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SubjectRe: Interesting scheduling times
Linus Torvalds writes:
>
>
> On Fri, 18 Sep 1998, Richard Gooch wrote:
> >
> > Actually, after some further testing, I get the same result! The first
> > time it's faster, then subsequent runs are slower (when using RT
> > processes). For non-RT processes, the times are stable. Strange.
>
> I haven't even looked at the benchmark you seem to be talking about - I
> use "lmbench" myself which I trust to be reasonably realistic. It
> certainly showed an effect of my FPU screwup, although it's not all that
> large on any reasonable system (it's probably horrible on a i386/i387
> combination where FP operations are slower).
>
> lmbench uses a set of pipes and passes a token around to force scheduling,
> and that should work fine. I'd be nervous about any other kind of
> scheduling benchmark.

Well, in fact the lmbench is not "realistic" in this case. Or at
least, it's measuring something different. Larry has most of the
processes sitting waiting on a pipe. That means most of his processes
are not on the run queue. Furthermore, using pipes includes the extra
overhead of shifting processes onto and off the run queue.

Last night I added a token passing scheme (via pipes) to my code, and
the thread switch time (on a Pentium/MMX 200) goes from 2.1 us to 5.0
us).

Anyway, I'll try to reproduce the slowdown-after-first effect on a few
different machines.

Regards,

Richard....

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