Re: sched_yield() makes OpenLDAP slow

[Robert Hancock <hancockr@shaw.ca>]

Howard Chu wrote:
> POSIX requires a reschedule to occur, as noted here:
> http://blog.firetree.net/2005/06/22/thread-yield-after-mutex-unlock/

No, it doesn't:

> 
> The relevant SUSv3 text is here
> http://www.opengroup.org/onlinepubs/000095399/functions/pthread_mutex_unlock.html 

"If there are threads blocked on the mutex object referenced by mutex 
when pthread_mutex_unlock() is called, resulting in the mutex becoming 
available, the scheduling policy shall determine which thread shall 
acquire the mutex."

This says nothing about requiring a reschedule. The "scheduling policy" 
can well decide that the thread which just released the mutex can 
re-acquire it.

> I suppose if pthread_mutex_unlock() actually behaved correctly we could 
> remove the other sched_yield() hacks that didn't belong there in the 
> first place and go on our merry way.

Generally, needing to implement hacks like this is a sign that there are 
problems with the synchronization design of the code (like a mutex which 
has excessive contention). Programs should not rely on the scheduling 
behavior of the kernel for proper operation when that behavior is not 
defined.

-- 
Robert Hancock      Saskatoon, SK, Canada
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