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? Is it possible to extend that use case too? Thanks, -- Peter Xu