications together can allocate. It is the sum of the physical memory + size of the page file.
    • If the page file has not a fixed size the OS might need to grow the page file for the requested memory which can take quite some time to ensure that the allocation request can be served from physical or page file baked memory.

    After reserving the address space and committing the memory the OS guarantees that your newly allocated memory can be served by the OS either from the page file or (more likely and performant) from physical memory.


    (RAM Image Source

    You see the dotted lines? The OS only guarantees that you can access the memory but it is still not assigned to your process. The OS has returned you a pointer to memory but you still do not have the memory in your process. All memory pages in your process are still empty! Now comes the expensive part. But this involves no API call at all. To force the OS to actually assign the memory to your process you only need to access it. When you access an empty page the CPU will trigger an exception (page fault) and call an back into the operating system. At this time the OS will actually assign the memory to your process working set “database” where the OS keeps track which physical pages are baked by real memory or the page file.

    The operation to add memory to your process working set is called page fault. If only RAM needs to be assigned to your process it is called soft page fault (fast). If you access paged out memory a hard page fault happens (slow, …. very slow) which will cause the OS to start a read operation from the hard disk which can be a lengthy undertaking.

    After the soft/hard page fault your application finally takes over and you can access the memory without any OS interference.


    If the system runs low on memory some least used memory pages are removed from your working set and its contents are put into the page file. I have indicated that with the dotted lines pointing to the page file in the picture above. The next time you access the memory you will get hard page faults which are the source of most sluggish system behavior. It has become much less of a problem if you are lucky enough to have the page file on an SSD which have pretty good random access times.

    Ok that was a pretty long excursion into the details of memory management. But why is the debug build so much faster and why is all of the memory after the allocation already in our process working if I use the debug build? When we examine the memory contents which was returned by allocation request we find some byte pattern (cd cd)


    If you look further what that means at you will find that the C-runtime initializes and and hence access the memory before returning the pointer to the calling code. That is the reason why the debug build was so much faster. The soft fault performance hit did happen already at allocation time because the memory was initialized to the CD CD byte pattern. That is one of the very few cases where the measured performance of a debug build is much better compared to a release build because the most expensive part of memory access has happened before we did start he measurement.

    Memory Copy And Soft Fault Performance Do NOT Scale On Windows

    That was an interesting case but I am getting just started. Lets suppose we want to read a large file from the disk as fast as possible. Most often the file was accessed already by the application some time ago and it is already in the file system cache. If you execute the read operation the OS has nothing to do except to copy the memory from the file system cache to the buffer in your application. When the buffer was just freshly allocated by new[] how fast can we get? Copying the data from the file system cache is a problem that calls for parallelization to speed up things. The interesting thing is how fast can we get if multiple threads are copying in parallel data from A to B.

    Since memory access has some hidden costs upon first access it makes sense to measure the memory copy performance for a freshly allocated array and a second time with the same one. The test essentially should do

    1. Allocate a large array e.g. 2000 MB
    2. Fill it with random data which will be our source array
    3. Allocate a 2000 MB target array
    4. Start 1-n threads
    5. Each thread copies a sub range of source to destination
    6. Measure the time stop step 4-5
    7. Repeat Step 4-6 for a second measurement

    First lets check the results on different CPUs when we hit a “warm” destination array which will exhibit no soft page faults.


    From these numbers we can deduce the “raw” memory copy performance which at some point saturates the CPU memory bus. The numbers obtained here differ by a large margin with the documented ones:

    CPU Theoretical Memory Bandwidth GB/s Max Memory Copy  Performance GB/s
    I7 4770K 25.6 9,3
    E5 2623 v3 59 10,3
    Xeon Gold 6148 ??? 40,8

    At least for my home machine (the 4770K) I get ca. 9.3 GB/s which is off by a large margin of my 25.6 GB/s. I assume that the maximum memory bandwidth was measured either for read or write operations, but not a parallel read/write operation which would mean that I can multiply my measured values with a factor two. Then I would arrive at ca. 18 GB/s which seems to be ok if I attribute the rest to the cache controller which needs also some memory bandwidth. The raw memory copy performance depends on the memory bus and the used memory modules. What happens when I replace from a full memory bank one module? You guessed it: I did another experiment with my home machine. As expected the memory copy performance did drop by 50% from 9,3 GB/s down to 4,7 GB/s which gives a strong hint that memory bandwidth saturating applications should run always on machines which have full memory banks.

    From the graph above it is clear that having more cores is better up to a certain point where the memory bandwidth is reached and more cores do not help anymore. The brand new Xeon Gold CPUs show an impressive performance where up to 10 memcopy threads did still add performance. Now lets change the use case and measure for the first access time which includes soft page faults.


    Well that is interesting. Adding more cores degrade the soft page fault performance  by a large margin. The speed gains added by parallel memcpy are far less than one would expect. To isolate the issue we can measure the page touch time (soft fault performance) with a similar application like the one above just with some multi threading added. We then get this


    The soft page fault performance of Windows 10 does not scale with the numbers of cores. Instead it decreases with the numbers of cores! There seems to be a magic number around 4 concurrent touch threads where we become faster but then the valley is left and the soft fault performance in all cases gets worse compared to the single threaded use case. If something unexpected like this happens it is time to use a profiler to drill deeper. Below is a CPU sampling graph which shows the parallel page touch with 1-40 threads for a 2000 MB array.


    As I have feared there is some internal locking in the Windows soft fault implementation which makes soft faulting a mostly single threaded operation. If multiple threads try to access the memory we get high CPU times in the method ExpAcquireSpinLockExclusiveAtDpcLevelInstrumented which wastes many CPU cycles until it finally gives up which ends in a highly contended lock. Remember: Locks do not scale. That is the reason why the small actual soft page fault work (yellow) is constant while the overhead causes by the lock explodes. This makes not much sense in my opinion when we get more and more cores to play with but the soft fault implementation still uses a process wide lock.

    Another view of the same date like above  named Flame Graph shows the overhead even better. The selected part below is the actual useful work and the huge rest is the overhead of the Spinlock.


    One would think that soft page fault is an essential OS service that should be implemented as fast as possible. Apparently that is not the case for Windows 8 and 10 ( The issue described there was solved by using VirtualLock which is essentially soft faulting the pages from one thread which is much better than to do it concurrently from many threads as you can see from the graph above. I do not know what you think but I have the strong feeling that Microsoft should improve the soft page fault code to finally enter the multi core era. It would be interesting to compare the numbers with Linux because  Linus Torvalds seems to be vigilantly looking at the soft page fault implementation in the Linux kernel.

    With SSDs getting nearly as fast as RAM the soft fault performance becomes a limiting factor in multithreaded applications which would otherwise be memory bus constrained.

    Even seemingly simple things like allocating and accessing memory can have dramatic impact on application performance and scalability. The test application from above is pretty simple but if you start asking why the performance is as it is and you try to optimize it you quickly reach operating system limits and not much later the limits of the hardware. That is actually a good thing because it means that your application is fast as hell.

    If you want to test the soft page fault/memcopy performance for yourself you can check out which should be pretty self explaining. That was the application I did use the produce the charts above.

    Update 1

    Windows 10 Fall Creators update contains a fix for the soft page fault performance. First some numbers from my home machine with 4 physical cores:


    The use case was copying a 2000 MB buffer into not yet touched memory with 1-N threads.

    Creators Update

    Due to the internal locking we did never get close to the actual memory bandwidth because the kernel was busy with its Spinlocks while updating the data structures to add the pages to our process working set. The copy performance did peak around 6,5 GB/s

    Fall Creators Update

    The soft page fault implementation is now much more scalable and we can now max out our memory bandwidth of 9,xGB/s with only 3 cores while we can fully distribute the soft page fault work across threads!


    When we zoom deeper into the soft page fault performance we find that even the single thread soft fault performance has become 43% faster and it scales much better now.



    Below is the Spinlock CPU vs actual work shown for Creators Update


    and here for Fall Creators Update


    The lock is still there but much less time is spent in locking things which is a good thing. How did the MS engineers improve the soft page fault implementation by nearly a factor two? To see this the WPA diff view is helpful along with some Excel magic. The diff view in WPA is nice but it still lacks in my opinion some important features

    • There is no way to mark methods which were removed or added except by drilling to the bottom of the call stacks which is very tedious
    • I want to filter only for added/removed methods which would be very helpful to spot the actual difference and not the consequence deeper in the call stacks

    Anyway. There is nothing Excel cannot fix for us. Below is the pimped diff view of the page touch performance test.


    What stands out is that the Fall Creators Update soft page fault implementation has far less method calls. The Windows Kernel internal List (all red marked list management methods) to maintain the page list was removed in favor of a more simple data structure with better cache locality and less locking. Besides getting rid of the kernel linked list the biggest change seems to be that by default the page size has been increased from 4 KB to 64 KB which means that the kernel needs to update 16 times less often the page table structures which seem to be the biggest change. At least that is my impression by noticing that the method MiGet64KPage consumes most CPU and looks like it was introduced with the Fall Creators Update.

    What about Windows Server?

    According to MS support the soft page fault fix should have made it into Windows Server 1709 which is a desktop less server which is best for container and cloud workloads. That is nice but I need a fix for Server 2016. The Windows Server 2016 soft page fault performance affects all applications, especially the memory hungry ones. Moving forward to a not compatible server edition which is still beta with a shorter long term support contract is not an option.

    It is pretty hard to get hard facts from MS support which issues is fixed with which OS version. The question: Is the issue in that ticket fixed in the build I am running? seems no longer to be easily answerable. That definitely should be improved.

    It Is Time For Time Travel

    Windbg is one of the most powerful yet underused tool in every Windows Developers toolbox. Some time ago a new fancier version of Windbg (Windbg Preview)  which looks like a todays modern application was released as Windows Store App. So far the UI has got a nice ribbon but under the hood no real changes were visible. Today a new version was released which supports a long time internally used tool called time travel tracer (TTT).

    The official documentation is quite good and here is the blog post announcing Time Travel Debugging

    If you have a hard to debug race condition you can now attach the Windbg and check the “Record process with Time Travel Debugging”


    The trace file can grow many GB in size which can make it cumbersome if you want to reproduce an error which needs some time to surface. Luckily Windbg is, although a Store App, still xcopy deployable. Search with Process Explorer where the debugger exe sits and copy the contents of the directory C:\Program Files\WindowsApps\Microsoft.WinDbg_1.0.13.0_x86__8wekyb3d8bbwe\ to your own tool folder like Windbg_1.0.13.0. Then you can run it e.g. from a memory stick or a network share as usual. After searching a bit in the debugger directories you will find the command line version of the Time Travel Trace tool located at

    • x64  Windbg_1.0.13\amd64\TTD\TTD.exe
    • x86 Windbg_1.0.13\amd64\TTD\wow64\TTD.exe

    Now you can go to your problematic machine where the problem occurs without the need to install a store app which might not be possible due to corporate policy/firewall/isolated test network, …. To record a time travel trace from the command line I normally use it in ring buffer mode with a 2 GB buffer which should cover a few seconds or minutes depending on the application activity.


    D:\Tools\Windbg_1.0.13\amd64\TTD>ttd -ring -maxfile 2048 -out c:\temp\ -launch D:\cppError.exe
    Microsoft (R) TTD 1.01.02
    Release: 10.0.16366.1000
    Copyright (C) Microsoft Corporation. All rights reserved.

    Launching D:\cppError.exe

    cppError.exe(x86) (1040): Tracing stopped after 31ms
      Ring trace dumped to c:\temp\


    You get a 2 GB file although the actually recorded data might be much smaller. If you have a short repro it might be better to skip the ring buffer setting.


    Once you have the data it is time to leave the crime scene pack the .run file and analyze it at your favorite developer machine. You can double click the .run file or fire up Windbg and select the Open Trace option. Once you have loaded the trace you can press g to let the application run until the first exception happens or you can set breakpoints. But if nothing is set the application will stop a the first exception with the actual source window and the current values of the local function:


    We find that whenever i is 5 we run into an issue which you could also have found also with a memory dump. But now you can travel back in time by entering p- to check the values were just before the crash. This is incredibly powerful to find the root cause how you did get into a situation. If Time Travel Debugging works it is a great tool. Just keep in mind that it makes the application around 10x times or more slower. You should not expect great performance if Time Travel recording is running.

    As far as I can tell it looks like Time Travel Tracing is built upon the Intel Processor Tracing feature which enables recording the full instruction stream along with all touched memory which is a really powerful feature.


    With every great tool there are things left to be desired.

    • Windbg supports no managed source code window
      • No managed breakpoint setting in source code
    • No managed local variables
    • No managed call stack
    • SOS.dll seems not to work at all with time travel traces

    The debugger shell seems to exist only in the x64 flavor which makes it impossible to load SOS.dll for 32 bit applications into Windbg because the bitness of SOS.dll must match with the bitness of the debugger executable. When I try to get a mixed mode stack inside Windbg SOS.dll can be loaded but it seems to be missing vital information. I really would like to use Windbg with time travel tracing support for managed code (regular .NET Framework and .NET Core) but until now this is a C/C++ fan boys tool only.


    Time Travel Tracing is a novel debugging technique which enables us developers to solve otherwise nearly impossible to find data races in a deterministic way. I hope that the Windbg team will add managed code support in the near future which would bring back feature parity between languages. Why should only C/C++ developer get all the great tools?

    The Mysterious Lost Memory Which Belongs To No Process

    Sometimes you experience sluggish performance for no apparent reason. That are usually the harder issues since it is not clear what is causing it. Things will become a bit messy but I will give some advice how you can pinpoint sluggish system performance with basic OS tools and some advanced tools which are rarely used in public.

    The Symptom

    A server machine did become very slow once the in use memory reported by task manager reached ca. 70% (e.g. 90/128 GB) of the available memory. There was still plenty of free memory but when a new application was started it was very slow and the C drive was very busy. A first suspicion was that the application dlls were loaded from disk again which did not really make sense since the application was started many times before. Everything should be in the file system cache since a long time.


    First Observations

    If you are on a Windows server and you want to view the disk IO graphs in Task Manager like on your normal Windows (8 and later) you need to enable the Disk IO graphs explicitly with

    C>diskperf -y 

    A closer look on disk IO in the task manager showed that the drive hosting the page file was from the very beginning having writes to the page file at ca. 90 MB/s. We certainly did page out a lot of data but why was this happening when only 50/128 GB were in use? 


    What was going on here?

    Advanced Tools

    To see where the memory is going to we can use RAMMap from Sysinternals. It can show system wide memory consumption of processes and the kernel as well. This is the only tool which can show also the lost memory because if it is allocated it must belong at least to the kernel which is also shown.


    Here we find that the modified memory is coming form Shareable memory. Shareable memory is memory which is not backed by a physical file. The only possibility I am aware of to create such memory is to create a file mapping with no file name which creates a page file backed shared memory section. The suspicion here is that we have lost memory to page file backed sections which did land for some reason in the Modified list which is the sign for the kernel to write them back to their backing store. In this case it is the page file which nicely supports our theory because we have high page file write rates.

    That is all nice but the question is which process is doing this? When I did look with Task manager through the processes I found no process with a very high Commit or Working set memory. If nothing helps try to reproduce the problem in isolation so you can better analyze it.

    Create A Repro

    The easiest repro is to create a file mapping, which is mapped, written to and then unmapped but the file mapping object is not closed.

    In C#  this is a few lines of code:

            private unsafe void Allocate()
                // CreateFileMapping
                var file = MemoryMappedFile.CreateNew(null, BytesToAlloc);
                Mappings.Add(file); // root object 
                var accessor = file.CreateViewAccessor(); // MapViewOfFile
                var handle = accessor.SafeMemoryMappedViewHandle;
                byte* ptr = null;
                handle.AcquirePointer(ref ptr);
                var pLong = (long*)ptr;
                Random rand = new Random();
                long nr;
                // fill page file allocated memory with random data which cannot be compressed
                // by Windows 10 memory compression to see the real impact of memory allocation.
                for (long i = 0; i < BytesToAlloc / 8L; i++)
                    nr = (long)((ulong)(rand.Next() << 32) | (ulong)rand.Next());
                    *(pLong + i) = nr;
                TotalAllocated += BytesToAlloc; 
                accessor.Dispose();        // UnmapViewOfFile
                Console.WriteLine($"Modifed: {ModifiedBytes:N0} bytes");

    The full source code can be found here!AhcFq7XO98yJgfZdGGK8ahaOrVnggg. With this sample application we can indeed nicely reproduce the issue that every time the memory is unmapped the kernel takes our mapped memory from the process working set away and puts it into the modified byte list.

    Now depending on the OS different things happen. On Server 2008-2016 the memory is immediately written to the page file. On Server 2016 you could enable memory compression also if you wish (see On Windows 10 machines on the other hand the memory is compressed by default and kept in memory which is much later written to disk once physical memory gets low. Since we have reproduced the symptom we can search for ways how to find the offending process more easily.

    Lets look in task manager when the process has eaten up all my physical memory:


    Nope not a single number which points towards high memory consumption. That is not surprising because the memory was already unmapped and no longer belongs to any process. But our process still has a file mapping handle open which is reason enough for the kernel to keep the data alive. Lets take a dump and look for file mapping objects. In kernel lingo these are called Section objects.

    Windbg – Dump Section Objects

    To dump all file mappings and their size we can use the !handle command. The first argument 0 means to dump all handles, 0xf to dump all handle information and limit the output to file mapping (Section) objects only.

    0:000>!handle 0 f Section
    Handle 0000000000000734
      Type             Section
      Attributes       0
      GrantedAccess    0xf0007:
      HandleCount      2
      PointerCount     65537
      Name             <none>
      Object specific information
        Section base address 0
        Section attributes 0x8000000
        Section max size 0x40000000
    24 handles of type Section

    We can sum up all open file mapping objects manually or use a pretty unknown Windbg extension called mex (see Mex is a Windbg extension written in managed code which is the reason for its name (managed extension). It is used by support engineers within MS quite a lot because it contains a lot of functionality in a single dll, which makes deployment pretty easy. It can even decompile C# code with some help from ILSpy from a memory dump which is pretty useful. When you have downloaded the zip file put the dll of the right bitness into the Windbg\winext folder where Windbg stores its extensions so you can load it simply by its name.

    Now we can make the tedious summing the sizes of all shared memory sections a pretty one liner:

    0:000>.load mex
    0:000> !sum !cut -c 22-35 !grep -r "size" !handle 0 f Section
    Total = 0n19327504385 (0x480025001)

    The !sum command is pretty self explaining. !cut will cut from each line the number by a fixed offset and length of all lines which are filtered for the lines which contain the size with !grep from the !handle command. This reminds me of old linux days where my command lines looked pretty similar like the Windbg commands above.

    From the dump it is clear that we have 19 GB of file mappings created.  But Task Manager is not showing anything. Even the trustworthy VMMap tool


    is not showing anything related to 19 GB of allocated shared memory. If you suspect only one process which is responsible for that you can use Windbg and check out if the process has a file mapping leak. If this is a problem you did either forget to close the file mapping, or someone was unmapping the shared memory too early.

    You can set breakpoints on Windows 8+ at kernelbase or on earlier Windows version at kernel32 where the methods are actually defined. Windows 8 introduced API sets ( which allows the OS guys to split formerly huge dlls into smaller parts where the implementation might be in a different dll. You need to be careful to set the breakpoints at the right location depending on the OS you are currently working on.

    0:000> bp kernelbase!CreateFileMappingW
    0:000> bp kernelbase!UnmapViewOfFile
    0:000> bp kernelbase!UnmapViewOfFileEx

    When you have hit a breakpoint and you are running managed code load the sos  dll get the call stacks

    .loadby sos clr

    Then it is easy to find e.g. who created the file mappings

    00007ffe`06280f20 4883ec48        sub     rsp,48h
    0:000> !ClrStack
    OS Thread Id: 0x59f8 (0)
            Child SP               IP Call Site
    000000aa64d3e720 00007ffe06280f20 [InlinedCallFrame: 000000aa64d3e720] Microsoft.Win32.UnsafeNativeMethods.CreateFileMapping(Microsoft.Win32.SafeHandles.SafeFileHandle, SECURITY_ATTRIBUTES, Int32, Int32, Int32, System.String)
    000000aa64d3e720 00007ffdd724c3e8 [InlinedCallFrame: 000000aa64d3e720] Microsoft.Win32.UnsafeNativeMethods.CreateFileMapping(Microsoft.Win32.SafeHandles.SafeFileHandle, SECURITY_ATTRIBUTES, Int32, Int32, Int32, System.String)
    000000aa64d3e6e0 00007ffdd724c3e8 DomainBoundILStubClass.IL_STUB_PInvoke(Microsoft.Win32.SafeHandles.SafeFileHandle, SECURITY_ATTRIBUTES, Int32, Int32, Int32, System.String)
    000000aa64d3e800 00007ffdd723014b System.IO.MemoryMappedFiles.MemoryMappedFile.CreateCore(Microsoft.Win32.SafeHandles.SafeFileHandle, System.String, System.IO.HandleInheritability, System.IO.MemoryMappedFiles.MemoryMappedFileSecurity, System.IO.MemoryMappedFiles.MemoryMappedFileAccess, System.IO.MemoryMappedFiles.MemoryMappedFileOptions, Int64)
    000000aa64d3e890 00007ffdd72169af System.IO.MemoryMappedFiles.MemoryMappedFile.CreateNew(System.String, Int64, System.IO.MemoryMappedFiles.MemoryMappedFileAccess, System.IO.MemoryMappedFiles.MemoryMappedFileOptions, System.IO.MemoryMappedFiles.MemoryMappedFileSecurity, System.IO.HandleInheritability)
    000000aa64d3e910 00007ffdd7436bd0 System.IO.MemoryMappedFiles.MemoryMappedFile.CreateNew(System.String, Int64)
    000000aa64d3e950 00007ffd8c1c08a0 FileMappingModifiedBytes.PageFileAllocator.Allocate() [\FileMappingModifiedBytes\FileMappingModifiedBytes\PageFileAllocator.cs @ 60]
    000000aa64d3ea50 00007ffd8c1c0736 FileMappingModifiedBytes.PageFileAllocator.SlowlyAllocateUntilOOM() [\FileMappingModifiedBytes\FileMappingModifiedBytes\PageFileAllocator.cs @ 49]
    000000aa64d3ead0 00007ffd8c1c05b1 FileMappingModifiedBytes.Program.Main(System.String[]) [\FileMappingModifiedBytes\FileMappingModifiedBytes\Program.cs @ 20]
    000000aa64d3ed80 00007ffdeb7b6a53 [GCFrame: 000000aa64d3ed80] 

    When the mappings are closed you get also the full call stack

    0:000> !ClrStack
    OS Thread Id: 0x59f8 (0)
            Child SP               IP Call Site
    000000aa64d3e328 00007ffe06285400 [InlinedCallFrame: 000000aa64d3e328] Microsoft.Win32.UnsafeNativeMethods.UnmapViewOfFile(IntPtr)
    000000aa64d3e328 00007ffdd724b23e [InlinedCallFrame: 000000aa64d3e328] Microsoft.Win32.UnsafeNativeMethods.UnmapViewOfFile(IntPtr)
    000000aa64d3e300 00007ffdd724b23e DomainBoundILStubClass.IL_STUB_PInvoke(IntPtr)
    000000aa64d3e3b0 00007ffdd72305a7 Microsoft.Win32.SafeHandles.SafeMemoryMappedViewHandle.ReleaseHandle()
    000000aa64d3e548 00007ffdeb7b6a53 [GCFrame: 000000aa64d3e548] 
    000000aa64d3e6b8 00007ffdeb7b6a53 [GCFrame: 000000aa64d3e6b8] 
    000000aa64d3e778 00007ffdeb7b6a53 [HelperMethodFrame_1OBJ: 000000aa64d3e778] System.Runtime.InteropServices.SafeHandle.InternalDispose()
    000000aa64d3e870 00007ffdd72309c1 System.IO.MemoryMappedFiles.MemoryMappedView.Dispose(Boolean)
    000000aa64d3e8a0 00007ffdd72307b9 System.IO.MemoryMappedFiles.MemoryMappedViewAccessor.Dispose(Boolean)
    000000aa64d3e8e0 00007ffdd723075a System.IO.MemoryMappedFiles.MemoryMappedViewAccessor.Dispose(Boolean)
    000000aa64d3e920 00007ffddab2b15a System.IO.UnmanagedMemoryAccessor.Dispose()
    000000aa64d3e950 00007ffd8c1c0a48 FileMappingModifiedBytes.PageFileAllocator.Allocate() [\FileMappingModifiedBytes\FileMappingModifiedBytes\PageFileAllocator.cs @ 83]
    000000aa64d3ea50 00007ffd8c1c0736 FileMappingModifiedBytes.PageFileAllocator.SlowlyAllocateUntilOOM() [\FileMappingModifiedBytes\FileMappingModifiedBytes\PageFileAllocator.cs @ 49]
    000000aa64d3ead0 00007ffd8c1c05b1 FileMappingModifiedBytes.Program.Main(System.String[]) [\FileMappingModifiedBytes\FileMappingModifiedBytes\Program.cs @ 20]
    000000aa64d3ed80 00007ffdeb7b6a53 [GCFrame: 000000aa64d3ed80] 

    Now we have enough information to come up with a

    Theory Of The Problem

    This is how we got into that situation

    1. Someone creates page file backed shared memory as file mapping.
    2. These file mappings are put into the address space of our process with MapViewOfFile.
    3. After usage the file mapping is removed from the process working set with UnmapViewOfFile. The file mapping object is closed much later or never.
    4. The kernel removes the memory pages from the process working set and moves them to the Modified List.
    5. No process owns that memory anymore and the OS is free to page it out to the page file.
    6. After the modified pages were written to disk they are put into the standby list, overwriting other previously cached things.

    The picture below shows the basic operations the kernel performs.


    The end result is that the unmapped memory from our process is written to the page file and at the same time it is put into the file system cache where it replaces older entries. This has far reaching implications. If you unmap the memory but do not close the file mapping object almost at the same time the OS will page the memory out if no other process has the same memory mapped. An innocent file mapping handle leak with large memory mappings can flush your complete file system cache at random times and there is no clear indicator why your or other application suddenly did become much slower.

    On Windows 10 Anniversary and later you can see the effect of flushing the file system cache more directly by looking at the task manager when the compressed memory goes up and up but the cached memory stored in the standby list is becoming less and less.


    Fresh paged out memory seems to have the highest priority in the file system cache which can overwrite anything else what was inside it. This is also true on server OS where you can check with Rammap that the file system cache was flushed. That explains why we were having so much read activity on the C drive because our file system was flushed and the application dlls needed to be loaded from disk again. If you have a large amount of compressed memory in Windows 10 but not much In use memory the chances are pretty good that you have a file mapping leak somewhere.

    How Things Look in ETW

    On my Windows 10 Anniversary machine I can nicely see that my paged out memory directly goes into the working set of MemCompression (Memory Compression in Process Explorer, Hacker)  which is not shown by the Windows task manager. At some point it is so huge that it needs to page out memory to my SSD with up to 400 MB/s. While MemCompression (purple) allocates more memory, the standby lists (green) becomes smaller and smaller.


    With ETW you can also find out who is responsible for the high modified bytes which are written to the page file with CPU Sampling tracing. That is very useful if you do not know which process is causing it and you cannot interrupt the server processes with memory dumps which might upset people working on the server. Since we know that based on our theory the modified bytes grow when we unmap the data we can simply view all callers of UnmapViewOfFile and follow the callers until you find the call stacks which have the best correlation between the growth of modified bytes and calling UnmapViewOfFile. With our tester application we get a perfect correlation:


    The blue bars map perfectly to the rise of modified bytes when we unmap a file section. You can also treat the file mapping issue as handle leak where you can trace with ETW Handle Leak tracing all Section objects which call CreateFileMapping which can be used to verify that no file mappings of this type exist anymore to verify that a fix works as expected. WPA is the perfect tool to visually correlate different metrics to check if there is a dependency. You can come up with a theory what depends on what, graph the different things underneath and then you see a pattern …. or not. Then it is time for the next theory.

    The Real Root Cause


    Problem solved? Only partially. This only explains sluggish behavior when the machine gets into high memory load which then flushes the file system cache. But there were issues right from the beginning even when there still was plenty of data in the file system cache. Can you spot based on the given data above where the problem is?




    We have a nice RAID on the machine but the pageouts were happening only with 100MB/s. That is not the write throughput one would expect from an expensive RAID. What I would expect is something like this


    but it was about 5 times slower than usually. Perhaps one disk did fail and the RAID was not scaling properly anymore? Nope. But there was a warning logged somewhere that the RAID battery has failed. The battery in the RAID is there to write all cached data from its internal DRAM cache to the disks in the case of a power failure. What happens if the battery fails? The RAID turns off its internal cache to prevent data loss if there is a power failure. We were therefore running the sever the whole time without a much wanted cache! I am not the one to take credit for finding the disabled RAID cache. It is all about teamwork where everyone looks at different aspects of a problem to (hopefully) find all issues. Thanks for finding that one. You know who you are.

    Luckily we do not need to wait to buy a new battery but we can enable the cache even when the battery is not present:


    That setting enables the RAID cache regardless of the state of the battery which is exactly what we want. In fact the battery was not broken but opening the server and dusting off the battery did solve the battery issue as well.

    It’s The Environment Stupid!

    When you write software it is not always the software which causes a change. It still runs on hardware which can change/fail too. You need to monitor not only yourself but also the environment where it is running. If you fail to take that into account you can search for a long time at the wrong or not most significant spot.

    Funny side note: I drive by car every day to work. When I did refill my windshield washing fluid yesterday I did notice an unauthorized hardware change:


    A marten


    seems to have brought into my car his own pet toys to make the place more cozy. Next action: Install some marten repellant to prevent more hardware changes:


    Have a nice day and keep your hardware marten safe.