linux-kernel - Re: [PATCH 0/1] mm, shmem: map few pages around fault address if they are in page cache

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Date:	Tue, 4 Mar 2014 02:50:01 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 03, 2014 at 03:37:07PM -0800, Andrew Morton wrote:
> On Mon, 3 Mar 2014 15:29:00 -0800 Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > 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.
> 
> Of course.
> 
> > 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.
> 
> When a pagefault hits a locked, not-uptodate page it is going to block.
> Once it wakes up we'd *like* to find lots of now-uptodate pages in
> that page's vicinity.  Obviously, that is happening, but not to the
> fullest possible extent.  We _could_ still achieve the 16x if readahead
> was cooperating in an ideal fashion.
> 
> I don't know what's going on in there to produce this consistent 3x
> factor.

In my VM numbers are different (fault in 1G):

cold cache: 2097352inputs+0outputs (2major+25048minor)pagefaults 0swaps
hot cache: 0inputs+0outputs (0major+16450minor)pagefaults 0swaps

~1.5x more page faults with cold cache comparing to hot cache.

BTW, moving do_fault_around() below __do_fault() doesn't make much better:

cold cache: 2097200inputs+0outputs (1major+24641minor)pagefaults 0swaps

-- 
 Kirill A. Shutemov
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