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SubjectRe: sched_yield() makes OpenLDAP slow
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.

-- Howard Chu
Chief Architect, Symas Corp.
Director, Highland Sun
OpenLDAP Core Team

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