linux-kernel - Re: allow non root users to set io priority "idle" ?

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Message-ID: <75b66ecd0708071926s519ebcf5t680c2ed8ab7ead8e@mail.gmail.com>
Date:	Tue, 7 Aug 2007 22:26:57 -0400
From:	"Lee Revell" <rlrevell@...-job.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	dragoran <drago01@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: allow non root users to set io priority "idle" ?

On 06 Aug 2007 13:11:01 +0200, Andi Kleen <andi@...stfloor.org> wrote:
> For IO I suppose the same could happen too. e.g. low priority
> task wants to write out a page and keeps it locked until the IO
> is finished. High priority task wants to access the page and has
> to wait until it is unlocked. Middle task generates an endless
> stream of IO that makes the idle priority writeout never finish.

I don't think it's a problem for high priority (RT) tasks - it's well
known in the real time Linux community that you never, ever do IO from
a thread that has to satisfy RT constraints.  A correct RT linux app
does its IO from a SCHED_NORMAL thread, with *plenty* of buffering,
and feeds the RT constrained SCHED_FIFO threads using a lock free
ringbuffer.

SCHED_IDLE starving SCHED_NORMAL is an issue of course.  But
SCHED_IDLE seems a lot more useful for read than write which I would
expect to take fewer locks.  For example I'd expect Beagle to want to
read at SCHED_IDLE but write out its indices at SCHED_NORMAL.

Would it make any sense to allow anyone to set SCHED_IDLE for reads
but require root to change IO priority for writes?

Lee
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