linux-kernel - Re: [RFC PATCH 00/14] sched: entity load-tracking re-work

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Date:	Tue, 13 Mar 2012 18:28:31 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Morten Rasmussen <Morten.Rasmussen@....com>
Cc:	Paul Turner <pjt@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Venki Pallipadi <venki@...gle.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Mike Galbraith <efault@....de>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ben Segall <bsegall@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Robin Randhawa <Robin.Randhawa@....com>
Subject: Re: [RFC PATCH 00/14] sched: entity load-tracking re-work

On Mon, 2012-03-12 at 10:39 +0000, Morten Rasmussen wrote:
> I have looked at traces of both runnable time and usage time trying to
> understand why you use runnable time as your load metric and not usage
> time which seems more intuitive. What I see is that runnable time
> depends on the total runqueue load. If you have many tasks on the
> runqueue they will wait longer and therefore have higher individual
> load_avg_contrib than they would if the were scheduled across more CPUs.
> Usage time is also affected by the number of tasks on the runqueue as
> more tasks means less CPU time. However, less usage can also just mean
> that the task does not execute very often. This would make a load
> contribution estimate based on usage time less accurate. Is this your
> reason for choosing runnable time? 

Exactly so, you cannot ever have more than 100% usage, so no matter how
many tasks you stick on a cpu, you'll never get over that 100% and thus
this is not a usable load metric.


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