Re: [RFC PATCH v1 1/1] migration: Disable postcopy + multifd migration

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Mar 30, 2023 at 10:36:11AM -0400, Peter Xu wrote:
> On Thu, Mar 30, 2023 at 03:20:14PM +0100, Daniel P. Berrangé wrote:
> > On Mon, Mar 27, 2023 at 01:15:18PM -0300, Leonardo Bras wrote:
> > > Since the introduction of multifd, it's possible to perform a multifd
> > > migration and finish it using postcopy.
> > > 
> > > A bug introduced by yank (fixed on cfc3bcf373) was previously preventing
> > > a successful use of this migration scenario, and now it should be
> > > working on most cases.
> > > 
> > > But since there is not enough testing/support nor any reported users for
> > > this scenario, we should disable this combination before it may cause any
> > > problems for users.
> > 
> > Clearly we don't have enough testing, but multifd+postcopy looks
> > like a clearly useful scenario that we should be supporting.
> > 
> > Every post-copy starts with at least one pre-copy iteration, and
> > using multifd for that will be important for big VMs where single
> > threaded pre-copy is going to be CPU bound. The greater amount we
> > can transfer in the pre-copy phase, the less page faults / latency
> > spikes postcopy is going to see.
> 
> If we're using 1-round precopy + postcopy approach, the amount of memory
> will be the same which is the guest mem size.
> 
> Multifd will make the round shorter so more chance of getting less
> re-dirtied pages during the iteration, but that effect is limited.  E.g.:
> 
>   - For a very idle guest, finishing 1st round in 1min or 3min may not
>     bring a large difference because most of the pages will be constant
>     anyway, or
> 
>   - For a very busy guest, probably similar amount of pages will be dirtied
>     no matter in 1min / 3min.  Multifd will bring a benefit here, but
>     busier the guest smaller the effect.

I don't feel like that follows. If we're bottlenecking mostly on CPU
but have sufficient network bandwidth, then multifd can be the difference
between needing to switch to post-copy or being successful in converging
in pre-copy.

IOW, without multifd we can expect 90% of guests will get stuck and need
a switch to post-copy, but with multifd 90% of the guest will complete
while in precopy mode and only 10% need switch to post-copy. That's good
because it means most guests will avoid the increased failure risk and
the period of increased page fault latency from post-copy.


> > In terms of migration usage, my personal recommendation to mgmt
> > apps would be that they should always enable the post-copy feature
> > when starting a migration. Even if they expect to try to get it to
> > complete using exclusively pre-copy in the common case, its useful
> > to have post-copy capability flag enabled, as a get out of jail
> > free card. ie if migration ends up getting stuck in non-convergance,
> > or they have a sudden need to urgently complete the migration it is
> > good to be able to flip to post-copy mode.
> 
> I fully agree.
> 
> It should not need to be enabled only if not capable, e.g., the dest host
> may not have privilege to initiate the userfaultfd (since QEMU postcopy
> requires kernel fault traps, so it's very likely).

Sure, the mgmt app (libvirt) should be checking support for userfaultfd
on both sides before permitting / trying to enable the feature.


> > I'd suggest that we instead add a multifd+postcopy test case to
> > migration-test.c and tackle any bugs it exposes. By blocking it
> > unconditionally we ensure no one will exercise it to expose any
> > further bugs.
> 
> That's doable.  But then we'd better also figure out how to identify the
> below two use cases of both features enabled:
> 
>   a. Enable multifd in precopy only, then switch to postcopy (currently
>   mostly working but buggy; I think Juan can provide more information here,
>   at least we need to rework multifd flush when switching, and test and
>   test over to make sure there's nothing else missing).
> 
>   b. Enable multifd in both precopy and postcopy phase (currently
>   definitely not supported)
> 
> So that mgmt app will be aware whether multifd will be enabled in postcopy
> or not.  Currently we can't identify it.
> 
> I assume we can say by default "mutlifd+postcopy" means a) above, but we
> need to document it, and when b) is wanted and implemented someday, we'll
> need some other flag/cap for it.

As I've mentioned a few times, I think we need to throw away the idea
of exposing capabilities that mgmt apps need to learn about, and make
the migration protocol fully bi-directional so src + dst QEMU can
directly negotiate features. Apps shouldn't have to care about the
day-to-day improvements in the migration impl to the extent that they
are today.

With regards,
Daniel
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