linux-kernel - Re: BFS vs. mainline scheduler benchmarks and measurements

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Date:	Wed, 09 Sep 2009 12:05:22 +0300
From:	Nikos Chantziaras <realnc@...or.de>
To:	Mike Galbraith <efault@....de>
CC:	Ingo Molnar <mingo@...e.hu>, Jens Axboe <jens.axboe@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Con Kolivas <kernel@...ivas.org>, linux-kernel@...r.kernel.org
Subject: Re: BFS vs. mainline scheduler benchmarks and measurements

On 09/09/2009 11:52 AM, Mike Galbraith wrote:
> On Wed, 2009-09-09 at 08:13 +0200, Ingo Molnar wrote:
>> * Jens Axboe<jens.axboe@...cle.com>  wrote:
>>
>>> On Tue, Sep 08 2009, Peter Zijlstra wrote:
>>>> On Tue, 2009-09-08 at 11:13 +0200, Jens Axboe wrote:
>>>>> And here's a newer version.
>>>>
>>>> I tinkered a bit with your proglet and finally found the
>>>> problem.
>>>>
>>>> You used a single pipe per child, this means the loop in
>>>> run_child() would consume what it just wrote out until it got
>>>> force preempted by the parent which would also get woken.
>>>>
>>>> This results in the child spinning a while (its full quota) and
>>>> only reporting the last timestamp to the parent.
>>>
>>> Oh doh, that's not well thought out. Well it was a quick hack :-)
>>> Thanks for the fixup, now it's at least usable to some degree.
>>
>> What kind of latencies does it report on your box?
>>
>> Our vanilla scheduler default latency targets are:
>>
>>    single-core: 20 msecs
>>      dual-core: 40 msecs
>>      quad-core: 60 msecs
>>      opto-core: 80 msecs
>>
>> You can enable CONFIG_SCHED_DEBUG=y and set it directly as well via
>> /proc/sys/kernel/sched_latency_ns:
>>
>>     echo 10000000>  /proc/sys/kernel/sched_latency_ns
>
> He would also need to lower min_granularity, otherwise, it'd be larger
> than the whole latency target.

Thank you for mentioning min_granularity.  After:

   echo 10000000 > /proc/sys/kernel/sched_latency_ns
   echo 2000000 > /proc/sys/kernel/sched_min_granularity_ns

I can clearly see an improvement: animations that are supposed to be 
fluid "skip" much less now, and in one occasion (simply moving the video 
window around) have been eliminated completely.  However, there seems to 
be a side effect from having CONFIG_SCHED_DEBUG enabled; things seem to 
be generally a tad more "jerky" with that option enabled, even when not 
even touching the latency and granularity defaults.

I'll try the patch you posted and see if this further improves things.
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