[Mono-dev] High threadpool CPU usage

[Ludovic Henry]

Hi Chris,

The first stacktrace you are observing is for threadpool thread parking. We
use this technique for threads that are currently not doing anything, to
keep them around for a little while (5-60 seconds) so if we have burst of
usage, we do not end up destroying and creating threads all the time.

The second stacktrace you are observing is, as you noted, when performing a
garbage collection, when the GC thread is suspending all the running
threads. So if you are witnessing this trace very often, it means a thread
is allocating memory very quickly triggering GC collection very often.

So I would need more information to tell you exactly why it would use 100%
CPU. Also which version of Mono are you running?

Thank you very much,
Ludovic

On Wed, May 25, 2016 at 8:30 PM Chris Swiedler <cswiedler at trionworlds.com>
wrote:

> We have a server app which is periodically going into a mode where all
> threadpool threads start running at very high CPU. I've run pstack when
> it's in this mode, and every time I do it, nearly all the threadpool
> threads have this stack:
>
> #0  pthread_cond_timedwait@@GLIBC_2.3.2 () at
> ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_timedwait.S:238
> #1  0x0000000000618817 in mono_cond_timedwait_ms (cond=cond at entry=0x7fe7ee1fddc0,
> mutex=0x241eb78, timeout_ms=<optimized out>) at mono-mutex.c:181
> #2  0x0000000000586f28 in worker_park () at threadpool-ms.c:509
> #3  worker_thread (data=<optimized out>) at threadpool-ms.c:607
> #4  0x00000000005841e9 in start_wrapper_internal (data=<optimized out>) at
> threads.c:725
> #5  start_wrapper (data=<optimized out>) at threads.c:772
> #6  0x0000000000621026 in inner_start_thread (arg=0x7fe831df4650) at
> mono-threads-posix.c:97
> #7  0x00007fe88a55edf5 in start_thread (arg=0x7fe7ee1fe700) at
> pthread_create.c:308
> #8  0x00007fe88a28c1ad in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:113
>
> Usually one thread will have a stack like this:
>
> #0  sem_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/sem_wait.S:85
> #1  0x000000000061aa37 in mono_sem_wait (sem=0x9542c0
> <suspend_ack_semaphore>, alertable=alertable at entry=0) at
> mono-semaphore.c:107
> #2  0x00000000005c77bd in sgen_wait_for_suspend_ack (count=count at entry=18)
> at sgen-os-posix.c:188
> #3  0x00000000005c78f9 in sgen_thread_handshake (suspend=suspend at entry=1)
> at sgen-os-posix.c:224
> #4  0x00000000005c7e92 in sgen_client_stop_world
> (generation=generation at entry=0) at sgen-stw.c:234
> #5  0x00000000005e6aca in sgen_stop_world (generation=0) at sgen-gc.c:3389
> #6  0x00000000005e6c29 in sgen_perform_collection (requested_size=4096,
> generation_to_collect=0, reason=0x6d9595 "Nursery full", wait_to_finish=0)
> at sgen-gc.c:2322#7  0x00000000005da62a in sgen_alloc_obj_nolock
> (vtable=vtable at entry=0x7fe85c0343c0, size=size at entry=128) at
> sgen-alloc.c:291
> #8  0x00000000005da913 in sgen_alloc_obj (vtable=vtable at entry=0x7fe85c0343c0,
> size=128) at sgen-alloc.c:457
> #9  0x00000000005c86e9 in mono_gc_alloc_obj (vtable=vtable at entry=0x7fe85c0343c0,
> size=<optimized out>) at sgen-mono.c:936
> #10 0x00000000005a8b54 in mono_object_new_alloc_specific
> (vtable=vtable at entry=0x7fe85c0343c0) at object.c:4385
> #11 0x00000000005a8bf0 in mono_object_new_specific (vtable=0x7fe85c0343c0)
> at object.c:4379
> #12 0x00000000005a8c8c in mono_object_new (domain=domain at entry=0x1ded1c0,
> klass=<optimized out>) at object.c:4318
> #13 0x00000000005ac1c9 in mono_async_result_new (domain=domain at entry=0x1ded1c0,
> handle=handle at entry=0x0, state=0x0, data=data at entry=0x0,
> object_data=object_data at entry=0x7fe8838af020) at object.c:5768
> #14 0x00000000005887ff in mono_threadpool_ms_begin_invoke
> (domain=0x1ded1c0, target=target at entry=0x7fe8838aee38, method=method at entry=0x2963d28,
> params=params at entry=0x7fe7ed9f8f10) at threadpool-ms.c:1300
> #15 0x000000000054b547 in mono_delegate_begin_invoke
> (delegate=0x7fe8838aee38, params=0x7fe7ed9f8f10) at marshal.c:2111
> #16 0x00000000416d29d8 in ?? ()
> #17 0x0000000000000000 in ?? ()
>
> Just from reading the first stack, it doesn't look like
> mono_cond_timedwait_ms would spin, at least as long as the timeout_ms
> wasn't 0. For the second stack, I don't know whether that's a normal
> garbage collection pass or (since we see it frequently) a sign that garbage
> collection is happening too often.
>
> Can anyone give me some pointers for where to dig more deeply?
>
> Thanks,
> chris
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