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

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Date:	Wed, 11 Jan 2012 16:30:12 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>
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 01/11/2012 04:10 PM, Johannes Weiner wrote:
> 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?

That is how I implemented it originally, but we need
to take the swap_lock so it is cleaner to implement
a helper function in swapfile.c :)

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