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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