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

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Message-ID: <484451F7.5090001@nortel.com>
Date:	Mon, 02 Jun 2008 14:03:03 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	vatsa@...ux.vnet.ibm.com
CC:	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, dhaval@...ux.vnet.ibm.com
Subject: Re: fair group scheduler not so fair?

Srivatsa Vaddagiri wrote:

> That seems to be pretty difficult to achieve with the per-cpu runqueue
> and smpnice based load balancing approach we have now.

Okay, thanks.

>>Initially I tried a simple setup with three hogs all in the default "sys" 
>>group.  Over multiple retries using 10-sec intervals, sometimes it gave 
>>roughly 67% for each task, other times it settled into a 100/50/50 split 
>>that remained stable over time.

> Was this with imbalance_pct set to 105? Does it make any difference if
> you change imbalance_pct to say 102?

It was set to 105 initially.  I later reproduced the problem with 102. 
For example, the following was with 102, with three tasks created in the 
sys class.  Based on the runtime, pid 2499 has been getting a cpu all to 
itself for over a minute.

  2499 cfriesen  20   0  3800  392  336 R 99.8  0.0   1:05.85 cat
  2496 cfriesen  20   0  3800  392  336 R 50.0  0.0   0:32.95 cat
  2498 cfriesen  20   0  3800  392  336 R 50.0  0.0   0:32.97 cat

The next run was much better, with sub-second fairness after a minute.

  2505 cfriesen  20   0  3800  392  336 R 68.2  0.0   1:00.32 cat
  2506 cfriesen  20   0  3800  392  336 R 66.9  0.0   0:59.85 cat
  2503 cfriesen  20   0  3800  392  336 R 64.2  0.0   1:00.21 cat

The lack of predictability is disturbing, as it implies some sensitivity 
to the specific test conditions.

>>With three groups, one task in each, I tried both 10 and 60 second 
>>intervals.  The longer interval looked better but was still up to 0.8% off:
> 
> 
> I honestly don't know if we can do better than 0.8%! In any case, I'd
> expect that it would require more drastic changes.

No problem.  It's still far superior than the SMP performance of CKRM,
which is what we're currently using (although heavily modified).

Chris

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