linux-kernel - Re: [PATCH] mm: cma: Discard clean pages during contiguous allocation instead of migration

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Date:	Wed, 12 Sep 2012 13:07:32 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Kyungmin Park <kmpark@...radead.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH] mm: cma: Discard clean pages during contiguous
 allocation instead of migration

On Tue, 11 Sep 2012 09:41:52 +0900
Minchan Kim <minchan@...nel.org> wrote:

> This patch drops clean cache pages instead of migration during
> alloc_contig_range() to minimise allocation latency by reducing the amount
> of migration is necessary. It's useful for CMA because latency of migration
> is more important than evicting the background processes working set.
> In addition, as pages are reclaimed then fewer free pages for migration
> targets are required so it avoids memory reclaiming to get free pages,
> which is a contributory factor to increased latency.
> 
> * from v1
>   * drop migrate_mode_t
>   * add reclaim_clean_pages_from_list instad of MIGRATE_DISCARD support - Mel
> 
> I measured elapsed time of __alloc_contig_migrate_range which migrates
> 10M in 40M movable zone in QEMU machine.
> 
> Before - 146ms, After - 7ms
> 
> ...
>
> @@ -758,7 +760,9 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  			wait_on_page_writeback(page);
>  		}
>  
> -		references = page_check_references(page, sc);
> +		if (!force_reclaim)
> +			references = page_check_references(page, sc);

grumble.  Could we please document `enum page_references' and
page_check_references()?

And the `force_reclaim' arg could do with some documentation.  It only
forces reclaim under certain circumstances.  They should be described,
and a reson should be provided.

Why didn't this patch use PAGEREF_RECLAIM_CLEAN?  It is possible for
someone to dirty one of these pages after we tested its cleanness and
we'll then go off and write it out, but we won't be reclaiming it?

>
> ...
>
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