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Why does the number of threads reported by WinDbg, Task Manager and VS Debugger differ?

Answer 1

+1 A:

My guess is the thread pool had periods of heavy load and then killed the threads when the load died down.

ChaosPandion 2010-02-10 22:52:42

When I collected the memory dump, my app had been idle for a long time (hours).

Sly 2010-02-11 13:09:49

Answer 2

+1 A:

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?

highphilosopher 2010-02-11 19:08:31

You're right, we are leaking managed Thread objects. I confirmed that using .Net Memory Profiler. Still, I'd like to understand why Visual Studio and the Task Manager don't agree on the number of threads...

Sly 2010-02-11 23:15:48

This may not be the answer that you wanted, but task manager reports some rather odd values sometimes. One good example is with memory. The memory counter in Task manager is you're application's working set. This is pretty much useless information because part of this memory is shared with other processes (depending on your app code, managed, or unmanaged, etc.). I try to steer clear of task manager for numbers, and stick to perfmon whenever possible.

highphilosopher 2010-03-01 22:51:49

Answer 3

+7 A:

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.

Brian Rasmussen 2010-02-19 09:06:05

@Brian : Thanks! How about the Visual Studio Debugger? Why does it report 7 threads and not 16 as Task Manager does? I expected that VS would either report 456 threads or 16.

Sly 2010-02-19 13:52:30

@Sly: Sorry I didn't address the VS question. I reckoned it was a typo since you were talking about WinDbg. My guess is that VS filters some of the CLR threads from the display.

Brian Rasmussen 2010-02-19 14:02:25

If I attach to a process while it is being debugged by VS, I see more threads in WinDbg using the ~ command than VS displays, so VS seems to filter some of them.

Brian Rasmussen 2010-02-19 14:06:10

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Why does the number of threads reported by WinDbg, Task Manager and VS Debugger differ?

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