linux-kernel - Re: SCHED_IDLE documentation

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Message-ID: <cfd18e0f0803050719y274d8b48r9dc2dcc0f9179e3c@mail.gmail.com>
Date:	Wed, 5 Mar 2008 16:19:01 +0100
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>
Cc:	"Ingo Molnar" <mingo@...e.hu>, "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 Tue, Mar 4, 2008 at 12:11 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>
>
>  On Mon, 2008-03-03 at 15:06 +0100, Michael Kerrisk wrote:
>  > Ingo,
>  >
>  > On Mon, Mar 3, 2008 at 1:52 PM, Ingo Molnar <mingo@...e.hu> wrote:
>  > >
>  > >  * Michael Kerrisk <mtk.manpages@...glemail.com> 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.
>  > >  >
>  > >  > So, suppose we have two CPU intensive jobs, one SCHED_OTHER and the
>  > >  > other SCHED_BATCH.  If they have the same nice value, will/should the
>  > >  > scheduler favour one over the other?
>  > >
>  > >  yes - SCHED_BATCH does not modify the CPU usage proportion for
>  > >  CPU-intense tasks, it's their nice value that controls the proportion.
>  > >  What it will influence is wakeup behavior - i.e. wakeup-intense
>  > >  workloads should schedule less with SCHED_BATCH. (but how that is done
>  > >  is really fluid and will probably tweaked in the future.)
>  > >
>  > >         Ingo
>  >
>  > So, I've tweaked the description of SCHED_BATCH in the
>  > sched_setscheduler.2 man page, and added some text describing
>  > SCHED_IDLE.  Relevant excepts below.  Does his look okay to you?
>  >
>  >        SCHED_OTHER is the default universal time-sharing  sched-
>  >        uler  policy  used  by  most  processes.   SCHED_BATCH is
>  >        intended  for  "batch"  style  execution  of   processes.
>  >        SCHED_IDLE  is  intended  for  running  very low priority
>  >        background jobs.  SCHED_FIFO and  SCHED_RR  are  intended
>  >        for  special time-critical applications that need precise
>  >        control over the way  in  which  runnable  processes  are
>  >        selected for execution.
>  >
>  >        Processes  scheduled  with  SCHED_OTHER,  SCHED_BATCH, or
>  >        SCHED_IDLE must be assigned the static priority 0.   Pro-
>  >        cesses  scheduled under SCHED_FIFO or SCHED_RR can have a
>  >        static priority in the range 1 to 99.
>  >        ...
>  >
>  >    SCHED_BATCH: Scheduling batch processes
>  >        (Since Linux 2.6.16.)  SCHED_BATCH can only  be  used  at
>  >        static   priority   0.    This   policy   is  similar  to
>  >        SCHED_OTHER, except that it will cause the  scheduler  to
>  >        always  assume that the process is CPU-intensive.  Conse-
>  >        quently, the scheduler  will  apply  a  small  scheduling
>  >        penalty  with  respect  to wakeup behaviour, so that this
>  >        process is mildly  disfavored  in  scheduling  decisions.
>  >        This policy is useful for workloads that are non-interac-
>  >        tive, but do not want to lower their nice value, and  for
>  >        workloads  that  want  a  deterministic scheduling policy
>  >        without interactivity causing extra preemptions  (between
>  >        the workload's tasks).
>  >
>  >    SCHED_IDLE: Scheduling very low priority jobs
>  >        (Since  Linux  2.6.23.)   SCHED_IDLE  can only be used at
>  >        static priority 0; the process nice value has  no  influ-
>  >        ence  for  this policy.  This policy is intended for run-
>  >        ning jobs at extremely low priority (lower  even  than  a
>  >        +19  nice value with the SCHED_OTHER or SCHED_BATCH poli-
>  >        cies).
>
>  Your SCHED_BATCH and SCHED_IDLE descriptions seem at odds, in that your
>  SCHED_IDLE description says you can run SCHED_BATCH +19, however your
>  SCHED_BATCH description says you can only run at nice 0.
>
>  To clarify SCHED_BATCH _can_ indeed use the full nice range.

Peter,

The problem is that the page is slightly confusing in that it talks
about two different types of priorities:

"static priorities" which can only be non-zero (1 to 99) for RR and
FIFO policies.

"dynamic priorities" (aka "nice value") which can be (in userspace) -20 to +19.

I tweaked the wording in the description of SCHED_BATCH a little to
make this clearer.

Thanks,

Michael

-- 
Michael Kerrisk
Maintainer of the Linux man-pages project
http://www.kernel.org/doc/man-pages/
Want to report a man-pages bug?  Look here:
http://www.kernel.org/doc/man-pages/reporting_bugs.html
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