On Mon, Mar 11, 2013 at 11:16:29AM +0800, Jaegeuk Hanse wrote: > 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? Modified pages get written out freshly, but in a multi-threaded application, the original page stays put until all threads have modified it or faulted it back in. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>