Date: Mon, 6 Jun 2011 15:55:17 +0100
From: Mel Gorman <mgorman@suse.de>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Minchan Kim <minchan.kim@gmail.com>, akpm@linux-foundation.org,
        Ury Stankevich <urykhy@gmail.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@kernel.org
Subject: Re: [PATCH] mm: compaction: Abort compaction if too many pages are
 isolated and caller is asynchronous
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On Mon, Jun 06, 2011 at 02:38:51PM +0200, Andrea Arcangeli wrote:
> On Mon, Jun 06, 2011 at 11:39:24AM +0100, Mel Gorman wrote:
> > Well spotted.
> > 
> > Acked-by: Mel Gorman <mgorman@suse.de>
> > 
> > Minor nit. swapper_space is rarely referred to outside of the swap
> > code. Might it be more readable to use
> > 
> > 	/*
> > 	 * swapcache is accounted as NR_FILE_PAGES but it is not
> > 	 * accounted as NR_SHMEM
> > 	 *
> > 	if (PageSwapBacked(page) && !PageSwapCache(page))
> 
> I thought the comparison on swapper_space would be faster as it was
> immediate vs register in CPU, instead of forcing a memory
> access. Otherwise I would have used the above. Now the test_bit is
> written in C and lockless so it's not likely to be very different
> considering the cacheline is hot in the CPU but it's still referencing
> memory instead register vs immediate comparison.

Ok, I had not considered that. That is a micro-optimisation but it's
there. I thought my version is more readable and migration is not
really a fast path but yours is still better.

-- 
Mel Gorman
SUSE Labs
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