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

> > @@ -2291,10 +2292,13 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
> > /*
> > * Determine the number of objects per slab
> > */
> > + if (oo_objects(s->oo))
> > + thrash_ratio = s->min_free_watermark * 100 / oo_objects(s->oo);
> > s->oo = oo_make(order, size);
> s->oo is set *after* you check it. Check oo_objects after the value has
> been set please.

It's actually right the way I implemented it, oo_objects(s->oo) will be 0
when this is called for kmem_cache_open() meaning there is no preexisting
slab_thrash_ratio. But this check is required when calculate_sizes() is
called from order_store() to adjust the slab_thrash_ratio for the new
objects per slab. The above check is saving the old thrash ratio so
the new s->min_free_watermark value can be set following the
oo_make(). This was mentioned in the changelog for this patch:

The value is stored in terms of the number of objects that the
ratio represents, not the ratio itself. This avoids costly
arithmetic in the slowpath for a calculation that could otherwise
be done only when `slab_thrash_ratio' or `order' is changed.

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