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    SubjectRe: Context switch times
    On Thu, 4 Oct 2001, Mike Kravetz wrote:

    > On Thu, Oct 04, 2001 at 10:42:37PM +0000, Linus Torvalds wrote:
    > > Could we try to hit just two? Probably, but it doesn't really matter,
    > > though: to make the lmbench scheduler benchmark go at full speed, you
    > > want to limit it to _one_ CPU, which is not sensible in real-life
    > > situations.
    >
    > Can you clarify? I agree that tuning the system for the best LMbench
    > performance is not a good thing to do! However, in general on an
    > 8 CPU system with only 2 'active' tasks I would think limiting the
    > tasks to 2 CPUs would be desirable for cache effects.
    >
    > I know that running LMbench with 2 active tasks on an 8 CPU system
    > results in those 2 tasks being 'round-robined' among all 8 CPUs.
    > Prior analysis leads me to believe the reason for this is due to
    > IPI latency. reschedule_idle() chooses the 'best/correct' CPU for
    > a task to run on, but before schedule() runs on that CPU another
    > CPU runs schedule() and the result is that the task runs on a
    > ?less desirable? CPU. The nature of the LMbench scheduler benchmark
    > makes this occur frequently. The real question is: how often
    > does this happen in real-life situations?

    Well, if you remember the first time this issue was discussed on the
    mailing list was due a real life situation not due a bench run.




    - Davide


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