linux-kernel - Re: [PATCH] mm: batch-free pcp list if possible

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Date:	Thu, 10 Feb 2011 13:00:37 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Namhyung Kim <namhyung@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] mm: batch-free pcp list if possible

On Thu, 10 Feb 2011 09:35:44 +0000
Mel Gorman <mel@....ul.ie> wrote:

> > What's the point in that?  What relationship does the number of
> > contiguous empty lists have with the number of pages to free from one
> > list?
> > 
> 
> The point is to avoid excessive checking of empty lists.

It seems pretty simple to me to skip the testing of empty lists
altogether.  I suggested one way, however I suspect a better approach
might be to maintain a count of the number of pages in each list and
then change free_pcppages_bulk() so that it calculates up-front the
number of pages to free from each list (equal proportion of each) then
sits in a tight loop freeing that number of pages.

It might be that the overhead of maintaining the per-list count makes
that not worthwhile.  It'll be hard to tell because the count
maintenance cost will be smeared all over the place.

I doubt if any of it matters much, compared to the cost of allocating,
populating and freeing a page.  I just want free_pcppages_bulk() to
stop hurting my brain ;)

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