* Wei Wang (wei.w.wang@xxxxxxxxx) wrote: > On 07/23/2018 10:36 PM, Dr. David Alan Gilbert wrote: > > * Michael S. Tsirkin (mst@xxxxxxxxxx) wrote: > > > On Fri, Jul 20, 2018 at 04:33:00PM +0800, 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 = 409ms vs 1757ms --> ~77% reduction > > > > (setting page poisoning zero and enabling ksm don't affect the > > > > comparison result) > > > > - Guest with Linux Compilation Workload (make bzImage -j4): > > > > - Live Migration Time (average) > > > > Optimization v.s. Legacy = 1407ms v.s. 2528ms --> ~44% reduction > > > > - Linux Compilation Time > > > > Optimization v.s. Legacy = 5min4s v.s. 5min12s > > > > --> no obvious difference > > > I'd like to see dgilbert's take on whether this kind of gain > > > justifies adding a PV interfaces, and what kind of guest workload > > > is appropriate. > > > > > > Cc'd. > > Well, 44% is great ... although the measurement is a bit weird. > > > > a) A 2 second downtime is very large; 300-500ms is more normal > > b) I'm not sure what the 'average' is - is that just between a bunch of > > repeated migrations? > > c) What load was running in the guest during the live migration? > > > > An interesting measurement to add would be to do the same test but > > with a VM with a lot more RAM but the same load; you'd hope the gain > > would be even better. > > It would be interesting, especially because the users who are interested > > are people creating VMs allocated with lots of extra memory (for the > > worst case) but most of the time migrating when it's fairly idle. > > > > Dave > > > > Hi Dave, > > The results of the added experiments have been shown in the v37 cover > letter. > Could you have a look at https://lkml.org/lkml/2018/8/27/29 . Thanks. OK, that's much better. The ~50% reducton with a 8G VM and a real workload is great, and it does what you expect when you put a lot more RAM in and see the 84% reduction on a guest with 128G RAM - 54s vs ~9s is a big win! (The migrate_set_speed is a bit high, since that's in bytes/s - but it's not important). That looks good, Thanks! Dave > Best, > Wei > -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
