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SubjectRe: [patch 1/3] slub: add per-cache slab thrash ratio
On Mon, 30 Mar 2009, Pekka Enberg wrote:

> > The slab_thrash_ratio for each cache do not have non-zero defaults
> > (yet?).
> If we're going to merge this code, I think it would be better to put a
> non-zero default there; otherwise we won't be able to hit potential
> performance regressions or bugs. Furthermore, the optimization is not
> very useful on large scale if people need to enable it themselves.
> Maybe stick 20 there and run tbench, sysbench, et al to see if it makes
> a difference? I'm cc'ing Mel in case he has some suggestions how to test
> it.

It won't cause a regression if sane SLAB_THRASHING_THRESHOLD and
slab_thrash_ratio values are set since the contention on list_lock will
always be slower than utilizing a more free cpu slab when its thrashing.

I agree that there should be a default value and I was originally going
to propose the following as the fourth patch in the series, but I wanted
to generate commentary on the approach first and there's always a
hesitation when changing the default behavior of the entire allocator for
workloads with very specific behavior that trigger this type of problem.

The fact that we need a tunable for this is unfortunate, but there doesn't
seem to be any other way to detect such situations and adjust the partial
list handling so that list_lock isn't contended so much and the allocation
slowpath to fastpath ratio isn't so high.

I'd be interested to hear other people's approaches.
diff --git a/mm/slub.c b/mm/slub.c
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -147,6 +147,12 @@

+ * Default slab thrash ratio, used to define when a slab is thrashing for a
+ * particular cpu.
+ */

@@ -2392,7 +2398,14 @@ static int kmem_cache_open(struct kmem_cache *s, gfp_t gfpflags,
set_min_partial(s, ilog2(s->size));
s->refcount = 1;
- s->min_free_watermark = 0;
+ s->min_free_watermark = oo_objects(s->oo) *
+ /*
+ * It doesn't make sense to define a slab as thrashing if its threshold
+ * is fewer than 4 objects.
+ */
+ if (s->min_free_watermark < 4)
+ s->min_free_watermark = 0;
s->remote_node_defrag_ratio = 1000;

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