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    SubjectRe: Context switch times
    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?

    --
    Mike
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