linux-kernel - Re: [PATCH v2 -mm] make swapin readahead skip over holes

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Message-ID: <20120111205041.GE24386@cmpxchg.org>
Date:	Wed, 11 Jan 2012 22:10:31 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	akpm@...ux-foundation.org, mel@....ul.ie, minchan.kim@...il.com
Subject: Re: [PATCH v2 -mm] make swapin readahead skip over holes

On Wed, Jan 11, 2012 at 02:30:44PM -0500, Rik van Riel wrote:
> Ever since abandoning the virtual scan of processes, for scalability
> reasons, swap space has been a little more fragmented than before.
> This can lead to the situation where a large memory user is killed,
> swap space ends up full of "holes" and swapin readahead is totally
> ineffective.
> 
> On my home system, after killing a leaky firefox it took over an
> hour to page just under 2GB of memory back in, slowing the virtual
> machines down to a crawl.
> 
> This patch makes swapin readahead simply skip over holes, instead
> of stopping at them.  This allows the system to swap things back in
> at rates of several MB/second, instead of a few hundred kB/second.
> 
> The checks done in valid_swaphandles are already done in 
> read_swap_cache_async as well, allowing us to remove a fair amount
> of code.

__swap_duplicate() also checks for whether the offset is within the
swap device range.  Do you think we could remove get_swap_cluster()
altogether and just try reading the aligned page_cluster range?
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