Re: [PATCH-for-9.1 v2 2/3] migration: Remove RDMA protocol handling

[Michael Galaxy]

Yu Zhang / Jinpu,

Any possibility (at your lesiure, and within the disclosure rules of 
your company, IONOS) if you could share any of your performance 
information to educate the group?

NICs have indeed changed, but not everybody has 100ge mellanox cards at 
their disposal. Some people don't.

- Michael

On 5/1/24 11:16, Peter Xu wrote:
> On Wed, May 01, 2024 at 04:59:38PM +0100, Daniel P. Berrangé wrote:
> > On Wed, May 01, 2024 at 11:31:13AM -0400, Peter Xu wrote:
> > > What I worry more is whether this is really what we want to keep rdma in
> > > qemu, and that's also why I was trying to request for some serious
> > > performance measurements comparing rdma v.s. nics.  And here when I said
> > > "we" I mean both QEMU community and any company that will support keeping
> > > rdma around.
> > >
> > > The problem is if NICs now are fast enough to perform at least equally
> > > against rdma, and if it has a lower cost of overall maintenance, does it
> > > mean that rdma migration will only be used by whoever wants to keep them in
> > > the products and existed already?  In that case we should simply ask new
> > > users to stick with tcp, and rdma users should only drop but not increase.
> > >
> > > It seems also destined that most new migration features will not support
> > > rdma: see how much we drop old features in migration now (which rdma
> > > _might_ still leverage, but maybe not), and how much we add mostly multifd
> > > relevant which will probably not apply to rdma at all.  So in general what
> > > I am worrying is a both-loss condition, if the company might be easier to
> > > either stick with an old qemu (depending on whether other new features are
> > > requested to be used besides RDMA alone), or do periodic rebase with RDMA
> > > downstream only.
> > I don't know much about the originals of RDMA support in QEMU and why
> > this particular design was taken. It is indeed a huge maint burden to
> > have a completely different code flow for RDMA with 4000+ lines of
> > custom protocol signalling which is barely understandable.
> >
> > I would note that /usr/include/rdma/rsocket.h provides a higher level
> > API that is a 1-1 match of the normal kernel 'sockets' API. If we had
> > leveraged that, then QIOChannelSocket class and the QAPI SocketAddress
> > type could almost[1] trivially have supported RDMA. There would have
> > been almost no RDMA code required in the migration subsystem, and all
> > the modern features like compression, multifd, post-copy, etc would
> > "just work".
> >
> > I guess the 'rsocket.h' shim may well limit some of the possible
> > performance gains, but it might still have been a better tradeoff
> > to have not quite so good peak performance, but with massively
> > less maint burden.
> My understanding so far is RDMA is sololy for performance but nothing else,
> then it's a question on whether rdma existing users would like to do so if
> it will run slower.
>
> Jinpu mentioned on the explicit usages of ib verbs but I am just mostly
> quotting that word as I don't really know such details:
>
> https://urldefense.com/v3/__https://lore.kernel.org/qemu-devel/CAMGffEm2TWJxOPcNQTQ1Sjytf5395dBzTCMYiKRqfxDzJwSN6A@mail.gmail.com/__;!!GjvTz_vk!W6-HGWM-XkF_52am249DrLIDQeZctVOHg72LvOHGUcwxqQM5mY0GNYYl-yNJslN7A5GfLOew9oW_kg$
>
> So not sure whether that applies here too, in that having qiochannel
> wrapper may not allow direct access to those ib verbs.
>
> Thanks,
>
> > With regards,
> > Daniel
> >
> > [1] "almost" trivially, because the poll() integration for rsockets
> >      requires a bit more magic sauce since rsockets FDs are not
> >      really FDs from the kernel's POV. Still, QIOCHannel likely can
> >      abstract that probme.
> > --
> > |: 
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> >        -o-    
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> >   :|
> > |: 
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> >           -o-            
> > https://urldefense.com/v3/__https://fstop138.berrange.com__;!!GjvTz_vk!W6-HGWM-XkF_52am249DrLIDQeZctVOHg72LvOHGUcwxqQM5mY0GNYYl-yNJslN7A5GfLOfPMofYqw$
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> > |: 
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> >   :|