From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx201.postini.com [74.125.245.201]) by kanga.kvack.org (Postfix) with SMTP id 4FA666B0005 for ; Sun, 10 Mar 2013 22:54:40 -0400 (EDT) Received: by mail-pb0-f43.google.com with SMTP id md12so3222263pbc.16 for ; Sun, 10 Mar 2013 19:54:39 -0700 (PDT) Message-ID: <513D4768.8050703@gmail.com> Date: Mon, 11 Mar 2013 10:54:32 +0800 From: Ric Mason 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 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: 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