Linux-kernel mailing list archive 1999-32,: Re: SMP Scheduling

# Re: SMP Scheduling

Horst von Brand (vonbrand@sleipnir.valparaiso.cl)
Mon, 09 Aug 1999 00:25:12 -0400

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Peter Waltenberg <peterw@dascom.com> said:
> On 09-Aug-99 Horst von Brand wrote:
> > Peter Waltenberg <peterw@dascom.com> said:
> > [...]

> >> Under 2.2 you see that one CPU hog hopping CPU's and at regular
> >> intervals. Using xosview to track load what you see is a picket fence
> >> effect. And there are more than "3 processes" running, more like 80
> >> on my machine, so running xosview alone shouldn't be enough to force
> >> this to happen and if it were, the other processes should be
> >> introducing enough noise to make the CPU swapping more erratic.

> > If you have that many processes running, your hog will have its state at
> > the CPU flushed anyway, so the CPU selected is irrelevant.

> >> This does seem to be "wrong", not so much that the process is changing
> >> CPU's, thats reasonable, but the fact that it's doing it with such
> >> regularity now.

> > File it under "random trivia" then ;-)

> I'd expect the process to be flushed, however in that case I'd expect it
> to be re-run on some random CPU. However that doesn't seem to happen, the
> process swaps CPU's at REGULAR intervals. The scheduler is supposedly
> designed so that a process will have a tendency to run on the same CPU.

This tendency is for a time quantum only, what you are seeing is clearly at
a much larger timescale. And the other processes will also tend to stick to
the same CPU, I can very well imagine that with the right mix this gives
wandering processes when you use round-robin scheduling. Essentially the
explanation given for three processes, just longer ;-)

> It's not that the process changes CPU's, it's that it's doing it at
> regular intervals that I find worrying.
> Yes, that could just be coincidence, or it could be a real problem.

AFAIKS, if you see the effect, it is at a timescale of seconds. An unneeded
migration a second is not that big a deal, is it?

    --
    Horst von Brand                             vonbrand@sleipnir.valparaiso.cl
    Casilla 9G, Viņa del Mar, Chile                               +56 32 672616

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