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    SubjectRe: SMP scalability: 8 -> 32 CPUs
    On Sat, 28 Nov 1998, Kurt Garloff wrote:
    > On Sat, Nov 28, 1998 at 12:50:03AM +0000, Eloy A. Paris wrote:
    > > Guys, excuse my ignorance, but does anyone know why Linus said in his
    > > last Linux Journal interview that right now it is fairly unrealistic
    > > to expect scaling from 8 to 32 CPUs? What's stopping Linux to scale in
    > > terms of CPUs?
    > Basically there are different mechanisms which limit the speed of operation:
    > (1) CPU(s) speed
    > (2) Memory speed (and size)
    > (3) I/O (Network, HD, VGA, ...)
    > By increasing the number of CPUs, you only do something about (1).
    > So only CPU bound processes may profit.
    > If you have a large number of independent CPU bound processes, they
    > will scale very nicely until they altogether hit the Memory or I/O
    > bound.

    Number (2) can be tackled with a number of scheduler adjustments:
    - longer time slices (for CPU-bound tasks only!)
    - make it harder to switch CPUs (depending on how long a process
    has slept)
    - more intelligent rescheduling in reschedule_idle() -- better
    burn a few hundred cycles there than completely mess up the
    cache of three CPUs and hogging the memory bus
    - in tune with this, move the bottleneck in other pieces of
    code from memory to CPU, better cache alignment of task
    struct and stuff

    If you have 8 CPUs, 8 CPU-bound tasks and one process
    taking 10% of one CPU, it might just be better to have
    one of the large tasks get 90% and the rest 100%.

    Switching around the one non-hog might create such an
    amount of cache/memory problems that it will make the
    CPU hog suffer more than the 10% in question.

    Weird counterintuitive things like this are what make
    the difference between a high-performance OS and your
    usual Redmond crap...


    Rik -- who prefers a certain OS with fairly non-intuitive innards.
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