[Linux-ia64] Re: web page on O(1) scheduler

From: Mike Galbraith <efault_at_gmx.de>
Date: 2003-05-25 19:17:49
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 



Received on Sun May 25 02:13:42 2003

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