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    SubjectRe: [patch] sched_[set|get]_affinity() syscall, 2.4.15-pre9
    On Fri, 23 Nov 2001, Ingo Molnar wrote:

    > On Thu, 22 Nov 2001, Mark Hahn wrote:
    > > only that it's nontrivial to estimate the migration costs, I think. at
    > > one point, around 2.3.3*, there was some effort at doing this - or
    > > something like it. specifically, the scheduler kept track of how long
    > > a process ran on average, and was slightly more willing to migrate a
    > > short-slice process than a long-slice. "short" was defined relative
    > > to cache size and a WAG at dram bandwidth.
    > yes. I added the avg_slice code, and i removed it as well - it was
    > hopeless to get it right and it was causing bad performance for certain
    > application sloads. Current CPUs simply do not support any good way of
    > tracking cache footprint of processes. There are methods that are an
    > approximation (eg. uninterrupted runtime and cache footprint are in a
    > monotonic relationship), but none of the methods (including cache traffic
    > machine counters) are good enough to cover all the important corner cases,
    > due to cache aliasing, MESI-invalidation and other effects.

    Uninterrupted run-time is a good approximation of a task's cache footprint.
    It's true, it's not 100% successful, processes like :

    for (;;);

    are uncorrectly classified but it's still way better than the method we're
    currently using ( PROC_CHANGE_PENALTY ).
    By taking the avg :

    AVG = (AVG + LAST) >> 1;

    run-time in jiffies is 1) fast 2) has a nice hysteresis property 3) gives
    you a pretty good estimation of the "nature" of the task.
    I'm currently using it as 1) classification for load balancing between
    CPUs 2) task's watermark value for your counter decay patch :


    if (p->counter > p->avg_jrun)
    else if (++p->timer_ticks >= p->counter) {
    p->counter = 0;
    p->timer_ticks = 0;
    p->need_resched = 1;

    In this way I/O bound tasks have a counter decay behavior like the
    standard scheduler while CPU bound ones preserve the priority inversion

    - Davide

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