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SubjectRe: [PATCH] mm/slab: Improve performance of gathering slabinfo stats
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Hi Joonsoo,

On 08/01/2016 05:55 PM, Joonsoo Kim wrote:
> Your patch updates these counters not only when a slabs are created and
> destroyed but also when object is allocated/freed from the slab. This
> would hurt runtime performance.
>

The counters are not updated for each object allocation/free - only if
that allocation/free results in that slab moving from one list
(free/partial/full) to another.

>> > slab lists for gathering slabinfo stats, resulting in a dramatic
>> > performance improvement. We tested this after growing the dentry cache to
>> > 70GB, and the performance improved from 2s to 2ms.
> Nice improvement. I can think of an altenative.
>
> I guess that improvement of your change comes from skipping to iterate
> n->slabs_full list. We can achieve it just with introducing only num_slabs.
> num_slabs can be updated when a slabs are created and destroyed.
>

Yes, slabs_full is typically the largest list.

> We can calculate num_slabs_full by following equation.
>
> num_slabs_full = num_slabs - num_slabs_partial - num_slabs_free
>
> Calculating both num_slabs_partial and num_slabs_free by iterating
> n->slabs_XXX list would not take too much time.

Yes, this would work too. We cannot avoid traversal of slabs_partial,
and slabs_free is usually a small list, so this should give us similar
performance benefits. But having separate counters could also be useful
for debugging, like the ones defined under CONFIG_DEBUG_SLAB/STATS.
Won't that help?

Thanks,
Aruna

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