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SubjectRe: sched_yield() makes OpenLDAP slow
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.
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