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SubjectRe: sched_yield() makes OpenLDAP slow
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On Sat, 20 Aug 2005, Robert Hancock wrote:

> Howard Chu wrote:
>> I'll note that we removed a number of the yield calls (that were in
>> OpenLDAP 2.2) for the 2.3 release, because I found that they were
>> redundant and causing unnecessary delays. My own test system is running
>> on a Linux 2.6.12.3 kernel (installed over a SuSE 9.2 x86_64 distro),
>> and OpenLDAP 2.3 runs perfectly well here, now that those redundant
>> calls have been removed. 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(). So again, for those of you claiming
>> "oh, all you need to do is use a condition variable or any of the other
>> POSIX synchronization primitives" - yes, that's a nice theory, but
>> reality says otherwise.
>
> I encountered a similar issue with some software that I wrote, and used
> a similar workaround, however this was basically because there wasn't
> enough time available at the time to redesign things to work properly.
> The problem here is essentially caused by the fact that the mutex is
> being locked for an excessively large proportion of the time and not
> letting other threads in. In the case I am thinking of, posting the
> messages to the thread that was hogging the mutex via a signaling queue
> would have been a better solution than using yield and having correct
> operation depend on undefined parts of thread scheduling behavior..
>
> --

> Robert Hancock Saskatoon, SK, Canada
> To email, remove "nospam" from hancockr@nospamshaw.ca
> Home Page: http://www.roberthancock.com/
>

I reported thet sched_yield() wasn't working (at least as expected)
back in March of 2004.

for(;;)
sched_yield();

... takes 100% CPU time as reported by `top`. It should take
practically 0. Somebody said that this was because `top` was
broken, others said that it was because I didn't know how to
code. Nevertheless, the problem was not fixed, even after
schedular changes were made for the current version.

One can execute:

usleep(0);
... instead of:
sched_yield();

... and Linux then performs exactly like other Unixes when
code is waiting on mutexes.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.12.5 on an i686 machine (5537.79 BogoMips).
Warning : 98.36% of all statistics are fiction.
.
I apologize for the following. I tried to kill it with the above dot :

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