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From: Nikita Danilov <nikita@clusterfs.com>
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Date: Sat, 20 Aug 2005 17:23:39 +0400
To: Howard Chu <hyc@symas.com>
Cc: Linux Kernel Mailing List <Linux-Kernel@Vger.Kernel.ORG>
Subject: Re: sched_yield() makes OpenLDAP slow
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Howard Chu writes:
 > Nikita Danilov wrote:

[...]

 > 
 > >  What prevents transaction monitor from using, say, condition
 > >  variables to "yield cpu"? That would have an additional advantage of
 > >  blocking thread precisely until specific event occurs, instead of
 > >  blocking for some vague indeterminate load and platform dependent
 > >  amount of time.
 > 
 > Condition variables offer no control over which thread is waken up. 

When only one thread waits on a condition variable, which is exactly a
scenario involved, --sorry if I weren't clear enough-- condition signal
provides precise control over which thread is woken up.

 > We're wandering into the design of the SleepyCat BerkeleyDB library 
 > here, and we don't exert any control over that either. BerkeleyDB 
 > doesn't appear to use pthread condition variables; it seems to construct 
 > its own synchronization mechanisms on top of mutexes (and yield calls). 

That returns us to the core of the problem: sched_yield() is used to
implement a synchronization primitive and non-portable assumptions are
made about its behavior: SUS defines that after sched_yield() thread
ceases to run on the CPU "until it again becomes the head of its thread
list", and "thread list" discipline is only defined for real-time
scheduling policies. E.g., 

int sched_yield(void)
{
       return 0;
}

and

int sched_yield(void)
{
       sleep(100);
       return 0;
}

are both valid sched_yield() implementation for non-rt (SCHED_OTHER)
threads.

Nikita.