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

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Date:	Thu, 26 Mar 2009 17:56:52 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	kosaki.motohiro@...fujitsu.com,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Nick Piggin <npiggin@...e.de>, 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%.

May I ask the number of the cpu of your test box.
In general, lock contention possibility depend on #ofCPUs.

So, I and lee mainly talked about large box.



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