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    SubjectRe: [2.4] heavy-load under swap space shortage
    On Sun, Mar 14, 2004 at 03:22:53PM -0800, Andrew Morton wrote:
    > Andrea Arcangeli <> wrote:
    > >
    > > >
    > > > Having a magic knob is a weak solution: the majority of people who are
    > > > affected by this problem won't know to turn it on.
    > >
    > > that's why I turned it _on_ by default in my tree ;)
    > So maybe Marcelo should apply this patch, and also turn it on by default.

    yes, I would suggest so. If anybody can find any swap-regression on
    small UP machines then reporting to us on l-k will be welcome. So far
    nobody could notice any swap difference at swap regime AFIK, and the
    improvement for the fast path is dramatic on the big smp boxes.

    > > There are workloads where adding anonymous pages to the lru is
    > > suboptimal for both the vm (cache shrinking) and the fast path too
    > > (lru_cache_add), not sure how 2.6 optimizes those bits, since with 2.6
    > > you're forced to add those pages to the lru somehow and that implies
    > > some form of locking.
    > Basically a bunch of tweeaks:
    > - Per-zone lru locks (which implicitly made them per-node)

    the 16-ways weren't numa, and these days 16-ways HT (8-ways phys) are
    not so uncommon anymore.

    > - Adding/removing sixteen pages for one taking of the lock.
    > - Making the lock irq-safe (it had to be done for other reasons, but
    > reduced contention by 30% on 4-way due to not having a CPU wander off to
    > service an interrupt while holding a critical lock).
    > - In page reclaim, snip 32 pages off the lru completely and drop the
    > lock while we go off and process them.

    sounds good, thanks.

    I don't see other ways to optimize it (and I never enjoyed too much the
    per-zone lru since it has some downside too with a worst case on 2G
    systems). peraphs a further optimization could be a transient per-cpu
    lru refiled only by the page reclaim (so absolutely lazy while lots of
    ram is free), but maybe that's already what you're doing when you say
    "Adding/removing sixteen pages for one taking of the lock". Though the
    fact you say "sixteen pages" sounds like it's not as lazy as it could
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