While my .Net 3.5 app was running, the Windows Task Manager shown that my app had 16 threads. I collected a memory dump for the process and opened it using WinDbg/SOS.
Running the !threads command reveals that I have :
ThreadCount: 456
UnstartedThread: 0
BackgroundThread: 6
PendingThread: 0
DeadThread: 449
Hosted Runtime: no
Here are the first few lines of the !threads output:
ID OSID ThreadOBJ State GC Context Domain Count APT Exception
0 1 2848 004366a8 6020 Enabled 11738178:11738778 0042a9f0 0 STA
2 2 1820 004430e0 b220 Enabled 00000000:00000000 0042a9f0 0 MTA (Finalizer)
7 5 2c38 055d6330 80a220 Enabled 00000000:00000000 0042a9f0 0 MTA (Threadpool Completion Port)
8 4 e18 04116900 180b220 Enabled 1157cdc8:1157e778 0042a9f0 0 MTA (Threadpool Worker)
XXXX 6 0 055f94b0 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 7 0 05649228 9820 Enabled 00000000:00000000 0042a9f0 0 MTA
XXXX 8 0 0567d4f8 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 9 0 05688d68 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX a 0 056fd680 9820 Enabled 00000000:00000000 0042a9f0 0 MTA
XXXX b 0 0575d7f0 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX c 0 056fd250 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX d 0 0572a780 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX e 0 0f082668 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX f 0 0f082a38 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 10 0 0570ca68 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 11 0 0570ce50 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
10 12 3fb0 0570d238 180b220 Enabled 00000000:00000000 0042a9f0 0 MTA (Threadpool Worker)
XXXX 13 0 0570d620 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 14 0 0570da08 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 15 0 0570ddf0 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 16 0 0570e1d8 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 17 0 0570e5c0 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 18 0 0579e540 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 19 0 0579e928 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 1a 0 0579ed10 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 1b 0 0579f0f8 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 1c 0 0579f4e0 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 1d 0 0579f8c8 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 1e 0 0579fcb0 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 1f 0 057a0098 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
XXXX 20 0 057a0480 9820 Enabled 00000000:00000000 0042a9f0 0 Ukn
If I attach the Visual Studio Debugger to the running process, the Threads windows shows 7 threads.
I have a few questions:
- Why does WinDbg say there are 456 threads and Task Manager says 16
- Why does Task Manager say there are 16 threads and Visual Studio Debugger says 7
- The !thread command shows that all those dead threads don't have a OSID value. Does that mean they are no longer known to the OS are and just .Net objects that are layout around? What is a dead thread exactly?
- Should I worry about the high number of dead threads?
EDIT Feb 11th 2010: Here is more information about my app. We use background threads to poll the server and perform other tasks. Those tasks are executed every few minutes. We do not use the .Net Thread Pool.
EDIT Feb 18th 2010: I fixed the managed Thread object leak in our program (thanks to @highphilosopher). However, my question about why WinDbg, Task Manager and VS Debugger don't agree on the number of threads is still unanswered. Can anyone explain?
EDIT Mar 1st 2010: I'm still interested in knowing why Task Manager and Visual Studio Debugger don't agree on the number of threads. Why does Visual studio filter some of the threads? What kind of threads does it filter out?
Task manager reports the total number of threads for your process while !threads reports the number of managed threads. If you use the ~ command in WinDbg you will see all thread threads for the process.
The output from !threads in your output shows a lot of dead threads. The threads listed with XXXX for id are threads that have terminated but the corresponding thread objects have not yet been collected. I.e. the reported number is much higher than the actual number of threads. The thread count numbers state that 449 of the 456 threads are dead.
I find the number of threads high and if the application has been idle it is odd that the're still around, but without further info it is hard to be more specific.
My guess is the thread pool had periods of heavy load and then killed the threads when the load died down.
What you're seeing sounds like you have a reference to these threads on a different thread. Since the thread is referenced it can't be Collected by the GC. The thread has completed execution of the method that it was sent to do so it is in neither a sleep state, nor runnable state, so it must be dead. Check your code for a Collection of threads or something similar. Perhaps an event that gets hooked to by the thread, but never unhooked?
Another reason why the thread counts will not match is that Task Manager collects information using noninvasive techniques. A debugger usually, and by default uses an invasive attach.
What this means, is that, any time you debug a running windows application, the debug API inject's a remote thread into the target application and call's KERNEL32!DebugBreak, which is an int3 (0xcc) instruction. At this point the process breaks and stop's executing, however, you now have at least one additional debug thread running.
来源：https://stackoverflow.com/questions/2240918/why-does-the-number-of-threads-reported-by-windbg-task-manager-and-vs-debugger