linux-kernel - Re: [PATCH 4/4] sched: bias to target cpu load to reduce task moving

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Date:	Tue, 7 Jan 2014 21:37:52 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Alex Shi <alex.shi@...aro.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	"fenghua.yu@...el.com" <fenghua.yu@...el.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"fengguang.wu@...el.com" <fengguang.wu@...el.com>,
	"james.hogan@...tec.com" <james.hogan@...tec.com>,
	"jason.low2@...com" <jason.low2@...com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"hanjun.guo@...aro.org" <hanjun.guo@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] sched: bias to target cpu load to reduce task moving

On Tue, Jan 07, 2014 at 03:16:32PM +0000, Morten Rasmussen wrote:
> From a load perspective wouldn't it be better to pick the least loaded
> cpu in the group? It is not cheap to implement, but in theory it should
> give less balancing within the group later an less unfairness until it
> happens.

I tried that; see 04f733b4afac5dc93ae9b0a8703c60b87def491e for why it
doesn't work.

> Rotating the cpu is probably good enough for most cases and certainly
> easier to implement.

Indeed.

> The bias continues after they first round of load balance by the other
> cpus?

The cost, yes. Even when perfectly balanced, we still get to iterate the
entire machine computing s[gd]_lb_stats to find out we're good and don't
need to move tasks about.

> Pulling everything to one cpu is not ideal from a performance point of
> view. You loose some available cpu cycles until the balance settles.
> However, it is not easy to do better and maintain scalability at the
> same time.

Right, its part of the cost we pay for scaling better. And rotating this
cost around a bit would alleviate the obvious bias.


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