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SubjectRe: sched_yield() makes OpenLDAP slow
Howard Chu writes:
> Lee Revell wrote:
> > On Sat, 2005-08-20 at 11:38 -0700, Howard Chu wrote:
> > > But I also found that I needed to add a new yield(), to work around
> > > yet another unexpected issue on this system - we have a number of
> > > threads waiting on a condition variable, and the thread holding the
> > > mutex signals the var, unlocks the mutex, and then immediately
> > > relocks it. The expectation here is that upon unlocking the mutex,
> > > the calling thread would block while some waiting thread (that just
> > > got signaled) would get to run. In fact what happened is that the
> > > calling thread unlocked and relocked the mutex without allowing any
> > > of the waiting threads to run. In this case the only solution was
> > > to insert a yield() after the mutex_unlock().
> >
> > That's exactly the behavior I would expect. Why would you expect
> > unlocking a mutex to cause a reschedule, if the calling thread still
> > has timeslice left?
> That's beside the point. Folks are making an assertion that
> sched_yield() is meaningless; this example demonstrates that there are
> cases where sched_yield() is essential.

It is not essential, it is non-portable.

Code you described is based on non-portable "expectations" about thread
scheduling. Linux implementation of pthreads fails to satisfy
them. Perfectly reasonable. Code is then "fixed" by adding sched_yield()
calls and introducing more non-portable assumptions. Again, there is no
guarantee this would work on any compliant implementation.

While "intuitive" semantics of sched_yield() is to yield CPU and to give
other runnable threads their chance to run, this is _not_ what standard
prescribes (for non-RT threads).

> --
> -- Howard Chu

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