linux-kernel - Re: CFS scheduler unfairly prefers pinned tasks

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Message-ID: <1444133838.2832.141.camel@gmail.com>
Date:	Tue, 06 Oct 2015 14:17:18 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	paul.szabo@...ney.edu.au
Cc:	linux-kernel@...r.kernel.org
Subject: Re: CFS scheduler unfairly prefers pinned tasks

On Tue, 2015-10-06 at 21:06 +1100, paul.szabo@...ney.edu.au wrote:

> And further... the CFS is meant to be fair, using things like vruntime
> to preempt, and throttling. Why are those pinned tasks not preempted or
> throttled?

Imagine you own a 8192 CPU box for a moment, all CPUs having one pinned
task, plus one extra unpinned task, and ponder what would have to happen
in order to meet your utilization expectation.  <time passes>  Right.

What you're seeing is not a bug.  No task can occupy more than one CPU
at a time, making space reservation on multiple CPUs a very bad idea.

	-Mike

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