Re: [PATCH] mm/slab: Improve performance of gathering slabinfo stats

[Aruna Ramakrishna]

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