linux-kernel - Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq

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Message-ID: <1361367148.10155.30.camel@laptop>
Date:	Wed, 20 Feb 2013 14:32:28 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Michael Wang <wangyun@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Mike Galbraith <efault@....de>,
	Andrew Morton <akpm@...ux-foundation.org>, alex.shi@...el.com,
	Ram Pai <linuxram@...ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq_fair()

On Wed, 2013-02-20 at 11:49 +0100, Ingo Molnar wrote:

> The changes look clean and reasoable, 

I don't necessarily agree, note that O(n^2) storage requirement that
Michael failed to highlight ;-)

> any ideas exactly *why* it speeds up?

That is indeed the most interesting part.. There's two parts to
select_task_rq_fair(), the 'regular' affine wakeup path, and the
fork/exec find_idlest_goo() path. At the very least we need to quantify
which of these two parts contributes most to the speedup.

In the power balancing discussion we already noted that the
find_idlest_goo() is in need of attention.

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