Re: SCHED_IDLE documentation

["Michael Kerrisk" <mtk.manpages@googlemail.com>]

Ingo,

On Mon, Mar 3, 2008 at 1:52 PM, Ingo Molnar <mingo@elte.hu> wrote:
>
>  * Michael Kerrisk <mtk.manpages@googlemail.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).

Cheers,

Michael

-- 
Michael Kerrisk
Maintainer of the Linux man-pages project
http://www.kernel.org/doc/man-pages/
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