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Date: Mon, 3 Mar 2014 15:29:00 -0800
Message-ID: <CA+55aFzvCF-tWg7qx2_Om+Y64uJy6ujEyWgcq87UkzSkfbVGqw@mail.gmail.com>
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>
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To: Andrew Morton <akpm@linux-foundation.org>
Cc: Ning Qu <quning@gmail.com>, Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Hugh Dickins <hughd@google.com>, Andi Kleen <ak@linux.intel.com>, Matthew Wilcox <matthew.r.wilcox@intel.com>, Dave Hansen <dave.hansen@linux.intel.com>, Alexander Viro <viro@zeniv.linux.org.uk>, Dave Chinner <david@fromorbit.com>, linux-mm <linux-mm@kvack.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>

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

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