Re: what is the purpose of SLAB and SLUB (was: Re: [PATCH v3] mm/slab: Improve performance of gathering slabinfo) stats

On Tue, Aug 23, 2016 at 05:38:08PM +0200, Michal Hocko wrote:

Do we have any documentation/study about which particular workloads
benefit from which allocator? It seems that most users will use whatever
the default or what their distribution uses. E.g. SLES kernel use SLAB
because this is what we used to have for ages and there was no strong
reason to change that default.

Yes, with the downside that a reliance on high-orders contended on the
zone lock which would not scale and could degrade over time. If there
were multiple compelling reasons then it would have been an easier
switch.

I did prototype high-order pcp caching up to PAGE_ALLOC_COSTLY_ORDER
but it pushed the size of per_cpu_pages over a cache line which could
be problematic in itself. I never finished off the work as fixing the
allocator for SLUB was not a priority. The prototype no longer applies as
it conflicts with the removal of the fair zone allocation policy.

If/when I get back to the page allocator, the priority would be a bulk
API for faster allocs of batches of order-0 pages instead of allocating
a large page and splitting.

-- 
Mel Gorman
SUSE Labs

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