Re: Linux 2.6.0-test6

[Nick Piggin]

Pedro Larroy wrote:

> On Mon, Sep 29, 2003 at 12:55:12PM -0400, Ed Sweetman wrote:
>
> > Nick Piggin wrote:
> >
> > > Rob Landley wrote:
> > >
> > >
> > > > On Sunday 28 September 2003 02:03, Con Kolivas wrote:
> > > >
> > > >
> > > > > On Sun, 28 Sep 2003 11:27, Linus Torvalds wrote:
> > > > >
> > > > > > from Andrew Morton. Most notably perhaps Con's scheduler changes that
> > > > >
> > > > > > have been discussed extensively and made it into the -mm tree for
> > > > > > testing.
> > > > > For those who are trying this for the first time, please note that the
> > > > > scheduler has been tuned to tell the difference between tasks of the 
> > > > > _same_
> > > > > nice level. This means do NOT renice X or it will make audio skip unless
> > > > > you also renice your audio application by the same amount. Lots of
> > > > > distributions have done this for the old 2.4 scheduler which could not
> > > > > treat equal "nice" levels as differently as the new scheduler does 
> > > > > and 2.6
> > > > > shouldn't need special treatment.
> > > > >
> > > > > So for testing note the following points:
> > > > >
> > > > > Make sure X is NOT reniced to -10 as many distributions are doing.
> > > > > Some shells spawn processes at nice +5 by default and this will make 
> > > > > audio
> > > > > apps suffer.
> > > > > Make sure your hard disk, graphics card and audio card are performing at
> > > > > equal standard to your 2.4 kernel (ie dma is working, graphics is fully
> > > > > accelerated etc).
> > > > I.E. with your new scheduler, priority levels actually have enough of 
> > > > an effect now that things that aren't reniced can be noticeably 
> > > > starved by things that are.
> > > AFAIK, Con's scheduler doesn't change the nice implementation at all.
> > > Possibly some of his changes amplify its problems, or, more likely they
> > > remove most other scheduler problems leaving this one noticable.
> > >
> > > If X is running at -20, and xmms at +19, xmms is supposed to still get
> > > 5% of the CPU. Should be enough to run fine. Unfortunately this is
> > > achieved by giving X very large timeslices, so xmms's scheduling latency
> > > becomes large. The interactivity bonuses don't help, either.
> > there are 40 positions between -20 and 19, that doesn't equal 5% steps. 
> > They don't even refer to % of cpu.  If i nice a process to -20 it 
> > doesn't get a given percentage of cpu just because it's -20. I may have 
> > other processes at -20 as well.  If you nice something to -20 and it is 
> > actually using that cpu then things that are +19 shouldn't run and wont 
> > run.  If I nice -20 vmstat 1, it's not going to starve xmms (or any 
> > better audio player).  -20 means starve all and it should do that when 
> > it actually makes use of the resources.
>
> Why not run xmms with SCHED_RR or SCHED_FIFO?

Well because playing an mp3 really is a pitiful task for modern CPUs,
and the standard scheduler should handle this fine. Also a music skip
isn't terribly important.

Realtime applications are difficult to make robust and they can easily
hang the system.

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