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Date: Mon, 3 Mar 2014 15:29:00 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Ning Qu <quning@...il.com>, Mel Gorman <mgorman@...e.de>,
Rik van Riel <riel@...hat.com>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Hugh Dickins <hughd@...gle.com>,
Andi Kleen <ak@...ux.intel.com>,
Matthew Wilcox <matthew.r.wilcox@...el.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Dave Chinner <david@...morbit.com>,
linux-mm <linux-mm@...ck.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/1] mm, shmem: map few pages around fault address if they
are in page cache
On Mon, Mar 3, 2014 at 2:38 PM, Andrew Morton <akpm@...ux-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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