linux-kernel - Re: fixed timeslice

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Message-Id: <1242055079.11251.282.camel@twins>
Date:	Mon, 11 May 2009 17:17:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason mclaughlin <mcjason@...il.com>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: fixed timeslice

On Mon, 2009-05-11 at 09:58 -0400, Jason mclaughlin wrote:
> can't scheduling be unfair when a fixed timeslice is used as the time
> up til a process can run?
> 
> won't it work out that if a program is using the harddrive, and
> another is using cpu time and using up it's timeslices, that the
> cpu user will give less runtime opportunity to the harddrive user
> because of a wait up until timeslice to use the harddrive again?
> 
> like, doesn't the length of a timeslice change the fairness of
> scheduling opportunity for harddrive use?
> 
> can't it span the time that something is ready to take from the
> harddrive, til the time the harddrive can be used again?
> 
> can't it anyways in some cases though no matter what, because of how
> using up til a timeslice is available sometimes when something wants
> to use the harddrive again,
> and because what wants to use the harddrive can be behind what uses a
> whole timeslice?

I'm rather confused. If a task is blocked on IO its not contending for
CPU resources and is thus irrelevant to the running tasks and their
fairness.

Also, only SCHED_RR has a fixed timeslice.
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