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SubjectRe: web page on O(1) scheduler
At 07:52 AM 5/22/2003 +0200, Mike Galbraith wrote:
>At 08:38 PM 5/21/2003 -0400, Rik van Riel wrote:
>>On Wed, 21 May 2003, Mike Galbraith wrote:
>> > At 11:49 PM 5/20/2003 -0700, David Mosberger wrote:
>> > >Recently, I started to look into some odd performance behaviors of the
>> > >O(1) scheduler. I decided to document what I found in a web page
>> > >at:
>> > >
>> > > http://www.hpl.hp.com/research/linux/kernel/o1.php
>> >
>> > The page mentions persistent starvation. My own explorations of this
>> > issue indicate that the primary source is always selecting the highest
>> > priority queue.
>>
>>It's deeper than that. The O(1) scheduler doesn't consider
>>actual CPU usage as a factor of CPU priority.
>
>Oh, there's no doubt in my mind that it's _much_ deeper than my little
>surface scratchings ;-)
>
>It does consider cpu usage though. Your run history is right there in
>your accumulated sleep_avg. Unfortunately (in some ways, fortunate in
>others.. conflict) that information can be diluted down to nothing
>instantly by new input from one wakeup.

Or did you mean that it misses a bunch of cpu usage? I went looking at cpu
usage, and...

Unless there's something seriously b0rked in the attached (don't _think_
so, but...;), trusting an irq that happens 1/ms to tick tasks isn't 100%
effective, even if you aren't context switching a zillion times
faster. The attached diff produced the attached log while running
test-starve.c. I get some pretty serious thievery even when doing ho-hum
stuff. We can deactivate tasks at _really_ high frequency, and besides, if
the timer interrupt doesn't fire while the last runnable task is active, he
can be missed for a while and have accumulated up nearly a full ms. It
seems to me that with the right timing, you could end up stealing a bunch
of cpu (my logs say it's happening a _lot_ under load. again, diff might
be b0rked)

-Mike [unhandled content-type:application/octet-stream][unhandled content-type:application/octet-stream]
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