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

[Jinpu Wang]

Hi Peter,

On Mon, Apr 8, 2024 at 6:18 PM Peter Xu <peterx@redhat.com> wrote:
>
> 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.
> > >
> > > >
> > > > > IMHO it's more important to know whether there are still users and 
> > > > > whether
> > > > > they would still like to see it around.
> > > >
> > > > > I admit RDMA migration was lack of testing(unit/CI test), which led 
> > > > > to the a few
> > > > > obvious bugs being noticed too late.
> > > >
> > > > Yes, we are a user of this subsystem. I was unaware of the lack of test 
> > > > coverage
> > > > for this part. As soon as 8.2 was released, I saw that many of the
> > > > migration test
> > > > cases failed and came to realize that there might be a bug between 8.1
> > > > and 8.2, but
> > > > was unable to confirm and report it quickly to you.
> > > >
> > > > The maintenance of this part could be too costly or difficult from
> > > > your point of view.
> > >
> > > It may or may not be too costly, it's just that we need real users of RDMA
> > > taking some care of it.  Having it broken easily for >1 releases 
> > > definitely
> > > is a sign of lack of users.  It is an implication to the community that we
> > > should consider dropping some features so that we can get the best use of
> > > the community resources for the things that may have a broader audience.
> > >
> > > One thing majorly missing is a RDMA tester to guard all the merges to not
> > > break RDMA paths, hopefully in CI.  That should not rely on RDMA hardwares
> > > but just to sanity check the migration+rdma code running all fine.  RDMA
> > > taught us the lesson so we're requesting CI coverage for all other new
> > > features that will be merged at least for migration subsystem, so that we
> > > plan to not merge anything that is not covered by CI unless extremely
> > > necessary in the future.
> > >
> > > For sure CI is not the only missing part, but I'd say we should start with
> > > it, then someone should also take care of the code even if only in
> > > maintenance mode (no new feature to add on top).
> > >
> > > >
> > > > My concern is, this plan will forces a few QEMU users (not sure how
> > > > many) like us
> > > > either to stick to the RDMA migration by using an increasingly older
> > > > version of QEMU,
> > > > or to abandon the currently used RDMA migration.
> > >
> > > RDMA doesn't get new features anyway, if there's specific use case for 
> > > RDMA
> > > migrations, would it work if such a scenario uses the old binary?  Is it
> > > possible to switch to the TCP protocol with some good NICs?
> > We have used rdma migration with HCA from Nvidia for years, our
> > experience is RDMA migration works better than tcp (over ipoib).
>
> Please bare with me, as I know little on rdma stuff.
>
> I'm actually pretty confused (and since a long time ago..) on why we need
> to operation with rdma contexts when ipoib seems to provide all the tcp
> layers.  I meant, can it work with the current "tcp:" protocol with ipoib
> even if there's rdma/ib hardwares underneath?  Is it because of performance
> improvements so that we must use a separate path comparing to generic
> "tcp:" protocol here?
using rdma protocol with ib verbs , we can leverage the full benefit of RDMA by
talking directly to NIC which bypasses the kernel overhead, less cpu
utilization and better performance.

While IPoIB is more for compatibility to  applications using tcp, but
can't get full benefit of RDMA.  When you have mix generation of IB
devices, there are performance issue on IPoIB, we've seen 40G HCA can
only reach 2 Gb/s on IPoIB, but with raw RDMA can reach full line
speed.

I just run a simple iperf3 test via ipoib and ib_send_bw on same hosts:

iperf 3.9
Linux ps404a-3 5.15.137-pserver #5.15.137-6~deb11 SMP Thu Jan 4
07:19:34 UTC 2024 x86_64
-----------------------------------------------------------
Server listening on 5201
-----------------------------------------------------------
Time: Tue, 09 Apr 2024 06:55:02 GMT
Accepted connection from 2a02:247f:401:4:2:0:b:3, port 41130
      Cookie: cer2hexlldrowclq6izh7gbg5toviffqbcwt
      TCP MSS: 0 (default)
[  5] local 2a02:247f:401:4:2:0:a:3 port 5201 connected to
2a02:247f:401:4:2:0:b:3 port 41136
Starting Test: protocol: TCP, 1 streams, 131072 byte blocks, omitting
0 seconds, 10 second test, tos 0
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec  1.80 GBytes  15.5 Gbits/sec
[  5]   1.00-2.00   sec  1.85 GBytes  15.9 Gbits/sec
[  5]   2.00-3.00   sec  1.88 GBytes  16.2 Gbits/sec
[  5]   3.00-4.00   sec  1.87 GBytes  16.1 Gbits/sec
[  5]   4.00-5.00   sec  1.88 GBytes  16.2 Gbits/sec
[  5]   5.00-6.00   sec  1.93 GBytes  16.6 Gbits/sec
[  5]   6.00-7.00   sec  2.00 GBytes  17.2 Gbits/sec
[  5]   7.00-8.00   sec  1.93 GBytes  16.6 Gbits/sec
[  5]   8.00-9.00   sec  1.86 GBytes  16.0 Gbits/sec
[  5]   9.00-10.00  sec  1.95 GBytes  16.8 Gbits/sec
[  5]  10.00-10.04  sec  85.2 MBytes  17.3 Gbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
Test Complete. Summary Results:
[ ID] Interval           Transfer     Bitrate
[  5] (sender statistics not available)
[  5]   0.00-10.04  sec  19.0 GBytes  16.3 Gbits/sec                  receiver
rcv_tcp_congestion cubic
iperf 3.9
Linux ps404a-3 5.15.137-pserver #5.15.137-6~deb11 SMP Thu Jan 4
07:19:34 UTC 2024 x86_64
-----------------------------------------------------------
Server listening on 5201
-----------------------------------------------------------
^Ciperf3: interrupt - the server has terminated
1 jwang@ps404a-3.stg:~$ sudo ib_send_bw -F -a

************************************
* Waiting for client to connect... *
************************************
---------------------------------------------------------------------------------------
                    Send BW Test
 Dual-port       : OFF Device         : mlx5_0
 Number of qps   : 1 Transport type : IB
 Connection type : RC Using SRQ      : OFF
 PCIe relax order: ON
 ibv_wr* API     : ON
 RX depth        : 512
 CQ Moderation   : 100
 Mtu             : 4096[B]
 Link type       : IB
 Max inline data : 0[B]
 rdma_cm QPs : OFF
 Data ex. method : Ethernet
---------------------------------------------------------------------------------------
 local address: LID 0x24 QPN 0x0174 PSN 0x300138
 remote address: LID 0x17 QPN 0x004a PSN 0xc54d6f
---------------------------------------------------------------------------------------
 #bytes     #iterations    BW peak[MB/sec]    BW average[MB/sec]   MsgRate[Mpps]
 2          1000             0.00               6.46       3.385977
 4          1000             0.00               10.38     2.721894
 8          1000             0.00               25.69     3.367830
 16         1000             0.00               41.46     2.716859
 32         1000             0.00               102.98    3.374577
 64         1000             0.00               206.12    3.377053
 128        1000             0.00               405.03    3.318007
 256        1000             0.00               821.52    3.364939
 512        1000             0.00               2150.78    4.404803
 1024       1000             0.00               4288.13    4.391044
 2048       1000             0.00               8518.25    4.361346
 4096       1000             0.00               11440.77    2.928836
 8192       1000             0.00               11526.45    1.475385
 16384      1000             0.00               11526.06    0.737668
 32768      1000             0.00               11524.86    0.368795
 65536      1000             0.00               11331.84    0.181309
 131072     1000             0.00               11524.75    0.092198
 262144     1000             0.00               11525.82    0.046103
 524288     1000             0.00               11524.70    0.023049
 1048576    1000             0.00               11510.84    0.011511
 2097152    1000             0.00               11524.58    0.005762
 4194304    1000             0.00               11514.26    0.002879
 8388608    1000             0.00               11511.01    0.001439
---------------------------------------------------------------------------------------

you can see with ipoib, it reaches 16 Gb/s using TCP, 1 streams,
131072 byte blocks
with RDMA at 4k+ message size it reaches 100 Gb/s


>
> >
> > Switching back to TCP will lead us to the old problems which was
> > solved by RDMA migration.
>
> Can you elaborate the problems, and why tcp won't work in this case?  They
> may not be directly relevant to the issue we're discussing, but I'm happy
> to learn more.
>
> What is the NICs you were testing before?  Did the test carry out with
> things like modern ones (50Gbps-200Gbps NICs), or the test was done when
> these hardwares are not common?
We use Mellanox/NVidia IB HCA from 40 Gb/s to 200 Gb/s mixed
generation across globe.
>
> Per my recent knowledge on the new Intel hardwares, at least the ones that
> support QPL, it's easy to achieve single core 50Gbps+.
In good case, I've also seen 50 Gbps + on Mellanox HCA.
>
> https://lore.kernel.org/r/PH7PR11MB5941A91AC1E514BCC32896A6A3342@PH7PR11MB5941.namprd11.prod.outlook.com
>
> Quote from Yuan:
>
>   Yes, I use iperf3 to check the bandwidth for one core, the bandwith is 
> 60Gbps.
>   [ ID] Interval           Transfer     Bitrate         Retr  Cwnd
>   [  5]   0.00-1.00   sec  7.00 GBytes  60.1 Gbits/sec    0   2.87 MBytes
>   [  5]   1.00-2.00   sec  7.05 GBytes  60.6 Gbits/sec    0   2.87 Mbytes
>
>   And in the live migration test, a multifd thread's CPU utilization is 
> almost 100%
>
> It boils down to what old problems were there with tcp first, though.
Yeah, this is the key reason we use RDMA. (low cpu ulitization and
better performance)
>
> >
> > >
> > > 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.

Is there document/link about the unittest/CI for migration tests, Why
are those tests missing?
Is it hard or very special to set up an environment for that? maybe we
can help in this regards.
>
> 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.

Thx for discussion and understanding.


Jinpu Wang
>
> Thanks,
>
> > >
> > > Thanks,
> > >
> > > --
> > > Peter Xu
> >
> > Thx!
> > Jinpu Wang
> > >
> >
>
> --
> Peter Xu
>