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    SubjectRe: ZVC: Increase threshold for larger processor configurationss
    On Thu, 29 Jun 2006 11:22:45 -0700 (PDT)
    Christoph Lameter <clameter@sgi.com> wrote:

    > On Wed, 28 Jun 2006, Andrew Morton wrote:
    >
    > > An alternative would be to calculate stat_threshold at runtime, based on
    > > the cpu_possible count (I guess). Or even:
    > >
    > > static inline int stat_threshold(void)
    > > {
    > > #if NR_CPUS <= 32
    > > return 32;
    > > #else
    > > return dynamically_calculated_stat_threshold;
    > > #endif
    > > }
    >
    > Thats one more memory reference. Hmmm... We could place the threshold in
    > the same cacheline. That would also open up the possbiliity of dynamically
    > calculating the threshold.

    yup.

    > > Did you consider my earlier suggestion about these counters? That, over the
    > > short-term, they tend to count in only one direction? So we can do
    > >
    > > if (x > STAT_THRESHOLD) {
    > > zone_page_state_add(x + STAT_THRESHOLD/2, zone, item);
    > > x = -STAT_THRESHOLD/2;
    > > } else if (x < -STAT_THRESHOLD) {
    > > zone_page_state_add(x - STAT_THRESHOLD/2, zone, item);
    > > x = STAT_THRESHOLD;
    > > }
    > >
    > > that'll give an decrease in contention while not consuming any extra
    > > storage and while (I think) increasing accuracy.
    >
    > Uhh... We are overcompensating right? Pretty funky idea that is new to me
    > and that would require some thought.

    See inbox ;)

    > This would basically increase the stepping by 50% if we are only going in
    > one direction.

    yes.

    > If we are doing a mixture of allocations and deallocations (or pages being
    > marked dirty / undirty, mapping unmapping) then this may potentially
    > increase the number of updates and therefore the cacheline contentions.

    Yes. I'd handwavingly contend that this is a rare situation.

    A lot of the counters only ever count in one direction! So we could even
    skew them by an entire STAT_THRESHOLD.
    -
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