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

[Mel Gorman <mgorman@suse.de>]

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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