On Wed, Jun 27, 2018 at 01:06:32PM +0200, David Hildenbrand wrote: > On 25.06.2018 14:05, Wei Wang wrote: > > This patch series is separated from the previous "Virtio-balloon > > Enhancement" series. The new feature, VIRTIO_BALLOON_F_FREE_PAGE_HINT, > > implemented by this series enables the virtio-balloon driver to report > > hints of guest free pages to the host. It can be used to accelerate live > > migration of VMs. Here is an introduction of this usage: > > > > Live migration needs to transfer the VM's memory from the source machine > > to the destination round by round. For the 1st round, all the VM's memory > > is transferred. From the 2nd round, only the pieces of memory that were > > written by the guest (after the 1st round) are transferred. One method > > that is popularly used by the hypervisor to track which part of memory is > > written is to write-protect all the guest memory. > > > > This feature enables the optimization by skipping the transfer of guest > > free pages during VM live migration. It is not concerned that the memory > > pages are used after they are given to the hypervisor as a hint of the > > free pages, because they will be tracked by the hypervisor and transferred > > in the subsequent round if they are used and written. > > > > * Tests > > - 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 = 284ms vs 1757ms --> ~84% reduction > > - Guest with Linux Compilation Workload (make bzImage -j4): > > - Live Migration Time (average) > > Optimization v.s. Legacy = 1402ms v.s. 2528ms --> ~44% reduction > > - Linux Compilation Time > > Optimization v.s. Legacy = 5min6s v.s. 5min12s > > --> no obvious difference > > > > Being in version 34 already, this whole thing still looks and feels like > a big hack to me. It might just be me, but especially if I read about > assumptions like "QEMU will not hotplug memory during migration". This > does not feel like a clean solution. > > I am still not sure if we really need this interface, especially as real > free page hinting might be on its way. > > a) we perform free page hinting by setting all free pages > (arch_free_page()) to zero. Migration will detect zero pages and > minimize #pages to migrate. I don't think this is a good idea but Michel > suggested to do a performance evaluation and Nitesh is looking into that > right now. Yes this test is needed I think. If we can get most of the benefit without PV interfaces, that's nice. Wei, I think you need this as part of your performance comparison too: set page poisoning value to 0 and enable KSM, compare with your patches. > b) we perform free page hinting using something that Nitesh proposed. We > get in QEMU blocks of free pages that we can MADV_FREE. In addition we > could e.g. clear the dirty bit of these pages in the dirty bitmap, to > hinder them from getting migrated. Right now the hinting mechanism is > synchronous (called from arch_free_page()) but we might be able to > convert it into something asynchronous. > > So we might be able to completely get rid of this interface. The way I see it, hinting during alloc/free will always add overhead which might be unacceptable for some people. So even with Nitesh's patches there's value in enabling / disabling hinting dynamically. And Wei's patches would then be useful to set the stage where we know the initial page state. > And looking at all the discussions and problems that already happened > during the development of this series, I think we should rather look > into how clean free page hinting might solve the same problem. I'm not sure I follow the logic. We found that neat tricks especially re-using the max order free page for reporting. > If it can't be solved using free page hinting, fair enough. I suspect Nitesh will need to find a way not to have mm code call out to random drivers or subsystems before that code is acceptable. -- MST
