linux-kernel - Re: [PATCH v3 09/22] sched: compute runnable load avg in cpu_load and cpu_avg_load_per

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Message-ID: <CA+55aFxbS-u1Zfwg2oKOmHq6FBF+dRRhrdssqc9PpC_gXoV3+g@mail.gmail.com>
Date:	Sun, 6 Jan 2013 10:31:12 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alex Shi <alex.shi@...el.com>
Cc:	Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>, namhyung@...nel.org,
	Mike Galbraith <efault@....de>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	preeti@...ux.vnet.ibm.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 09/22] sched: compute runnable load avg in cpu_load and cpu_avg_load_per_task

On Sat, Jan 5, 2013 at 11:54 PM, Alex Shi <alex.shi@...el.com> wrote:
>
> I just looked into the aim9 benchmark, in this case it forks 2000 tasks,
> after all tasks ready, aim9 give a signal than all tasks burst waking up
> and run until all finished.
> Since each of tasks are finished very quickly, a imbalanced empty cpu
> may goes to sleep till a regular balancing give it some new tasks. That
> causes the performance dropping. cause more idle entering.

Sounds like for AIM (and possibly for other really bursty loads), we
might want to do some load-balancing at wakeup time by *just* looking
at the number of running tasks, rather than at the load average. Hmm?

The load average is fundamentally always going to run behind a bit,
and while you want to use it for long-term balancing, a short-term you
might want to do just a "if we have a huge amount of runnable
processes, do a load balancing *now*". Where "huge amount" should
probably be relative to the long-term load balancing (ie comparing the
number of runnable processes on this CPU right *now* with the load
average over the last second or so would show a clear spike, and a
reason for quick action).

           Linus
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