linux-kernel - Re: [RFC] sched: Limit idle_balance() when it is being used too frequently

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Message-ID: <20130718093218.GH27075@twins.programming.kicks-ass.net>
Date:	Thu, 18 Jul 2013 11:32:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>, Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Mel Gorman <mgorman@...e.de>, aswin@...com,
	scott.norton@...com, chegu_vinod@...com
Subject: Re: [RFC] sched: Limit idle_balance() when it is being used too
 frequently

On Wed, Jul 17, 2013 at 09:02:24PM -0700, Jason Low wrote:

> I ran a few AIM7 workloads for the 8 socket HT enabled case and I needed
> to set N to more than 20 in order to get the big performance gains.
> 
> One thing that I thought of was to have N be based on how often idle
> balance attempts does not pull task(s).
> 
> For example, N can be calculated based on the number of idle balance
> attempts for the CPU  since the last "successful" idle balance attempt.
> So if the previous 30 idle balance attempts resulted in no tasks moved,
> then n = 30 / 5. So idle balance gets less time to run as the number of
> unneeded idle balance attempts increases, and thus N will not be set too
> high during situations where idle balancing is "successful" more often.
> Any comments on this idea?

It would be good to get a solid explanation for why we need such high N.
But yes that might work.


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