From: Jaegeuk Hanse <jaegeuk.hanse@gmail.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Raymond Jennings <shentino@gmail.com>,
Linux Memory Management List <linux-mm@kvack.org>
Subject: Re: Swap defragging
Date: Mon, 11 Mar 2013 11:16:29 +0800 [thread overview]
Message-ID: <513D4C8D.6080106@gmail.com> (raw)
In-Reply-To: <20130308023511.GD23767@cmpxchg.org>
Hi Johannes,
On 03/08/2013 10:35 AM, Johannes Weiner wrote:
> On Thu, Mar 07, 2013 at 06:07:23PM -0800, Raymond Jennings wrote:
>> Just a two cent question, but is there any merit to having the kernel
>> defragment swap space?
> That is a good question.
>
> Swap does fragment quite a bit, and there are several reasons for
> that.
>
> We swap pages in our LRU list order, but this list is sorted by first
> access, not by access frequency (not quite that cookie cutter, but the
> ordering is certainly fairly coarse). This means that the pages may
> already be in suboptimal order for swap in at the time of swap out.
>
> Once written to disk, the layout tends to stick. One reason is that
> we actually try to not free swap slots unless there is a shortage of
> swap space to save future swap out IO (grep for vm_swap_full()). The
Since anonymous page will be swap out if it's dirty and the contents of
the page and data store in swap area is not equal now, why can avoid
future swap out IO?
> other reason is that if a page shared among multiple threads is
> swapped out, it can not be removed from swap until all threads have
> faulted the page back in because of page table entries still referring
> to the swap slot on disk. In a multi-threaded application, this is
> rather unlikely.
>
> So even though the referencing order of the application might change,
> the disk layout won't. But adjusting the disk layout speculatively
> increases disk IO, so it could be hard to prove that you came up with
> a net improvement.
>
> --
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next prev parent reply other threads:[~2013-03-11 3:16 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-03-08 2:07 Swap defragging Raymond Jennings
2013-03-08 2:35 ` Johannes Weiner
2013-03-08 3:01 ` Raymond Jennings
2013-03-09 2:00 ` Will Huck
2013-03-12 16:52 ` Johannes Weiner
2013-03-13 0:46 ` Will Huck
2013-03-13 1:31 ` Will Huck
2013-03-11 2:54 ` Ric Mason
2013-03-12 16:53 ` Johannes Weiner
2013-03-11 3:11 ` Simon Jeons
2013-03-12 17:03 ` Johannes Weiner
2013-03-11 3:16 ` Jaegeuk Hanse [this message]
2013-03-12 17:08 ` Johannes Weiner
2013-03-13 3:47 ` Jaegeuk Hanse
2013-03-11 6:24 ` Will Huck
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