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    SubjectRe: RSDL v0.31
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    On Fri, 2007-03-23 at 09:50 +1100, Con Kolivas wrote:

    > Now to figure out some meaningful cheap way of improving this accounting.

    The accounting is easy iff tick resolution is good enough, the deadline
    mechanism is harder. I did the "quota follows task" thing, but nothing
    good happens. That just ensured that the deadline mechanism kicks in
    constantly because tick theft is a fact of tick-based life. A
    reasonable fudge factor would help, but...

    I see problems wrt with trying to implement the deadline mechanism.

    As implemented, it can't identify who is doing the stealing (which
    happens constantly, even if userland if 100% hog) because of tick
    resolution accounting. If you can't identify the culprit, you can't
    enforce the quota, and quotas which are not enforced are, strictly
    speaking, not quotas. At tick time, you can only close the barn door
    after the cow has been stolen, and the thief can theoretically visit
    your barn an infinite number of times while you aren't watching the
    door. ("don't blink" scenarios, and tick is backward-assward blink)

    You can count nanoseconds in schedule, and store the actual usage, but
    then you still have the problem of inaccuracies in sched_clock() from
    cross-cpu wakeup and migration. Cross-cpu wakeups happen quite a lot.
    If sched_clock() _were_ absolutely accurate, you wouldn't need the
    runqueue deadline mechanism, because at slice tick time you can see
    everything you will ever see without moving enforcement directly into
    the most critical of paths.

    IMHO, unless it can be demonstrated that timeslice theft is a problem
    with a real-life scenario, you'd be better off dropping the queue
    ticking. Time slices are a deadline mechanism, and in practice the god
    of randomness ensures that even fast movers do get caught often enough
    to make ticking tasks sufficient.

    (that was a very long-winded reply to one sentence because I spent a lot
    of time looking into this very subject and came to the conclusion that
    you can't get there from here. fwiw, ymmv and all that of course;)

    > Thanks again!

    You're welcome.

    -Mike

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