linux-kernel - Re: [PATCH v4] sched: Fast idling of CPU when system is partially loaded

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Date:	Mon, 23 Jun 2014 15:22:28 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] sched: Fast idling of CPU when system is partially
 loaded

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On 06/23/2014 03:16 PM, Tim Chen wrote:
> Thanks to the review from Jason, Andi and Peter. I've updated the
> code as Peter suggested with simplified logic.
> 
> When a system is lightly loaded (i.e. no more than 1 job per cpu), 
> attempt to pull job to a cpu before putting it to idle is
> unnecessary and can be skipped.  This patch adds an indicator so
> the scheduler can know when there's no more than 1 active job is on
> any CPU in the system to skip needless job pulls.
> 
> On a 4 socket machine with a request/response kind of workload
> from clients, we saw about 0.13 msec delay when we go through a
> full load balance to try pull job from all the other cpus.  While
> 0.1 msec was spent on processing the request and generating a
> response, the 0.13 msec load balance overhead was actually more
> than the actual work being done. This overhead can be skipped much
> of the time for lightly loaded systems.
> 
> With this patch, we tested with a netperf request/response workload
> that has the server busy with half the cpus in a 4 socket system.
> We found the patch eliminated 75% of the load balance attempts
> before idling a cpu.
> 
> The overhead of setting/clearing the indicator is low as we already
> gather the necessary info while we call add_nr_running and
> update_sd_lb_stats. We switch to full load balance load immediately
> if any cpu got more than one job on its run queue in
> add_nr_running.  We'll clear the indicator to avoid load balance
> when we detect no cpu's have more than one job when we scan the
> work queues in update_sg_lb_stats.  We are aggressive in turning on
> the load balance and opportunistic in skipping the load balance.
> 
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com> Acked-by:
> Jason Low <jason.low2@...com>

Acked-by: Rik van Riel <riel@...hat.com>


- -- 
All rights reversed
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