Messages in this thread | | | Date | Fri, 23 Jan 2009 14:23:14 +0000 (GMT) | From | Hugh Dickins <> | Subject | Re: [patch] SLQB slab allocator |
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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@veritas.com> 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.
Hugh
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