Re: sched_yield() makes OpenLDAP slow
From: Nikita Danilov
Date: Sat Aug 20 2005 - 08:26:16 EST
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.,
are both valid sched_yield() implementation for non-rt (SCHED_OTHER)
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/