On Fri, Jun 01, 2018 at 12:58:24PM +0800, Peter Xu wrote: > On Tue, Apr 24, 2018 at 02:13:43PM +0800, Wei Wang wrote: > > This is the deivce part implementation to add a new feature, > > VIRTIO_BALLOON_F_FREE_PAGE_HINT to the virtio-balloon device. The device > > receives the guest free page hints from the driver and clears the > > corresponding bits in the dirty bitmap, so that those free pages are > > not transferred by the migration thread to the destination. > > > > - Test Environment > > Host: Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz > > Guest: 8G RAM, 4 vCPU > > Migration setup: migrate_set_speed 100G, migrate_set_downtime 2 second > > > > - Test Results > > - Idle Guest Live Migration Time (results are averaged over 10 runs): > > - Optimization v.s. Legacy = 271ms vs 1769ms --> ~86% reduction > > - Guest with Linux Compilation Workload (make bzImage -j4): > > - Live Migration Time (average) > > Optimization v.s. Legacy = 1265ms v.s. 2634ms --> ~51% reduction > > - Linux Compilation Time > > Optimization v.s. Legacy = 4min56s v.s. 5min3s > > --> no obvious difference > > > > - Source Code > > - QEMU: https://github.com/wei-w-wang/qemu-free-page-lm.git > > - Linux: https://github.com/wei-w-wang/linux-free-page-lm.git > > Hi, Wei, > > I have a very high-level question to the series. > > IIUC the core idea for this series is that we can avoid sending some > of the pages if we know that we don't need to send them. I think this > is based on the fact that on the destination side all the pages are by > default zero after they are malloced. While before this series, IIUC > any migration will send every single page to destination, no matter > whether it's zeroed or not. So I'm uncertain about whether this will > affect the received bitmap on the destination side. Say, before this > series, the received bitmap will directly cover the whole RAM bitmap > after migration is finished, now it's won't. Will there be any side > effect? I don't see obvious issue now, but just raise this question > up. > > Meanwhile, this reminds me about a more funny idea: whether we can > just avoid sending the zero pages directly from QEMU's perspective. > In other words, can we just do nothing if save_zero_page() detected > that the page is zero (I guess the is_zero_range() can be fast too, > but I don't know exactly how fast it is)? And how that would be > differed from this page hinting way in either performance and other > aspects.
I noticed a problem (after I wrote the above paragraph 5 minutes ago...): when a page was valid and sent to the destination (with non-zero data), however after a while that page was zeroed. Then if we don't send zero pages at all, we won't send the page after it's zeroed. Then on the destination side we'll have a stale non-zero page. Is my understanding correct? Will that be a problem to this series too where a valid page can be possibly freed and hinted? > > I haven't digged into the kernel patches yet so I have totally no idea > on the detailed implementation of the page hinting. Please feel free > to correct me if there is obvious misunderstandings. > > Regards, > > -- > Peter Xu -- Peter Xu
