Re: [uPATCH] SMP scheduling fix (?)

Neil Conway (nconway.list@ukaea.org.uk)
Fri, 15 Jan 1999 14:00:29 +0000

Robert M. Hyatt wrote:
>
> On Fri, 15 Jan 1999, Rik van Riel wrote:
>
> > On Thu, 14 Jan 1999, Robert M. Hyatt wrote:
> >
> > > This would make a good discussion point, since it is a topic I am
> > > highly interested in. Take a process whose nice value is 0. And
> > > a process that has been niced to +10. What is the expectation there?
> >
> > The expectation is that the +10 niced program gets half the
> > CPU time from the 'normal' process.
>
> ok... then '20'??? IE it would seem that that might be 1/4th using
> the same 'what number is next in this series?' :) That's not bad,

Rather simpler than that... A nice value of 20 means you get about
1/20th as much as a process with a nice value of 0. 5% is too big for
lots of things, and this is one of the reasons I like Rik et al.'s
patches for sched_idle.

Does anyone know why Linus doesn't like sched_idle things? (*Does* he
dislike them??).

Neil

PS: Having bravely said the above without checking, I foolishly decided
to check. It wasn't the case. Running two CPU-hogs, one non-niced and
the other "nice +20", I got not the 95/5 split (actually 20/21 vs. 1/21
was what I expected), but a roughly 91.5/8.5 split. This means that the
niced process got about one eleventh of the CPU time.

This doesn't square with what I read in the source code. "nice +20"
should give the niced process (i386 values here) ONE time-slice in every
round, while the non-niced should have 20 timeslices.

Argh.

(That's on my 2.0.35 desktop box btw. Quick inspection suggests 2.1.131
is better behaved but not sure.)

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