On Thu, Mar 10, 2016 at 01:41:16AM +0000, Li, Liang Z wrote: > > > > > > 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. > > > > > > > > > > I am considering to drop the pagecache before getting the free pages. > > > > > > > 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 > > > > > > I didn't say communicating is slow. Even this is very slow, my > > > solution use bitmap instead of PFNs, there is fewer data traffic, so it's > > faster than the existing balloon which use PFNs. > > > > By how much? > > > > Haven't measured yet. > To identify a page, 1 bit is needed if using bitmap, 4 Bytes(32bit) is needed if using PFN, > > For a guest with 8GB RAM, the corresponding free page bitmap size is 256KB. > And the corresponding total PFNs size is 8192KB. Assuming the inflating size > is 7GB, the total PFNs size is 7168KB. Yes but this is not how balloon works, instead, it will reuse a single 4K page multiple times. We can also trade off more memory for speed if we want to, it's completely up to guest. > > Maybe this is not the point. > > Liang > > > > 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, 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>