Message-ID: <43C9511D.70500@symas.com>
Date: Sat, 14 Jan 2006 11:29:33 -0800
From: Howard Chu <hyc@symas.com>
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To: linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: sched_yield() makes OpenLDAP slow
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Resurrecting a dead horse...
> *From: *Lee Revell
> *Date: * Sat Aug 20 2005 - 15:57:36 EST
>
> ------------------------------------------------------------------------
> 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?
>
> Lee

POSIX requires a reschedule to occur, as noted here:
http://blog.firetree.net/2005/06/22/thread-yield-after-mutex-unlock/

The relevant SUSv3 text is here
http://www.opengroup.org/onlinepubs/000095399/functions/pthread_mutex_unlock.html

I suppose if pthread_mutex_unlock() actually behaved correctly we could 
remove the other sched_yield() hacks that didn't belong there in the 
first place and go on our merry way.

-- 
  -- 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/

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