linux-kernel - Re: [patch v5 09/15] sched: add power aware scheduling in fork/exec/wake

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Date:	Thu, 21 Feb 2013 22:40:07 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	torvalds@...ux-foundation.org, mingo@...hat.com,
	tglx@...utronix.de, akpm@...ux-foundation.org,
	arjan@...ux.intel.com, bp@...en8.de, pjt@...gle.com,
	namhyung@...nel.org, efault@....de, vincent.guittot@...aro.org,
	gregkh@...uxfoundation.org, preeti@...ux.vnet.ibm.com,
	viresh.kumar@...aro.org, linux-kernel@...r.kernel.org,
	morten.rasmussen@....com
Subject: Re: [patch v5 09/15] sched: add power aware scheduling in fork/exec/wake

On 02/21/2013 09:33 PM, Peter Zijlstra wrote:
> On Wed, 2013-02-20 at 22:23 +0800, Alex Shi wrote:
>>> But but but,... nr_running is completely unrelated to utilization.
>>>
>>
>> Actually, I also hesitated on the name, how about using nr_running to
>> replace group_util directly?
> 
> 
> The name is a secondary issue, first you need to explain why you think
> nr_running is a useful metric at all.
> 
> You can have a high nr_running and a low utilization (a burst of
> wakeups, each waking a process that'll instantly go to sleep again), or
> low nr_running and high utilization (a single process cpu bound
> process).

It is true in periodic balance. But in fork/exec/waking timing, the
incoming processes usually need to do something before sleep again.

I use nr_running to measure how the group busy, due to 3 reasons:
1, the current performance policy doesn't use utilization too.
2, the power policy don't care load weight.
3, I tested some benchmarks, kbuild/tbench/hackbench/aim7 etc, some
benchmark results looks clear bad when use utilization. if my memory
right, the hackbench/aim7 both looks bad. I had tried many ways to
engage utilization into this balance, like use utilization only, or use
utilization * nr_running etc. but still can not find a way to recover
the lose. But with nr_running, the performance seems doesn't lose much
with power policy.

> 
> There is absolutely no relation between utilization and nr_running,
> building something on that assumption is just wrong and broken.

I just had tried all my benchmarks dbench/loop netperf/specjbb/sysbench
etc seems the performance/power testing result are all acceptable.

> 
> 
> 

-- 
Thanks
    Alex
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