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

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Date:	Wed, 11 Jan 2012 16:58:14 -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 04:42 PM, Johannes Weiner wrote:
> On Wed, Jan 11, 2012 at 04:30:12PM -0500, Rik van Riel wrote:
>> 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 :)
>
> AFAICS, it's only needed to validate the offset against si->max, but
> this too is done in __swap_duplicate().
>
> What's otherwise left is just rounding down swp_offset(entry) and
> adding 1<<  page_cluster to it, that shouldn't need the swap_lock?
>
> Am I missing something?

Good point.  I guess we could get rid of get_swap_cluster
afterall and let __swap_duplicate deal with everything.

Andrew, Mel, is that ok with you?
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