Re: [PATCH RFC 00/26] Multifd 🔀 device state transfer support with VFIO consumer

[Peter Xu <peterx@redhat.com>]

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