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

[Nick Piggin]

linux-os (Dick Johnson) wrote:
> On Thu, 26 Jan 2006, Nick Piggin wrote:

> > What cases has sched_yield mucked up for you, and why do you
> > think the problem is sched_yield mucking up? Can you solve it
> > using mutexes?
> >
> > Thanks,
> > Nick
>
>
> Somebody wrote code that used Linux Threads. We didn't know
> why it was so slow so I was asked to investigate. It was
> a user-interface where high-speed image data gets put into
> a buffer (using DMA) and one thread manipulates it. Another
> thread copies and crunches the data, then displays it. The
> writer insisted that he was doing the correct thing, however
> the response sucked big time. I ran top and found that the
> threaded processes were always grabbing big chunks of
> CPU time. Searching for every instance of sched_yield(), I
> was going to replace it with a diagnostic. However, the code
> ran beautifully when the 'fprintf(stderr, "Message\n"' was
> in the code! The call to write() sleeps. That gave the
> CPU to somebody who was starving. The 'quick-fix" was
> to replace sched_yield() with usleep(0).
>
> The permanent fix was to not use threads at all.

This sounds like a trivial producer consumer problem that you
would find in any basic books on synchronisation, threading, or
operating systems.

If it was not a realtime system, then I can't believe it has any
usages of sched_yield in there at all. If it is a realtime system,
then replacing them with something else could easily have broken
it.

Also, I'm not sure that you can rely on write or usleep for 0
microseconds to sleep.

> Cheers,
> Dick Johnson
> Penguin : Linux version 2.6.13.4 on an i686 machine (5589.66 BogoMips).
> Warning : 98.36% of all statistics are fiction.
> .
>
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>
> Thank you.

Any chance you can get rid of that crazy disclaimer when posting
to lkml, please?

Thanks,
Nick

--
SUSE Labs, Novell Inc.
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