linux-kernel - Re: [patch v5 10/15] sched: packing transitory tasks in wake/exec power balancing

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Message-ID: <1361346030.5919.63.camel@marge.simpson.net>
Date:	Wed, 20 Feb 2013 08:40:30 +0100
From:	Mike Galbraith <efault@....de>
To:	Alex Shi <alex.shi@...el.com>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	torvalds@...ux-foundation.org, mingo@...hat.com,
	peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, arjan@...ux.intel.com, bp@...en8.de,
	pjt@...gle.com, namhyung@...nel.org, 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 10/15] sched: packing transitory tasks in wake/exec
 power balancing

On Wed, 2013-02-20 at 13:55 +0800, Alex Shi wrote:

> Joonsoo Kim suggests not packing exec task, since the old task utils is
> possibly unuseable.

(I'm stumbling around in rtmutex PI land, all dazed and confused, so
forgive me if my peripheral following of this thread is off target;)

Hm, possibly.  Future behavior is always undefined, trying to predict
always a gamble... so it looks to me like not packing on exec places a
bet against the user, who chose to wager that powersaving will happen
and it won't cost him too much, if you don't always try to pack despite
any risks.  The user placed a bet on powersaving, not burst performance.

Same for the fork, if you spread to accommodate a potential burst, you
bin the power wager, so maybe it's not in his best interest.. fork/exec
is common, if it's happening frequently, you'll bin the potential power
win frequently, reducing effectiveness, and silently trading power for
performance when the user asked to trade performance for a lower
electric bill.

Dunno, just a thought, but I'd say for powersaving policy, you have to
go just for broke and hope it works out.  You can't know it won't, but
you'll toss potential winnings every time you don't roll the dice.

-Mike

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