* Roman Kagan (rkagan@xxxxxxxxxxxxx) wrote: > On Fri, Mar 04, 2016 at 08:23:09AM +0000, Li, Liang Z wrote: > > > On Thu, Mar 03, 2016 at 05:46:15PM +0000, Dr. David Alan Gilbert wrote: > > > > * Liang Li (liang.z.li@xxxxxxxxx) wrote: > > > > > The current QEMU live migration implementation mark the all the > > > > > guest's RAM pages as dirtied in the ram bulk stage, all these pages > > > > > will be processed and that takes quit a lot of CPU cycles. > > > > > > > > > > From guest's point of view, it doesn't care about the content in > > > > > free pages. We can make use of this fact and skip processing the > > > > > free pages in the ram bulk stage, it can save a lot CPU cycles and > > > > > reduce the network traffic significantly while speed up the live > > > > > migration process obviously. > > > > > > > > > > This patch set is the QEMU side implementation. > > > > > > > > > > The virtio-balloon is extended so that QEMU can get the free pages > > > > > information from the guest through virtio. > > > > > > > > > > After getting the free pages information (a bitmap), QEMU can use it > > > > > to filter out the guest's free pages in the ram bulk stage. This > > > > > make the live migration process much more efficient. > > > > > > > > Hi, > > > > An interesting solution; I know a few different people have been > > > > looking at how to speed up ballooned VM migration. > > > > > > > > I wonder if it would be possible to avoid the kernel changes by > > > > parsing /proc/self/pagemap - if that can be used to detect > > > > unmapped/zero mapped pages in the guest ram, would it achieve the > > > same result? > > > > > > Yes I was about to suggest the same thing: it's simple and makes use of the > > > existing infrastructure. And you wouldn't need to care if the pages were > > > unmapped by ballooning or anything else (alternative balloon > > > implementations, not yet touched by the guest, etc.). Besides, you wouldn't > > > need to synchronize with the guest. > > > > > > Roman. > > > > The unmapped/zero mapped pages can be detected by parsing /proc/self/pagemap, > > but the free pages can't be detected by this. Imaging an application allocates a large amount > > of memory , after using, it frees the memory, then live migration happens. All these free pages > > will be process and sent to the destination, it's not optimal. > > First, the likelihood of such a situation is marginal, there's no point > optimizing for it specifically. > > And second, even if that happens, you inflate the balloon right before > the migration and the free memory will get umapped very quickly, so this > case is covered nicely by the same technique that works for more > realistic cases, too. Although I wonder which is cheaper; that would be fairly expensive for the guest wouldn't it? And you'd somehow have to kick the guest before migration to do the ballooning - and how long would you wait for it to finish? Dave > > Roman. -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK -- 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>