On Sat, Sep 26, 2020 at 4:50 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > On 25/09/20 19:30, Ben Gardon wrote: > > Oh, thank you for explaining that. I didn't realize the goal here was > > to improve LM performance. I was under the impression that this was to > > give VMs a better experience on startup for fast scaling or something. > > In your testing with live migration how has this affected the > > distribution of time between the phases of live migration? Just for > > terminology (since I'm not sure how standard it is across the > > industry) I think of a live migration as consisting of 3 stages: > > precopy, blackout, and postcopy. In precopy we're tracking the VM's > > working set via dirty logging and sending the contents of its memory > > to the target host. In blackout we pause the vCPUs on the source, copy > > minimal data to the target, and resume the vCPUs on the target. In > > postcopy we may still have some pages that have not been copied to the > > target and so request those in response to vCPU page faults via user > > fault fd or some other mechanism. > > > > Does EPT pre-population preclude the use of a postcopy phase? > > I think so. > > As a quick recap, turn postcopy migration handles two kinds of > pages---they can be copied to the destination either in background > (stuff that was dirty when userspace decided to transition to the > blackout phase) or on-demand (relayed from KVM to userspace via > get_user_pages and userfaultfd). Normally only on-demand pages would be > served through userfaultfd, while with prepopulation every missing page > would be faulted in from the kernel through userfaultfd. In practice > this would just extend the blackout phase. > > Paolo > Yep, you are right, based on current implementation it doesn't support the postcopy. Thanks for the suggestion, we will try to fill the gap with proper EPT population during the post-copy. > > I would > > expect that to make the blackout phase really long. Has that not been > > a problem for you? > > > > I love the idea of partial EPT pre-population during precopy if you > > could still handle postcopy and just pre-populate as memory came in. > > >
