On Mon, Mar 07, 2016 at 01:40:06PM +0200, Michael S. Tsirkin wrote: > On Mon, Mar 07, 2016 at 06:49:19AM +0000, Li, Liang Z wrote: > > > > No. And it's exactly what I mean. The ballooned memory is still > > > > processed during live migration without skipping. The live migration code is > > > in migration/ram.c. > > > > > > So if guest acknowledged VIRTIO_BALLOON_F_MUST_TELL_HOST, we can > > > teach qemu to skip these pages. > > > Want to write a patch to do this? > > > > > > > Yes, we really can teach qemu to skip these pages and it's not hard. > > The problem is the poor performance, this PV solution > > Balloon is always PV. And do not call patches solutions please. > > > is aimed to make it more > > efficient and reduce the performance impact on guest. > > We need to get a bit beyond this. You are making multiple > changes, it seems to make sense to split it all up, and analyse each > change separately. Couldn't agree more. There are three stages in this optimization: 1) choosing which pages to skip 2) communicating them from guest to host 3) skip transferring uninteresting pages to the remote side on migration For (3) there seems to be a low-hanging fruit to amend migration/ram.c:iz_zero_range() to consult /proc/self/pagemap. This would work for guest RAM that hasn't been touched yet or which has been ballooned out. For (1) I've been trying to make a point that skipping clean pages is much more likely to result in noticable benefit than free pages only. As for (2), we do seem to have a problem with the existing balloon: according to your measurements it's very slow; besides, I guess it plays badly with transparent huge pages (as both the guest and the host work with one 4k page at a time). This is a problem for other use cases of balloon (e.g. as a facility for resource management); tackling that appears a more natural application for optimization efforts. Thanks, Roman. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
