linux-kernel - Re: [RFC] sched: unused cpu in affine workload

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Message-ID: <20160404213450.GE3448@twins.programming.kicks-ass.net>
Date:	Mon, 4 Apr 2016 23:34:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...hat.com>,
	James Hartsock <hartsjc@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Kirill Tkhai <ktkhai@...allels.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] sched: unused cpu in affine workload

On Mon, Apr 04, 2016 at 03:45:16PM -0400, Rik van Riel wrote:
> An optimal solution is NP hard.
> 
> Heuristics that "move tasks with pressure" may be
> much more doable, and lead to perfectly satisfactory
> results, especially if most migrations happen within
> a socket (and the same shared L3 cache).

Right; trick will be finding something that mostly works without making
the regular balance paths increase in complexity.

As per the argument in kernel/sched/fair.c:5694 the current
load-balancing averages out to O(n), and I would very much like to keep
it that way.