> Subject: Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration > optimization > > On Fri, Mar 04, 2016 at 09:08:44AM +0000, Li, Liang Z wrote: > > > On Fri, Mar 04, 2016 at 01:52:53AM +0000, Li, Liang Z wrote: > > > > > 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? > > > > > > > > Only detect the unmapped/zero mapped pages is not enough. > Consider > > > the > > > > situation like case 2, it can't achieve the same result. > > > > > > Your case 2 doesn't exist in the real world. If people could stop > > > their main memory consumer in the guest prior to migration they > > > wouldn't need live migration at all. > > > > The case 2 is just a simplified scenario, not a real case. > > As long as the guest's memory usage does not keep increasing, or not > > always run out, it can be covered by the case 2. > > The memory usage will keep increasing due to ever growing caches, etc, so > you'll be left with very little free memory fairly soon. > I don't think so. > > > I tend to think you can safely assume there's no free memory in the > > > guest, so there's little point optimizing for it. > > > > If this is true, we should not inflate the balloon either. > > We certainly should if there's "available" memory, i.e. not free but cheap to > reclaim. > What's your mean by "available" memory? if they are not free, I don't think it's cheap. > > > OTOH it makes perfect sense optimizing for the unmapped memory > > > that's made up, in particular, by the ballon, and consider inflating > > > the balloon right before migration unless you already maintain it at > > > the optimal size for other reasons (like e.g. a global resource manager > optimizing the VM density). > > > > > > > Yes, I believe the current balloon works and it's simple. Do you take the > performance impact for consideration? > > For and 8G guest, it takes about 5s to inflating the balloon. But it > > only takes 20ms to traverse the free_list and construct the free pages > bitmap. > > I don't have any feeling of how important the difference is. And if the > limiting factor for balloon inflation speed is the granularity of communication > it may be worth optimizing that, because quick balloon reaction may be > important in certain resource management scenarios. > > > By inflating the balloon, all the guest's pages are still be processed (zero > page checking). > > Not sure what you mean. If you describe the current state of affairs that's > exactly the suggested optimization point: skip unmapped pages. > You'd better check the live migration code. > > The only advantage of ' inflating the balloon before live migration' is simple, > nothing more. > > That's a big advantage. Another one is that it does something useful in real- > world scenarios. > I don't think the heave performance impaction is something useful in real world scenarios. Liang > Roman. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
