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    SubjectRe: [patch] SLQB slab allocator
    On Thu, 22 Jan 2009, Pekka Enberg wrote:
    > On Wed, Jan 21, 2009 at 8:10 PM, Hugh Dickins <hugh@veritas.com> wrote:
    > > I was initially _very_ impressed by how well it did on my venerable
    > > tmpfs loop swapping loads, where I'd expected next to no effect; but
    > > that turned out to be because on three machines I'd been using SLUB,
    > > without remembering how default slub_max_order got raised from 1 to 3
    > > in 2.6.26 (hmm, and Documentation/vm/slub.txt not updated).
    > >
    > > That's been making SLUB behave pretty badly (e.g. elapsed time 30%
    > > more than SLAB) with swapping loads on most of my machines. Though
    > > oddly one seems immune, and another takes four times as long: guess
    > > it depends on how close to thrashing, but probably more to investigate
    > > there. I think my original SLUB versus SLAB comparisons were done on
    > > the immune one: as I remember, SLUB and SLAB were equivalent on those
    > > loads when SLUB came in, but even with boot option slub_max_order=1,
    > > SLUB is still slower than SLAB on such tests (e.g. 2% slower).
    > > FWIW - swapping loads are not what anybody should tune for.
    >
    > What kind of machine are you seeing this on? It sounds like it could
    > be a side-effect from commit 9b2cd506e5f2117f94c28a0040bf5da058105316
    > ("slub: Calculate min_objects based on number of processors").

    Thanks, yes, that could well account for the residual difference: the
    machines in question have 2 or 4 cpus, so the old slub_min_objects=4
    has effectively become slub_min_objects=12 or slub_min_objects=16.

    I'm now trying with slub_max_order=1 slub_min_objects=4 on the boot
    lines (though I'll need to curtail tests on a couple of machines),
    and will report back later.

    It's great that SLUB provides these knobs; not so great that it needs them.

    Hugh


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