linux-kernel - Re: fair group scheduler not so fair?

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Date:	Thu, 22 May 2008 14:02:19 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	linux-kernel@...r.kernel.org, vatsa@...ux.vnet.ibm.com,
	mingo@...e.hu, pj@....com
Subject: Re: fair group scheduler not so fair?

Peter Zijlstra wrote:

> Could you see if the patches found here:
> 
>  http://programming.kicks-ass.net/kernel-patches/sched-smp-group-fixes/
> 
> make any difference for you?

Not much difference.  In the following case pid 2438 is in group "a" and
pids 2439/2440 are in group "b".  Pid 2438 still gets stuck with only 66%.

  2439 cfriesen  20   0  3800  392  336 R 99.7  0.0   3:17.37 cat 

  2438 cfriesen  20   0  3800  392  336 R 66.2  0.0   2:33.63 cat 

  2440 cfriesen  20   0  3800  392  336 R 33.6  0.0   1:47.53 cat 



With 3 tasks in group a, 2 in group b, it's still pretty poor:

  2514 cfriesen  20   0  3800  392  336 R 52.5  0.0   0:48.11 cat 

  2515 cfriesen  20   0  3800  392  336 R 50.2  0.0   0:42.53 cat 

  2439 cfriesen  20   0  3800  392  336 R 35.4  0.0   4:37.07 cat 

  2438 cfriesen  20   0  3800  392  336 R 33.3  0.0   3:34.97 cat 

  2440 cfriesen  20   0  3800  392  336 R 28.3  0.0   2:26.17 cat 




If I boot with "nosmp" it behaves more or less as expected:

3 tasks in default:
  2427 cfriesen  20   0  3800  392  336 R 33.7  0.0   0:36.54 cat 

  2429 cfriesen  20   0  3800  392  336 R 33.5  0.0   0:35.63 cat 

  2428 cfriesen  20   0  3800  392  336 R 32.9  0.0   0:35.84 cat 


1 task in a, 2 in b:
  2427 cfriesen  20   0  3800  392  336 R 49.8  0.0   1:45.74 cat 

  2428 cfriesen  20   0  3800  392  336 R 25.0  0.0   1:38.65 cat 

  2429 cfriesen  20   0  3800  392  336 R 25.0  0.0   1:38.18 cat 


3 tasks in a, 2 in b:
  2521 cfriesen  20   0  3800  392  336 R 25.2  0.0   0:08.52 cat 

  2522 cfriesen  20   0  3800  392  336 R 25.2  0.0   0:08.23 cat 

  2427 cfriesen  20   0  3800  392  336 R 16.6  0.0   1:59.39 cat 

  2429 cfriesen  20   0  3800  392  336 R 16.6  0.0   1:47.63 cat 

  2428 cfriesen  20   0  3800  392  336 R 16.4  0.0   1:48.65 cat 



I haven't really dug into the scheduler yet (although that's next), but 
based on these results it doesn't really look like the load balancer is 
properly group-aware.

Chris
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