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    SubjectRe: [PATCH 00/25] Cleanup and optimise the page allocator V5
    On Fri, Mar 20, 2009 at 11:00:42AM -0400, Christoph Lameter wrote:
    > On Fri, 20 Mar 2009, Mel Gorman wrote:
    > > The lock contention on some machines goes up for the the zone->lru_lock
    > > and zone->lock locks which can regress some workloads even though others on
    > > the same machine still go faster. For netperf, a lock called slock-AF_INET
    > > seemed very important although I didn't look too closely other than noting
    > > contention went up. The zone->lock gets hammered a lot by high order allocs
    > > and frees coming from SLUB which are not covered by the PCP allocator in
    > > this patchset. zone->lru_lock goes up is less clear but as it's page cache
    > > releases but overall contention may be up because CPUs are spending less
    > > time with interrupts disabled and more time trying to do real work but
    > > contending on the locks.
    > We can tune SLUB to buffer more pages if the lru lock becomes too hot.

    hmm, I'm missing something in your reasoning. The contention I saw for

    &zone->lru_lock 37350 [<ffffffff8029d6fe>] ____pagevec_lru_add+0x9c/0x172
    &zone->lru_lock 55423 [<ffffffff8029d377>] release_pages+0x10a/0x21b
    &zone->lru_lock 402 [<ffffffff8029d9d9>] activate_page+0x4f/0x147
    &zone->lru_lock 6 [<ffffffff8029dbbd>] put_page+0x94/0x122

    So I just assumed it was LRU pages being taken off and freed that was
    causing the contention. Can SLUB affect that?

    Maybe you meant zone->lock and SLUB could tune buffers more to avoid
    that if that lock was hot. That is one alternative but the later patches
    proposed an alternative whereby high-order and compound pages could be
    stored on the PCP lists. Compound only really helps SLUB but high-order
    also helped stacks, signal handlers and the like so it seemed like a
    good idea one way or the other. Course, this meant a search of the PCP
    lists or increasing the size of the PCP structure - swings and
    roundabouts :/

    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab

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