system scope threads entering STOP state

Guy Helmer ghelmer at
Fri Jul 15 13:36:00 GMT 2005

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.

Would an "uninterruptible wait" include system calls like a write(2) of 
a large buffer?  That would explain it...


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

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