linux-kernel - Re: [PATCH 3/3] sched, fair: Stop searching for tasks in newidle balance if there are runnable tasks

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Message-ID: <20140424165219.GX11096@twins.programming.kicks-ass.net>
Date:	Thu, 24 Apr 2014 18:52:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	daniel.lezcano@...aro.org, alex.shi@...aro.org,
	preeti@...ux.vnet.ibm.com, efault@....de,
	vincent.guittot@...aro.org, morten.rasmussen@....com, aswin@...com,
	chegu_vinod@...com
Subject: Re: [PATCH 3/3] sched, fair: Stop searching for tasks in newidle
 balance if there are runnable tasks

On Thu, Apr 24, 2014 at 09:43:09AM -0700, Jason Low wrote:
> If the below patch is what you were referring to, I believe this
> can help too. This was also something that I was testing out before
> we went with those patches which compares avg_idle with idle balance
> cost. I recall seeing somewhere around a +7% performance improvement
> in at least least 1 of the AIM7 workloads. I can do some more testing
> with this.

Yes, exactly that.

I can't remember the details, but I suspect we feared the less agressive
idle balance due to that patch (it will only pull a single task, instead
of multiple) would cause more idle_balance invocations and thereby
decrease throughput.

So I suppose something with _many_ bursty threads which leads to severe
inequalities would be the workload to trigger that.

Not sure we've ever seen that.. maybe Mike remembers, he seems to have a
head for such details.
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