From: Ric Mason <ric.masonn@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 10:54:32 +0800 [thread overview]
Message-ID: <513D4768.8050703@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
If all the swap slots will be freed when swapoff?
> swap space to save future swap out IO (grep for vm_swap_full()). The
> 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.
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org. For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2013-03-11 2:54 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 [this message]
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
2013-03-12 17:08 ` Johannes Weiner
2013-03-13 3:47 ` Jaegeuk Hanse
2013-03-11 6:24 ` Will Huck
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=513D4768.8050703@gmail.com \
--to=ric.masonn@gmail.com \
--cc=hannes@cmpxchg.org \
--cc=linux-mm@kvack.org \
--cc=shentino@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).