linux-kernel - Re: SCHED_IDLE documentation

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Date:	Mon, 3 Mar 2008 11:12:52 +0100
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Arnd Bergmann" <arnd@...db.de>, "Christoph Hellwig" <hch@....de>,
	cbe-oss-dev@...abs.org, "Jeremy Kerr" <jk@...abs.org>,
	linux-kernel@...r.kernel.org
Subject: Re: SCHED_IDLE documentation

On Mon, Mar 3, 2008 at 11:04 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Arnd Bergmann <arnd@...db.de> wrote:
>
> > On Monday 03 March 2008, Ingo Molnar wrote:
> > > > * What's the difference between SCHED_IDLE and SCHED_BATCH?
> > >
> > > SCHED_BATCH can still have nice levels from -20 to +19, it is a modified
> > > SCHED_OTHER/SCHED_NORMAL for "throughput oriented" workloads.
> > >
> > > SCHED_IDLE overrides the nice settings and it means a "super idle"
> > > workload.
> >
> > Does that mean that a SCHED_IDLE task still runs some of the time if
> > you have a CPU hog running on +19, or can any other process starve the
> > SCHED_IDLE task?
>
> yes, even SCHED_IDLE tasks get some CPU time - so complete starvation
> should not be possible. We dont really want to define the exact amount
> of time they need. (we might want to change that in the future) But it
> will always be less prio than nice +19 :-) You can think of it as if it
> was "nice +30".
>
> > What happens if you have two SCHED_IDLE tasks on a single CPU, do they
> > get equal share, or will they just run as batch jobs?
>
> they get equal share.
>
> you can try it out too: if you have 2.6.23 or newer kernels then just
> pick up schedtool and use "schedtool -I" to run SCHED_IDLE tasks.

Thanks for the answers Ingo!

Cheers,

Michael

-- 
Michael Kerrisk
Maintainer of the Linux man-pages project
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