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

[Markus Armbruster <armbru@redhat.com>]

Peter Xu <peterx@redhat.com> writes:

> On Mon, Apr 08, 2024 at 04:07:20PM +0200, Jinpu Wang wrote:
>> Hi Peter,
>
> Jinpu,
>
> Thanks for joining the discussion.
>
>> 
>> On Tue, Apr 2, 2024 at 11:24 PM Peter Xu <peterx@redhat.com> wrote:
>> >
>> > On Mon, Apr 01, 2024 at 11:26:25PM +0200, Yu Zhang wrote:
>> > > Hello Peter und Zhjian,
>> > >
>> > > Thank you so much for letting me know about this. I'm also a bit surprised at
>> > > the plan for deprecating the RDMA migration subsystem.
>> >
>> > It's not too late, since it looks like we do have users not yet notified
>> > from this, we'll redo the deprecation procedure even if it'll be the final
>> > plan, and it'll be 2 releases after this.

[...]

>> > Per our best knowledge, RDMA users are rare, and please let anyone know if
>> > you are aware of such users.  IIUC the major reason why RDMA stopped being
>> > the trend is because the network is not like ten years ago; I don't think I
>> > have good knowledge in RDMA at all nor network, but my understanding is
>> > it's pretty easy to fetch modern NIC to outperform RDMAs, then it may make
>> > little sense to maintain multiple protocols, considering RDMA migration
>> > code is so special so that it has the most custom code comparing to other
>> > protocols.
>> +cc some guys from Huawei.
>> 
>> I'm surprised RDMA users are rare,  I guess maybe many are just
>> working with different code base.
>
> Yes, please cc whoever might be interested (or surprised.. :) to know this,
> and let's be open to all possibilities.
>
> I don't think it makes sense if there're a lot of users of a feature then
> we deprecate that without a good reason.  However there's always the
> resource limitation issue we're facing, so it could still have the
> possibility that this gets deprecated if nobody is working on our upstream
> branch. Say, if people use private branches anyway to support rdma without
> collaborating upstream, keeping such feature upstream then may not make
> much sense either, unless there's some way to collaborate.  We'll see.
>
> It seems there can still be people joining this discussion.  I'll hold off
> a bit on merging this patch to provide enough window for anyone to chim in.

Users are not enough.  Only maintainers are.

At some point, people cared enough about RDMA in QEMU to contribute the
code.  That's why have the code.

To keep the code, we need people who care enough about RDMA in QEMU to
maintain it.  Without such people, the case for keeping it remains
dangerously weak, and no amount of talk or even benchmarks can change
that.