On Thu, Sep 05, 2024 at 07:45:43PM +0300, Avihai Horon wrote: > > Does it also mean then that the currently reported stop-size - precopy-size > > will be very close to the constant non-iterable data size? > > It's not constant, while the VM is running it can change.
I wonder how heavy is VFIO_DEVICE_FEATURE_MIG_DATA_SIZE ioctl. I just gave it a quick shot with a busy VM migrating and estimate() is invoked only every ~100ms. VFIO might be different, but I wonder whether we can fetch stop-size in estimate() somehow, so it's still a pretty fast estimate() meanwhile we avoid the rest of exact() calls (which are destined to be useless without VFIO). IIUC so far the estimate()/exact() was because ram sync is heavy when exact(). When idle it's 80+ms now for 32G VM with current master (which has a bug and I'm fixing it up [1]..), even if after the fix it's 3ms (I think both numbers contain dirty bitmap sync for both vfio and kvm). So in that case maybe we can still try fetching stop-size only for both estimate() and exact(), but only sync bitmap in exact(). [1] https://lore.kernel.org/r/[email protected] Thanks, -- Peter Xu
