Re: [patch] SLQB slab allocator

From: Pekka Enberg
Date: Thu Jan 22 2009 - 05:01:40 EST

Hi Hugh,

On Wed, Jan 21, 2009 at 8:10 PM, Hugh Dickins <hugh@xxxxxxxxxxx> 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").

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