On Wed, Apr 24, 2024 at 12:25:08AM +0200, Maciej S. Szmigiero wrote: > On 24.04.2024 00:20, Peter Xu wrote: > > On Tue, Apr 23, 2024 at 06:15:35PM +0200, Maciej S. Szmigiero wrote: > > > On 19.04.2024 17:31, Peter Xu wrote: > > > > On Fri, Apr 19, 2024 at 11:07:21AM +0100, Daniel P. BerrangΓ© wrote: > > > > > On Thu, Apr 18, 2024 at 04:02:49PM -0400, Peter Xu wrote: > > > > > > On Thu, Apr 18, 2024 at 08:14:15PM +0200, Maciej S. Szmigiero wrote: > > > > > > > I think one of the reasons for these results is that mixed (RAM + > > > > > > > device > > > > > > > state) multifd channels participate in the RAM sync process > > > > > > > (MULTIFD_FLAG_SYNC) whereas device state dedicated channels don't. > > > > > > > > > > > > Firstly, I'm wondering whether we can have better names for these > > > > > > new > > > > > > hooks. Currently (only comment on the async* stuff): > > > > > > > > > > > > - complete_precopy_async > > > > > > - complete_precopy > > > > > > - complete_precopy_async_wait > > > > > > > > > > > > But perhaps better: > > > > > > > > > > > > - complete_precopy_begin > > > > > > - complete_precopy > > > > > > - complete_precopy_end > > > > > > > > > > > > ? > > > > > > > > > > > > As I don't see why the device must do something with async in such > > > > > > hook. > > > > > > To me it's more like you're splitting one process into multiple, > > > > > > then > > > > > > begin/end sounds more generic. > > > > > > > > > > > > Then, if with that in mind, IIUC we can already split > > > > > > ram_save_complete() > > > > > > into >1 phases too. For example, I would be curious whether the > > > > > > performance > > > > > > will go back to normal if we offloading multifd_send_sync_main() > > > > > > into the > > > > > > complete_precopy_end(), because we really only need one shot of > > > > > > that, and I > > > > > > am quite surprised it already greatly affects VFIO dumping its own > > > > > > things. > > > > > > > > > > > > I would even ask one step further as what Dan was asking: have you > > > > > > thought > > > > > > about dumping VFIO states via multifd even during iterations? > > > > > > Would that > > > > > > help even more than this series (which IIUC only helps during the > > > > > > blackout > > > > > > phase)? > > > > > > > > > > To dump during RAM iteration, the VFIO device will need to have > > > > > dirty tracking and iterate on its state, because the guest CPUs > > > > > will still be running potentially changing VFIO state. That seems > > > > > impractical in the general case. > > > > > > > > We already do such interations in vfio_save_iterate()? > > > > > > > > My understanding is the recent VFIO work is based on the fact that the > > > > VFIO > > > > device can track device state changes more or less (besides being able > > > > to > > > > save/load full states). E.g. I still remember in our QE tests some old > > > > devices report much more dirty pages than expected during the iterations > > > > when we were looking into such issue that a huge amount of dirty pages > > > > reported. But newer models seem to have fixed that and report much > > > > less. > > > > > > > > That issue was about GPU not NICs, though, and IIUC a major portion of > > > > such > > > > tracking used to be for GPU vRAMs. So maybe I was mixing up these, and > > > > maybe they work differently. > > > > > > The device which this series was developed against (Mellanox ConnectX-7) > > > is already transferring its live state before the VM gets stopped (via > > > save_live_iterate SaveVMHandler). > > > > > > It's just that in addition to the live state it has more than 400 MiB > > > of state that cannot be transferred while the VM is still running. > > > And that fact hurts a lot with respect to the migration downtime. > > > > > > AFAIK it's a very similar story for (some) GPUs. > > > > So during iteration phase VFIO cannot yet leverage the multifd channels > > when with this series, am I right? > > That's right. > > > Is it possible to extend that use case too? > > I guess so, but since this phase (iteration while the VM is still > running)Β doesn't impact downtime it is much less critical.
But it affects the bandwidth, e.g. even with multifd enabled, the device iteration data will still bottleneck at ~15Gbps on a common system setup the best case, even if the hosts are 100Gbps direct connected. Would that be a concern in the future too, or it's known problem and it won't be fixed anyway? I remember Avihai used to have plan to look into similar issues, I hope this is exactly what he is looking for. Otherwise changing migration protocol from time to time is cumbersome; we always need to provide a flag to make sure old systems migrates in the old ways, new systems run the new ways, and for such a relatively major change I'd want to double check on how far away we can support offload VFIO iterations data to multifd. Thanks, -- Peter Xu
