On Fri, Apr 26, 2024 at 07:35:36PM +0200, Maciej S. Szmigiero wrote: > On 24.04.2024 00:27, Peter Xu wrote: > > On Tue, Apr 23, 2024 at 06:14:18PM +0200, Maciej S. Szmigiero wrote: > > > We don't lose any genericity since by default the transfer is done via > > > mixed RAM / device state multifd channels from a shared pool. > > > > > > It's only when x-multifd-channels-device-state is set to value > 0 then > > > the requested multifd channel counts gets dedicated to device state. > > > > > > It could be seen as a fine-tuning option for cases where tests show that > > > it provides some benefits to the particular workload - just like many > > > other existing migration options are. > > > > > > 14% downtime improvement is too much to waste - I'm not sure that's only > > > due to avoiding RAM syncs, it's possible that there are other subtle > > > performance interactions too. > > > > > > For even more genericity this option could be named like > > > x-multifd-channels-map and contain an array of channel settings like > > > "ram,ram,ram,device-state,device-state". > > > Then a possible future other uses of multifd channels wouldn't even need > > > a new dedicated option. > > > > Yeah I understand such option would only provide more options. > > > > However as long as such option got introduced, user will start to do their > > own "optimizations" on how to provision the multifd channels, and IMHO > > it'll be great if we as developer can be crystal clear on why it needs to > > be introduced in the first place, rather than making all channels open to > > all purposes. > > > > So I don't think I'm strongly against such parameter, but I want to double > > check we really understand what's behind this to justify such parameter. > > Meanwhile I'd be always be pretty caucious on introducing any migration > > parameters, due to the compatibility nightmares. The less parameter the > > better.. > > Ack, I am also curious why dedicated device state multifd channels bring > such downtime improvement. > > > > > > > > > > > > > > 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. > > > > > > Ack, I will rename these hooks to begin/end. > > > > > > > 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. > > > > > > AFAIK there's already just one multifd_send_sync_main() during downtime - > > > the one called from save_live_complete_precopy SaveVMHandler. > > > > > > In order to truly never interfere with device state transfer the sync > > > would > > > need to be ordered after the device state transfer is complete - that is, > > > after VFIO complete_precopy_end (complete_precopy_async_wait) handler > > > returns. > > > > Do you think it'll be worthwhile give it a shot, even if we can't decide > > yet on the order of end()s to be called? > > Upon a closer inspection it looks like that there are, in fact, *two* > RAM syncs done during the downtime - besides the one at the end of > ram_save_complete() there's another on in find_dirty_block(). This function > is called earlier from ram_save_complete() -> ram_find_and_save_block().
Fabiano and I used to discuss this when he's working on the mapped-ram feature, and afaiu the flush in complete() is not needed when the other one existed. I tried to remove it and at least the qtests run all well: @@ -3415,10 +3415,6 @@ static int ram_save_complete(QEMUFile *f, void *opaque) } } - if (migrate_multifd() && !migrate_multifd_flush_after_each_section() && - !migrate_mapped_ram()) { - qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH); - } qemu_put_be64(f, RAM_SAVE_FLAG_EOS); return qemu_fflush(f); } > > Unfortunately, skipping that intermediate sync in find_dirty_block() and > moving the one from the end of ram_save_complete() to another SaveVMHandler > that's called only after VFIO device state transfer doesn't actually > improve downtime (at least not on its own). > > > It'll be great if we could look into these issues instead of workarounds, > > and figure out what affected the performance behind, and also whether that > > can be fixed without such parameter. > > I've been looking at this and added some measurements around device state > queuing for submission in multifd_queue_device_state(). > > To my surprise, the mixed RAM / device state config of 15/0 has *much* > lower total queuing time of around 100 msec compared to the dedicated > device state channels 15/4 config with total queuing time of around > 300 msec. Did it account device only, or device+RAM? I'd expect RAM enqueue time grows in 15/0 due to the sharing with device threads. However even if so it may not be that fair a comparison, as the cpu resources aren't equal. It's fairer if we compare 15/0 (mixed) v.s. 10/5 (dedicated), for example. > > Despite that, the 15/4 config still has significantly lower overall > downtime. > > This means that any reason for the downtime difference is probably on > the receive / load side of the migration rather than on the save / > send side. > > I guess the reason for the lower device state queuing time in the 15/0 > case is that this data could be sent via any of the 15 multifd channels > rather than just the 4 dedicated ones in the 15/4 case. Agree. > > Nevertheless, I will continue to look at this problem to at least find > some explanation for the difference in downtime that dedicated device > state multifd channels bring. Thanks for looking at this. -- Peter Xu