Re: [PATCH 0/3] mm: hugetlbfs: fix hugetlbfs optimization v2

[Khalid Aziz]

On 11/15/2013 10:47 AM, Andrea Arcangeli wrote:
> Hi,
>
> 1/3 is a bugfix so it should be applied more urgently. 1/3 is not as
> fast as the current upstream code in the hugetlbfs + directio extreme
> 8GB/sec benchmark (but 3/3 should fill the gap later). The code is
> identical to the one I posted in v1 just rebased on upstream and was
> developed in collaboration with Khalid who already tested it.
>
> 2/3 and 3/3 had very little testing yet, and they're incremental
> optimization. 2/3 is minor and most certainly worth applying later.
>
> 3/3 instead complicates things a bit and adds more branches to the THP
> fast paths, so it should only be applied if the benchmarks of
> hugetlbfs + directio show that it is very worthwhile (that has not
> been verified yet). If it's not worthwhile 3/3 should be dropped (and
> the gap should be filled in some other way if the gap is not caused by
> the _mapcount mangling as I guessed). Ideally this should bring even
> more performance than current upstream code, as current upstream code
> still increased the _mapcount in gup_fast by mistake, while this
> eliminates the locked op on the tail page cacheline in gup_fast too
> (which is required for correctness too).

Hi Andrea,

I ran directio benchmark and here are the performance numbers (MBytes/sec):

Block size        3.12         3.12+patch 1      3.12+patch 1,2,3
----------        ----         ------------      ----------------
1M                8467           8114              7648
64K               4049           4043              4175

Performance numbers with 64K reads look good but there is further 
deterioration with 1M reads.

--
Khalid
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