Re: pthread_mutex_unlock (was Re: sched_yield() makes OpenLDAP slow)

From: Howard Chu
Date: Thu Jan 26 2006 - 09:23:11 EST


Nick Piggin wrote:
Howard Chu wrote:
But then we have to deal with you folks' bizarre notion that sched_yield() can legitimately be a no-op, which also defies the POSIX spec. Again, in SUSv3 "The /sched_yield/() function shall force the running thread to relinquish the processor until it again becomes the head of its thread list. It takes no arguments." There is no language

How many times have we been over this? What do you think the "head of
its thread list" might mean?

here saying "sched_yield *may* do nothing at all." There are of course

There is language saying SCHED_OTHER is arbitrary, including how the
thread list is implemented and how a task might become on the head of
it.

They obviously don't need to redefine exactly what sched_yield may do
under each scheduling policy, do they?

As Dave Butenhof says so often, threading is a cooperative programming model, not a competitive one. The sched_yield function exists for a specific purpose, to let one thread decide to allow some other thread to run. No matter what the scheduling policy, or even if there is no scheduling policy at all, the expectation is that the current thread will not continue to run unless there are no other runnable threads in the same process. The other important point here is that the yielding thread is only cooperating with other threads in its process. The 2.6 kernel behavior effectively causes the entire process to give up its time slice, since the yielding thread has to wait for other processes in the system before it can run again. Again, if folks wanted process scheduling behavior they would have used fork().

By the way, I've already raised an objection with the Open Group asking for more clarification here.
http://www.opengroup.org/austin/aardvark/latest/xshbug2.txt request number 120.

--
-- 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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