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    SubjectRe: [RFC tg_shares_up improvements - v1 00/12] [RFC tg_shares_up - v1 00/12] Reducing cost of tg->shares distribution
    On Sat, Oct 16, 2010 at 12:46 PM, Peter Zijlstra <> wrote:
    > On Fri, 2010-10-15 at 21:43 -0700, wrote:
    >> Hi all,
    >> Peter previously posted a patchset that attempted to improve the problem of
    >> task_group share distribution.  This is something that has been a long-time
    >> pain point for group scheduling.  The existing algorithm considers
    >> distributions on a per-cpu-per-domain basis and carries a fairly high update
    >> overhead, especially on larger machines.
    >> I was previously looking at improving this using Fenwick trees to allow a
    >> single sum without the exorbitant cost but then Peter's idea above was better :).
    >> The kernel is that by monitoring the average contribution to load on a
    >> per-cpu-per-taskgroup basis we can distribute the weight for which we are
    >> expected to consume.
    >> This set extends the original posting with a focus on increased fairness and
    >> reduced convergence (to true average) time.  In particular the case of large
    >> over-commit in the case of a distributed wake-up is a concern which is now
    >> fairly well addressed.
    >> Obviously everything's experimental but it should be stable/fair.
    > I like what you've done with it, my only worry is 10/12 where you allow
    > for extra updates to the global state -- I think they should be fairly
    > limited in number, and I can see the need for the update if we get too
    > far out of whack, but it is something to look at while testing this
    > stuff.

    So my original answer here was to only update when there was load and
    it was > n% delta which stops 1 thread waking up and sleeping from
    thrashing it, but the 2 thread case is just as obviously broken for
    any n. It needs a rate limit but I'm sort of loathe to introduce
    _another_ set of timestamps. I don't suppose there's much harm in
    doing so though and I don't think it's going to be clean to overload
    one of the existing ones so perhaps another counter is the answer.

    I'll make sure this is addressed in v2.

    >> TODO:
    >> - Validate any RT interaction
    > I don't think there's anything to worry about there, the only
    > interaction which there is between this and the rt scheduling classes is
    > the initial sharing of the load-avg window, but you 'cure' that in 7/12.
    > (I think that sysctl wants a _us postfix someplace and we thus want some
    > NSEC_PER_USEC multiplication in there).

    Yes -- updated, thanks.

    >> - Continue collecting/analyzing performance and fairness data
    > Yes please ;-), I'll try and run this on some machines as well.
    >> - Should the shares period just be the sched_latency?
    > Interesting idea.. lets keep it a separate sysctl for now for easy
    > tuning, if things settle down and we're still good in that range we can
    > consider merging them.
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