On 29.04.2024 17:09, Peter Xu wrote:
On Fri, Apr 26, 2024 at 07:34:09PM +0200, Maciej S. Szmigiero wrote:
On 24.04.2024 00:35, Peter Xu wrote:
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 think any improvements to the migration performance are good, even if
they don't impact downtime.
It's just that this patch set focuses on the downtime phase as the more
critical thing.
After this gets improved there's no reason why not to look at improving
performance of the VM live phase too if it brings sensible improvements.
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.
The device state transfer is indicated by a new flag in the multifd
header (MULTIFD_FLAG_DEVICE_STATE).
If we are to use multifd channels for VM live phase transfers these
could simply re-use the same flag type.
Right, and that's also my major purpose of such request to consider both
issues.
If supporting iterators can be easy on top of this, I am thinking whether
we should do this in one shot. The problem is even if the flag type can be
reused, old/new qemu binaries may not be compatible and may not migrate
well when:
- The old qemu only supports the downtime optimizations
- The new qemu supports both downtime + iteration optimizations
I think the situation here will be the same as with any new flag
affecting the migration wire protocol - if the old version of QEMU
doesn't support that flag then it has to be kept at its backward-compatible
setting for migration to succeed.
IIUC, at least the device threads are currently created only at the end of
migration when switching over for the downtime-only optimization (aka, this
series). Then it means it won't be compatible with a new QEMU as the
threads there will need to be created before iteration starts to take
iteration data. So I believe we'll need yet another flag to tune the
behavior of such, one for each optimizations (downtime v.s. data during
iterations). If they work mostly similarly, I want to avoid two flags.
It'll be chaos for user to see such similar flags and they'll be pretty
confusing.
The VFIO loading threads are created from vfio_load_setup(), which is
called at the very beginning of the migration, so they should be already
there.
However, they aren't currently prepared to receive VM live phase data.
If possible, I wish we can spend some time looking into that if they're so
close, and if it's low hanging fruit when on top of this series, maybe we
can consider doing that in one shot.
I'm still trying to figure out the complete explanation why dedicated
device state channels improve downtime as there was a bunch of holidays
last week here.
I will have a look later what would it take to add VM live phase multifd
device state transfer support and also how invasive it would be as I
think it's better to keep the number of code conflicts in a patch set
to a manageable size as it reduces the chance of accidentally
introducing regressions when forward-porting the patch set to the git master.
Thanks,
Thanks,
Maciej
