linux-kernel - Re: busted CFS group load balancer?

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Message-ID: <4921DFD8.9060509@nortel.com>
Date:	Mon, 17 Nov 2008 15:19:20 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Ken Chen <kenchen@...gle.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: busted CFS group load balancer?

Ken Chen wrote:
> On Mon, Nov 17, 2008 at 7:37 AM, Chris Friesen wrote:
>>> It appears that the fair-group load balancer in 2.6.27 does not work
>>> properly.
>> There was an issue fixed post 2.6.27 where the load balancer didn't work
>> properly if there was one task per group per cpu.  You might try
>> backporting commit 38736f4 and see if that helps.
> 
> Tested git commit 38736f4, it doesn't fix the problem I'm seeing.
> 

I plugged in the same weights into my test app (groups 1 and 2 instead 
of ant/bee) and got the results below for a 10-sec run.  The "actual" 
numbers give the overall average and then the values for each hog 
separately.  In this case we see that both tasks in group 2 ended up 
sharing a cpu with one of the tasks from group 1.

   group      actual(%)      expected(%)  ctx switches   max_latency(ms)
       1  99.69(99.38/99.99)   99.81       160/262          4/0
       2   0.31( 0.31/0.31)     0.19       32/33         391/375

I've only got a 2-way system.  If the results really are that much worse 
on larger systems, then that's going to cause problems for us as well. 
I'll see if I can get some time on a bigger machine.

Chris
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