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

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Message-ID: <20080530102121.GG12836@linux.vnet.ibm.com>
Date:	Fri, 30 May 2008 15:51:21 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
Cc:	Chris Friesen <cfriesen@...tel.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, a.p.zijlstra@...llo.nl, pj@....com,
	Balbir Singh <balbir@...ibm.com>,
	aneesh.kumar@...ux.vnet.ibm.com
Subject: Re: fair group scheduler not so fair?

On Fri, May 30, 2008 at 12:13:24PM +0530, Dhaval Giani wrote:
> > Also, although the long-term results are good, the shorter-term fairness 
> > isn't great.  Is there a tuneable that would allow for a tradeoff between 
> > performance and fairness?  I have people that are looking for within 4% 
> > fairness over a 1sec interval.
> >
> 
> How fair does smp fairness look for a !group scenario? I don't expect
> group schould be able to do much better.

Just tested this combo for !group case:

	1 nice0 (weight = 1024)
	2 nice3 (each weight = 526)
	3 nice5 (each weight = 335)

You'd expect nice0 to get (on a 2 cpu system):

	2 * 1024 / (1024 + 2*526 + 3*335) = 66.47

This is what I see over a 10sec interval (error = 6%):

 4386 root      20   0  1384  228  176 R 60.4  0.0   3:06.75  1 nice0
 4387 root      23   3  1384  232  176 R 37.9  0.0   1:57.03  0 nice3
 4388 root      23   3  1384  228  176 R 37.9  0.0   1:57.24  0 nice3
 4390 root      25   5  1384  228  176 R 24.1  0.0   1:14.62  0 nice5
 4391 root      25   5  1384  228  176 R 19.8  0.0   1:01.26  1 nice5
 4389 root      25   5  1384  228  176 R 19.7  0.0   1:01.12  1 nice5

Over 120sec interval (error still as high as 6%):

 4386 root      20   0  1384  228  176 R 60.4  0.0   6:13.95  1 nice0
 4388 root      23   3  1384  228  176 R 37.9  0.0   3:54.69  0 nice3
 4387 root      23   3  1384  232  176 R 37.9  0.0   3:54.44  0 nice3
 4390 root      25   5  1384  228  176 R 24.2  0.0   2:29.45  0 nice5
 4391 root      25   5  1384  228  176 R 19.8  0.0   2:02.56  1 nice5
 4389 root      25   5  1384  228  176 R 19.8  0.0   2:02.44  1 nice5

The high error could be because of interference from other tasks. Anyway I 
dont think !group case is better at achieving fairness over shorter
intervals.


-- 
Regards,
vatsa
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