linux-kernel - Re: [patch 3/3] mm: keep pages from unevictable mappings off the LRU lists

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Message-ID: <20090326000100.GA5404@cmpxchg.org>
Date:	Thu, 26 Mar 2009 01:01:00 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Nick Piggin <npiggin@...e.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	MinChan Kim <minchan.kim@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>
Subject: Re: [patch 3/3] mm: keep pages from unevictable mappings off the LRU lists

On Mon, Mar 23, 2009 at 10:53:27AM +0000, David Howells wrote:
> Johannes Weiner <hannes@...xchg.org> wrote:
> 
> > -		if (page_is_file_cache(page))
> > +		if (mapping_unevictable(mapping))
> > +			add_page_to_unevictable_list(page);
> > +		else if (page_is_file_cache(page))
> 
> It would be nice to avoid adding an extra test and branch in here.  This
> function is used a lot, and quite often we know the answer to the first test
> before we even get here.

Yes, I thought about that too.  So I mounted a tmpfs and dd'd
/dev/zero to a file on it until it ran out of space (around 900M,
without swapping), deleted the file again.  I did this in a tight loop
and profiled it.

I couldn't think of a way that would excercise add_to_page_cache_lru()
more, I hope I didn't overlook anything, please correct if I am wrong.

If I was not, than the extra checking for unevictable mappings doesn't
make a measurable difference.  The function on the vanilla kernel had
a share of 0.2033%, on the patched kernel 0.1953%.

	Hannes
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