linux-kernel - Re: [RFC PATCH v3 5/6] sched: pack the idle load balance

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Date:	Wed, 27 Mar 2013 21:32:30 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org, mingo@...nel.org,
	linux@....linux.org.uk, pjt@...gle.com, santosh.shilimkar@...com,
	morten.rasmussen@....com, chander.kashyap@...aro.org,
	cmetcalf@...era.com, tony.luck@...el.com,
	preeti@...ux.vnet.ibm.com, paulmck@...ux.vnet.ibm.com,
	tglx@...utronix.de, len.brown@...el.com, arjan@...ux.intel.com,
	amit.kucheria@...aro.org, corbet@....net
Subject: Re: [RFC PATCH v3 5/6] sched: pack the idle load balance

On 03/27/2013 06:30 PM, Vincent Guittot wrote:
>> Arguing the performance/power balance does no much sense without
>> > detailed scenario. We just want to seek a flexible compromise way.
>> > But fixed buddy cpu is not flexible. and it may lose many possible
>> > powersaving fit scenarios on x86 system. Like if 2 SMT cpu can handle
>> > all tasks, we don't need to wake another core. or if 2 cores in one
>> > socket can handle tasks, we also don't need to wakeup another socket.
> Using 2 SMT for all tasks implies to accept latency and to share
> resources like cache and memory bandwidth so it means that you also
> accept some potential performance decrease which implies that someone
> must select this mode with a knob.
> The primary goal of the patchset is not to select between powersaving
> and performance but to stay in performance mode. We pack the small
> tasks in one CPU so the performance will not decrease but the low load
> scenario will consume less power. Then, I can add another step which
> will be more power saving aggressive with a potential cost of
> performance and i this case the buddy CPU will be updated dynamically
> according to the system load
> 

Predication of small task behavior is often wrong. so for performance
purpose, packing task is a bad idea.

-- 
Thanks
    Alex
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