One thing to keep in mind here (despite me not having any hardware to
test) was that one of the original goals here
in the RDMA implementation was not simply raw throughput nor raw
latency, but a lack of CPU utilization in kernel
space due to the offload. While it is entirely possible that newer
hardware w/ TCP might compete, the significant
reductions in CPU usage in the TCP/IP stack were a big win at the time.
Just something to consider while you're doing the testing........
- Michael
On 5/9/24 03:58, Zheng Chuan wrote:
Hi, Peter，Lei，Jinpu.
On 2024/5/8 0:28, Peter Xu wrote:
On Tue, May 07, 2024 at 01:50:43AM +0000, Gonglei (Arei) wrote:
Hello,
-----Original Message-----
From: Peter Xu [mailto:peterx@xxxxxxxxxx]
Sent: Monday, May 6, 2024 11:18 PM
To: Gonglei (Arei) <arei.gonglei@xxxxxxxxxx>
Cc: Daniel P. Berrangé <berrange@xxxxxxxxxx>; Markus Armbruster
<armbru@xxxxxxxxxx>; Michael Galaxy <mgalaxy@xxxxxxxxxx>; Yu Zhang
<yu.zhang@xxxxxxxxx>; Zhijian Li (Fujitsu) <lizhijian@xxxxxxxxxxx>; Jinpu Wang
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Xiexiangyou <xiexiangyou@xxxxxxxxxx>
Subject: Re: [PATCH-for-9.1 v2 2/3] migration: Remove RDMA protocol handling
On Mon, May 06, 2024 at 02:06:28AM +0000, Gonglei (Arei) wrote:
Hi, Peter
Hey, Lei,
Happy to see you around again after years.
Haha, me too.
RDMA features high bandwidth, low latency (in non-blocking lossless
network), and direct remote memory access by bypassing the CPU (As you
know, CPU resources are expensive for cloud vendors, which is one of
the reasons why we introduced offload cards.), which TCP does not have.
It's another cost to use offload cards, v.s. preparing more cpu resources?
Software and hardware offload converged architecture is the way to go for all cloud vendors
(Including comprehensive benefits in terms of performance, cost, security, and innovation speed),
it's not just a matter of adding the resource of a DPU card.
In some scenarios where fast live migration is needed (extremely short
interruption duration and migration duration) is very useful. To this
end, we have also developed RDMA support for multifd.
Will any of you upstream that work? I'm curious how intrusive would it be
when adding it to multifd, if it can keep only 5 exported functions like what
rdma.h does right now it'll be pretty nice. We also want to make sure it works
with arbitrary sized loads and buffers, e.g. vfio is considering to add IO loads to
multifd channels too.
In fact, we sent the patchset to the community in 2021. Pls see:
https://urldefense.com/v3/__https://lore.kernel.org/all/20210203185906.GT2950@work-vm/T/__;!!GjvTz_vk!VfP_SV-8uRya7rBdopv8OUJkmnSi44Ktpqq1E7sr_Xcwt6zvveW51qboWOBSTChdUG1hJwfAl7HZl4NUEGc$
Yes, I have sent the patchset of multifd support for rdma migration by taking over my colleague, and also
sorry for not keeping on this work at that time due to some reasons.
And also I am strongly agree with Lei that the RDMA protocol has some special advantages against with TCP
in some scenario, and we are indeed to use it in our product.
I wasn't aware of that for sure in the past..
Multifd has changed quite a bit in the last 9.0 release, that may not apply
anymore. One thing to mention is please look at Dan's comment on possible
use of rsocket.h:
https://urldefense.com/v3/__https://lore.kernel.org/all/ZjJm6rcqS5EhoKgK@xxxxxxxxxx/__;!!GjvTz_vk!VfP_SV-8uRya7rBdopv8OUJkmnSi44Ktpqq1E7sr_Xcwt6zvveW51qboWOBSTChdUG1hJwfAl7HZ0CFSE-o$
And Jinpu did help provide an initial test result over the library:
https://urldefense.com/v3/__https://lore.kernel.org/qemu-devel/CAMGffEk8wiKNQmoUYxcaTHGtiEm2dwoCF_W7T0vMcD-i30tUkA@xxxxxxxxxxxxxx/__;!!GjvTz_vk!VfP_SV-8uRya7rBdopv8OUJkmnSi44Ktpqq1E7sr_Xcwt6zvveW51qboWOBSTChdUG1hJwfAl7HZxPNcdb4$
It looks like we have a chance to apply that in QEMU.
One thing to note that the question here is not about a pure performance
comparison between rdma and nics only. It's about help us make a decision
on whether to drop rdma, iow, even if rdma performs well, the community still
has the right to drop it if nobody can actively work and maintain it.
It's just that if nics can perform as good it's more a reason to drop, unless
companies can help to provide good support and work together.
We are happy to provide the necessary review and maintenance work for RDMA
if the community needs it.
CC'ing Chuan Zheng.
I'm not sure whether you and Jinpu's team would like to work together and
provide a final solution for rdma over multifd. It could be much simpler
than the original 2021 proposal if the rsocket API will work out.
Thanks,
That's a good news to see the socket abstraction for RDMA!
When I was developed the series above, the most pain is the RDMA migration has no QIOChannel abstraction and i need to take a 'fake channel'
for it which is awkward in code implementation.
So, as far as I know, we can do this by
i. the first thing is that we need to evaluate the rsocket is good enough to satisfy our QIOChannel fundamental abstraction
ii. if it works right, then we will continue to see if it can give us opportunity to hide the detail of rdma protocol
into rsocket by remove most of code in rdma.c and also some hack in migration main process.
iii. implement the advanced features like multi-fd and multi-uri for rdma migration.
Since I am not familiar with rsocket, I need some times to look at it and do some quick verify with rdma migration based on rsocket.
But, yes, I am willing to involved in this refactor work and to see if we can make this migration feature more better:）
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