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SubjectRe: pthread_mutex_unlock (was Re: sched_yield() makes OpenLDAP slow)
Lee Revell wrote:
> On Wed, 2006-01-25 at 10:26 -0800, Howard Chu wrote:
>> The SUSv3 text seems pretty clear. It says "WHEN
>> pthread_mutex_unlock()
>> is called, ... the scheduling policy SHALL decide ..." It doesn't say
>> MAY, and it doesn't say "some undefined time after the call."
> This does NOT require pthread_mutex_unlock() to cause the scheduler to
> immediately pick a new runnable process. It only says it's up the the
> scheduling POLICY what to do. The policy could be "let the unlocking
> thread finish its timeslice then reschedule".

This is obviously some very old ground.

Kaz's post clearly interprets the POSIX spec differently from you. The
policy can decide *which of the waiting threads* gets the mutex, but the
releasing thread is totally out of the picture. For good or bad, the
current pthread_mutex_unlock() is not POSIX-compliant. Now then, if
we're forced to live with that, for efficiency's sake, that's OK,
assuming that valid workarounds exist, such as inserting a sched_yield()
after the unlock.

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
here saying "sched_yield *may* do nothing at all." There are of course
cases where it will have no effect, such as when called in a
single-threaded program, but those are the exceptions that define the
rule. Otherwise, the expectation is that some other runnable thread will
acquire the CPU. Again, note that sched_yield() is a core function of
the Threads specification, while scheduling policies are an optional
feature. The function's core behavior (give up the CPU and make some
other runnable thread run) is invariant; the current thread gives up the
CPU regardless of which scheduling policy is in effect or even if
scheduling policies are implemented at all. The only behavior that's
open to implementors is which *of the other runnable threads* is chosen
to take the place of the current thread.

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

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