Messages in this thread | | | Date | Wed, 5 Mar 2008 16:19:01 +0100 | From | "Michael Kerrisk" <> | Subject | Re: SCHED_IDLE documentation |
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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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