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Subject: Re: [PATCH 0/1] mm, shmem: map few pages around fault address if they
 are in page cache
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Ning Qu <quning@gmail.com>, Mel Gorman <mgorman@suse.de>,
        Rik van Riel <riel@redhat.com>,
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On Mon, Mar 3, 2014 at 2:38 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
>
> When the file is uncached, results are peculiar:
>
> 0.00user 2.84system 0:50.90elapsed 5%CPU (0avgtext+0avgdata 4198096maxresident)k
> 0inputs+0outputs (1major+49666minor)pagefaults 0swaps
>
> That's approximately 3x more minor faults.

This is not peculiar.

When the file is uncached, some pages will obviously be under IO due
to readahead etc. And the fault-around code very much on purpose will
*not* try to wait for those pages, so any busy pages will just simply
not be faulted-around.

So you should still have fewer minor faults than faulting on *every*
page (ie the non-fault-around case), but I would very much expect that
fault-around will not see the full "one sixteenth" reduction in minor
faults.

And the order of IO will not matter, since the read-ahead is
asynchronous wrt the page-faults.

             Linus