linux-kernel - Re: [PATCH] mm: add parameter to disable faultaround

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Date:	Tue, 25 Nov 2014 13:48:46 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Chanho Min <chanho.min@....com>
Cc:	'Andrew Morton' <akpm@...ux-foundation.org>,
	"'Kirill A. Shutemov'" <kirill.shutemov@...ux.intel.com>,
	'Hugh Dickins' <hughd@...gle.com>,
	'Michal Hocko' <mhocko@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, 'HyoJun Im' <hyojun.im@....com>,
	'Gunho Lee' <gunho.lee@....com>,
	'Wonhong Kwon' <wonhong.kwon@....com>
Subject: Re: [PATCH] mm: add parameter to disable faultaround

On Tue, Nov 25, 2014 at 08:19:40PM +0900, Chanho Min wrote:
> > > The faultaround improves the file read performance, whereas pages which
> > > can be dropped by drop_caches are reduced. On some systems, The amount of
> > > freeable pages under memory pressure is more important than read
> > > performance.
> > 
> > The faultaround pages *are* freeable.  Perhaps you meant "free" here.
> > 
> > Please tell us a great deal about the problem which you are trying to
> > solve.  What sort of system, what sort of workload, what is bad about
> > the behaviour which you are observing, etc.
> 
> We are trying to solve two issues.
> 
> We drop page caches by writing to /proc/sys/vm/drop_caches at specific point
> and make suspend-to-disk image. The size of this image is increased if faultaround
> is worked.

drop_caches should never be used outside debugging process. If you use it
as part of usual workflow you're doing something wrong.

I'm not aware about details on how suspend-to-disk works, but I don't see
much point in saving page cache pages into suspend-to-disk image. Dirty
pages should be write out and we can read them back after resume on first
use. Possible exception is mlocked pages.

> Under memory pressure, we want to drop many page caches as possible.
> But, The number of dropped pages are reduced compared to non-faultaround kernel.

The reason why you see more pages in page cache after drop_pages with
faultaround enabled is that drop_pages doesn't touch mapped pages. And
with faultaround we obviously have more pages mapped.

It's not a reason to have faultaround disable. You should take a closer
look on suspend process.

-- 
 Kirill A. Shutemov
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