Re: sched_yield() makes OpenLDAP slow

[Howard Chu <hyc@symas.com>]

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.  http://www.symas.com
  Director, Highland Sun        http://highlandsun.com/hyc
  OpenLDAP Core Team            http://www.openldap.org/project/