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    SubjectRe: CFS review


    On Tue, 28 Aug 2007, Al Boldi wrote:
    >
    > No need for framebuffer. All you need is X using the X.org vesa-driver.
    > Then start gears like this:
    >
    > # gears & gears & gears &
    >
    > Then lay them out side by side to see the periodic stallings for ~10sec.

    I don't think this is a good test.

    Why?

    If you're not using direct rendering, what you have is the X server doing
    all the rendering, which in turn means that what you are testing is quite
    possibly not so much about the *kernel* scheduling, but about *X-server*
    scheduling!

    I'm sure the kernel scheduler has an impact, but what's more likely to be
    going on is that you're seeing effects that are indirect, and not
    necessarily at all even "good".

    For example, if the X server is the scheduling point, it's entirely
    possible that it ends up showing effects that are more due to the queueing
    of the X command stream than due to the scheduler - and that those
    stalls are simply due to *that*.

    One thing to try is to run the X connection in synchronous mode, which
    minimizes queueing issues. I don't know if gears has a flag to turn on
    synchronous X messaging, though. Many X programs take the "[+-]sync" flag
    to turn on synchronous mode, iirc.

    Linus
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