system scope threads entering STOP state

Guy Helmer ghelmer at
Wed Aug 17 16:18:10 GMT 2005

Julian Elischer wrote:

> Guy Helmer wrote:
>> Julian Elischer wrote:
>>> Guy Helmer wrote:
>>>> I have a long-running multithreaded process on FreeBSD 5.4 (SMP, 
>>>> PREEMTPION, SCHED_4BSD) linked with libpthread and I'm creating the 
>>>> threads with attribute PTHREAD_SCOPE_SYSTEM.  The threads need to 
>>>> be processing input in near-real-time or its input buffers overflow.
>>>> I've modified the program so that a thread can fork/execl/waitpid 
>>>> (without WNOHANG) to use an external program for further processing 
>>>> on a batch of input (sometimes via a pipe, other times via writing 
>>>> to a file).  However, even under a light input load, the program is 
>>>> now dropping input.  While running top(1) in thread mode, I 
>>>> occasionally find all the program's threads are in the STOP state 
>>>> for several consecutive seconds.  Is there anything related to the 
>>>> frequent use of fork, execve, or wait4 that would be likely to 
>>>> cause such a situation?  I'm not seeing anything obvious in my 
>>>> reading of the kernel sources.
>>> duirng a fork the parent process is in a variant of the  "STOPPED" 
>>> state, or, rather, if you
>>> look at top -H you should see that all teh threads except for that 
>>> doing the fork, are in
>>> the STOPPED state.
>>> This is because while a thread is forking the process needs to be 
>>> single threaded so that
>>> there is a consistent image to be copied to teh child.
>>> the single threaded state is also enterred for exit() and execve(), 
>>> though that should not affect your program.
>>> I can't imagine why the state would persist for any length of time, 
>>> unless there is another thread
>>> that is in an uninterruptible wait. In that case the other threads 
>>> have to wait for it to complete
>>> what it is doing and come back.  I have considerred whether such a 
>>> thread should not be considerred
>>> "already suspended" and in fact some earlier versions of the code 
>>> did that, however it leads to some
>>> inconsistancies and the danger that such a thread will be suspended 
>>> holding some resource
>>> that it should not hold for any length of time.
>> Thanks for the explanation.  I was [aware] that the other threads 
>> would be stopped during a fork(2) but it looked to me like the STOP 
>> would be brief.
>> Would an "uninterruptible wait" include system calls like a write(2) 
>> of a large buffer?  That would explain it...
> it's hard to say.. Possibly yes, if it had to allocate buffer space. 
> However this is a question for
> others..
> Is it possible to duplicate this on request?

[where did the past month go?]

I think I found the culprit - I think the process in question was 
actually dumping core and it is a large process - between 50MB and 100MB 
- so that would explain the 10+ seconds all the threads were in the STOP 
state.  It was difficult to notice while running top(1) since a watchdog 
process immediately restarts the multi-threaded process if it exits due 
to things like segfaults, and I was paying attention to the state 
column, not the PID column.

Sorry for what was a bit of a wild-goose chase,

Guy Helmer, Ph.D.
Principal System Architect
Palisade Systems, Inc.

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