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    SubjectRe: SCHED_IDLE documentation
    On Tue, Mar 4, 2008 at 12:11 PM, Peter Zijlstra <a.p.zijlstra@chello.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@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).
    >
    > 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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