Re: [patch] SLQB slab allocator

From: Hugh Dickins
Date: Fri Jan 23 2009 - 09:23:53 EST

On Thu, 22 Jan 2009, Hugh Dickins wrote:
> On Thu, 22 Jan 2009, Pekka Enberg wrote:
> > On Wed, Jan 21, 2009 at 8:10 PM, Hugh Dickins <hugh@xxxxxxxxxxx> wrote:
> > >
> > > 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.

Yes, slub_max_order=1 with slub_min_objects=4 certainly helps this
swapping load. I've not tried slub_max_order=0, but I'm running
with 8kB stacks, so order 1 seems a reasonable choice.

I can't say where I pulled that "e.g. 2% slower" from: on different
machines slub was 5% or 10% or 20% slower than slab and slqb even with
slub_max_order=1 (but not significantly slower on the "immune" machine).
How much slub_min_objects=4 helps again varies widely, between halving
or eliminating the difference.

But I think it's more important that I focus on the worst case machine,
try to understand what's going on there.

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