Re: [PATCH v2 1/2] mm, thp: move invariant bug check out of loop in __split_huge_page_map

[Waiman Long]

On 06/18/2014 08:24 AM, Kirill A. Shutemov wrote:
> On Tue, Jun 17, 2014 at 06:37:58PM -0400, Waiman Long wrote:
> > In the __split_huge_page_map() function, the check for
> > page_mapcount(page) is invariant within the for loop. Because of the
> > fact that the macro is implemented using atomic_read(), the redundant
> > check cannot be optimized away by the compiler leading to unnecessary
> > read to the page structure.
> >
> > This patch moves the invariant bug check out of the loop so that it
> > will be done only once. On a 3.16-rc1 based kernel, the execution
> > time of a microbenchmark that broke up 1000 transparent huge pages
> > using munmap() had an execution time of 38,245us and 38,548us with
> > and without the patch respectively. The performance gain is about 1%.
> For this low difference it would be nice to average over few runs +
> stddev. It can easily can be a noise.

The timing data was the average of 5 runs with a SD of 100-200us.
> > Signed-off-by: Waiman Long<Waiman.Long@xxxxxx>
> But okay:
>
> Acked-by: Kirill A. Shutemov<kirill.shutemov@xxxxxxxxxxxxxxx>

Thank for the review.

-Longman
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