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    SubjectRe: [PATCH] 2.6.0 batch scheduling, HT aware

    Pavel Machek wrote:

    >>I've done a resync and update of my batch scheduling that is also hyper-thread
    >>What is batch scheduling? Specifying a task as batch allows it to only use cpu
    >>time if there is idle time available, rather than having a proportion of the
    >>cpu time based on niceness.
    >>Why do I need hyper-thread aware batch scheduling?
    >>If you have a hyperthread (P4HT) processor and run it as two logical cpus you
    >>can have a very low priority task running that can consume 50% of your
    >>physical cpu's capacity no matter how high priority tasks you are running.
    >>For example if you use the distributed computing client setiathome you will
    >>be effectively be running at half your cpu's speed even if you run setiathome
    >>at nice 20. Batch scheduling for normal cpus allows only idle time to be used
    >>for batch tasks, and for HT cpus only allows idle time when both logical cpus
    >>are idle.
    >BTW this is going to be an issue even on normal (non-HT)
    >systems. Imagine memory-bound scientific task on CPU0 and nice -20
    >memory-bound seti&home at CPU1. Even without hyperthreading, your
    >scientific task is going to run at 50% of speed and seti&home is going
    >to get second half. Oops.
    >Something similar can happen with disk, but we are moving out of
    >cpu-scheduler arena with that.
    >[I do not have SMP nearby to demonstrate it, anybody wanting to
    >benchmark a bit?]
    > Pavel
    heh...and the situation gets even worse when you add cpus, with 16way
    you get only 1/16 of the speed ;)


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