linux-kernel - Re: SCHED: Is task migration necessary in sched

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Date:	Wed, 23 Dec 2009 11:53:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rakib Mullick <rakib.mullick@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: SCHED: Is task migration necessary in sched_exec().

On Wed, 2009-12-23 at 16:46 +0600, Rakib Mullick wrote:
> On 12/23/09, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Wed, 2009-12-23 at 16:14 +0600, Rakib Mullick wrote:
> >
> >
> > Well, if there's an imbalance the 'slow' load-balancer will move it
> >  around eventually anyway, and since it will then have build up a larger
> >  cache footprint it will be even more expensive.
> >
> If there is an imbalance - then the overloaded task will moved. 

There is no overloaded task, its the runqueue that is overloaded wrt to
other runqueues. The load-balancer has to pick a 'random' task and pray.
Current heuristics try to pick a task that hasn't been on the cpu for a
while, because for those the effective cache footprint is minimal.

> Why the _current_ task? 

Because at exec it has effective 0 cache footprint, and is thus an ideal
victim to move about.

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