system scope threads entering STOP state

Julian Elischer julian at
Fri Jul 15 20:16:33 GMT 2005

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 that the other threads would be 
> stopped during a fork(2) but it looked to me like the STOP would be 
> brief.

You were *what*?   "aware"?,  "suspicious"?    :-)

> 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

Is it possible to duplicate this on request?

> Thanks,
> Guy

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