From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx197.postini.com [74.125.245.197]) by kanga.kvack.org (Postfix) with SMTP id B40836B0005 for ; Sun, 10 Mar 2013 23:16:34 -0400 (EDT) Received: by mail-da0-f47.google.com with SMTP id s35so113407dak.6 for ; Sun, 10 Mar 2013 20:16:34 -0700 (PDT) Message-ID: <513D4C8D.6080106@gmail.com> Date: Mon, 11 Mar 2013 11:16:29 +0800 From: Jaegeuk Hanse MIME-Version: 1.0 Subject: Re: Swap defragging References: <20130308023511.GD23767@cmpxchg.org> In-Reply-To: <20130308023511.GD23767@cmpxchg.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: Raymond Jennings , Linux Memory Management List 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. > > -- > 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: email@kvack.org -- 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: email@kvack.org