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    SubjectRe: [PATCH v5 3/7] sched: set initial value of runnable avg for new forked task
    On Wed, May 8, 2013 at 4:34 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    > On Tue, May 07, 2013 at 04:20:55AM -0700, Paul Turner wrote:
    >> Yes, 1024 was only intended as a starting point. We could also
    >> arbitrarily pick something larger, the key is that we pick
    >> _something_.
    >>
    >> If we wanted to be more exacting about it we could just give them a
    >> sched_slice() worth; this would have a few obvious "nice" properties
    >> (pun intended).
    >
    > Oh I see I misunderstood again :/ Its not about the effective load but weight
    > of the initial effective load wrt adjustment.
    >
    > Previous schedulers didn't have this aspect at all, so no experience from me
    > here. Paul would be the one, since he's ran longest with this stuff.
    >
    > That said, I would tend to keep it shorter rather than longer so that it would
    > adjust quicker to whatever it really wanted to be.
    >
    > Morten says the load is unstable specifically on loaded systems.

    Here, Morten was (I believe) referring to the stability at task startup.

    To be clear:
    Because we have such a small runnable period denominator at this point
    a single changed observation (for an equivalently behaving thread)
    could have a very large effect. e.g. fork/exec -- happen to take a
    major #pf, observe a "relatively" long initial block.

    By associating an initial period (along with our full load_contrib)
    here, we're making the denominator larger so that these effects are
    less pronounced; achieving better convergence towards what our load
    contribution should actually be.

    Also: We do this conservatively, by converging down, not up.

    > I would think
    > this is because we'd experience scheduling latency, we're runnable more pushing
    > things up. But if we're really an idle task at heart we'd not run again for a
    > long while, pushing things down again.

    Exactly, this is why we must be careful to use instaneous weights
    about wake-up decisions. Interactive and background tasks are largely
    idle.

    While this is exactly how we want them to be perceived from a
    load-balance perspective it's important to keep in mind that while
    wake-up placement has a very important role in the overall balance of
    a system, it is not playing quite the same game as the load-balancer.

    >
    > So on that point Paul's suggestion of maybe starting with __sched_slice() might
    > make sense because it increases the weight of the initial avg with nr_running.
    > Not sure really, we'll have to play and see what works best for a number of
    > workloads.


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