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Date: Tue, 31 May 2022 15:02:03 +0800
From: liuyacan@...p.netease.com
To: tonylu@...ux.alibaba.com
Cc: davem@...emloft.net, edumazet@...gle.com, kgraul@...ux.ibm.com,
kuba@...nel.org, linux-kernel@...r.kernel.org,
linux-s390@...r.kernel.org, liuyacan@...p.netease.com,
netdev@...r.kernel.org, pabeni@...hat.com, ubraun@...ux.ibm.com
Subject: Re: SMC-R problem under multithread
> > > Hi experts,
> > >
> > > I recently used memcached to test the performance of SMC-R relative to TCP, but the results
> > > are confusing me. When using multithread on the server side, the performance of SMC-R is not as good as TCP.
> > >
> > > Specifically, I tested 4 scenarios with server thread: 1\2\4\8. The client uses 8threads fixedly.
> > >
> > > server: (smc_run) memcached -t 1 -m 16384 -p [SERVER-PORT] -U 0 -F -c 10240 -o modern
> > > client: (smc-run) memtier_benchmark -s [SERVER-IP] -p [SERVER-PORT] -P memcache_text --random-data --data-size=100 --data-size-pattern=S --key-minimum=30 --key-maximum=100 -n 5000000 -t 8
> > >
> > > The result is as follows:
> > >
> > > SMC-R:
> > >
> > > server-thread ops/sec client-cpu server-cpu
> > > 1 242k 220% 97%
> > > 2 362k 241% 128%
> > > 4 378k 242% 160%
> > > 8 395k 242% 210%
> > >
> > > TCP:
> > > server-thread ops/sec client-cpu server-cpu
> > > 1 185k 224% 100%
> > > 2 435k 479% 200%
> > > 4 780k 731% 400%
> > > 8 938k 800% 659%
> > >
> > > It can be seen that as the number of threads increases, the performance increase of SMC-R is much slower than that of TCP.
> > >
> > > Am I doing something wrong? Or is it only when CPU resources are tight that SMC-R has a significant advantage ?
> > >
> > > Any suggestions are welcome.
> >
> > Hi, Tony.
> >
> > Inline.
> >
> > > Hi Yacan,
> > >
> > > This result matches some of our scenarios to some extent. Let's talk
> > > about this result first.
> > >
> > > Based on your benchmark, the biggest factor affecting performance seems
> > > that the CPU resource is limited. As the number of threads increased,
> > > neither CPU usage nor performance metrics improved, and CPU is limited
> > > to about 200-250%. To make it clear, could you please give out more
> > > metrics about per-CPU (usr / sys / hi / si) and memcached process usage.
> >
> > Now, I use taskset to limit memcached to use cpu21~cpu28. The result is as follows:
> >
> > TCP 1 thread
> > %Cpu21 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu22 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu23 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu24 : 0.0 us, 0.0 sy, 0.0 ni, 99.7 id, 0.0 wa, 0.0 hi, 0.3 si, 0.0 st
> > %Cpu25 : 14.3 us, 76.3 sy, 0.0 ni, 0.0 id, 0.0 wa, 0.0 hi, 9.3 si, 0.0 st
> > %Cpu26 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu27 : 1.0 us, 0.0 sy, 0.0 ni, 98.0 id, 0.0 wa, 0.0 hi, 1.0 si, 0.0 st
> > %Cpu28 : 0.0 us, 0.0 sy, 0.0 ni, 99.7 id, 0.0 wa, 0.0 hi, 0.3 si, 0.0 st
> >
> > SMC-R 1 thread
> > %Cpu21 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu22 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu23 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu24 : 0.0 us, 2.8 sy, 0.0 ni, 17.2 id, 0.0 wa, 0.0 hi, 79.9 si, 0.0 st
> > %Cpu25 : 18.9 us, 74.2 sy, 0.0 ni, 7.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu26 : 2.9 us, 0.3 sy, 0.0 ni, 96.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu27 : 0.3 us, 0.0 sy, 0.0 ni, 99.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu28 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> >
> > TCP 2 thread
> > %Cpu21 : 12.0 us, 81.7 sy, 0.0 ni, 6.3 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu22 : 11.0 us, 80.0 sy, 0.0 ni, 9.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu23 : 3.0 us, 12.6 sy, 0.0 ni, 84.4 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu24 : 0.0 us, 0.0 sy, 0.0 ni, 98.3 id, 0.0 wa, 0.0 hi, 1.7 si, 0.0 st
> > %Cpu25 : 0.0 us, 0.0 sy, 0.0 ni, 96.5 id, 0.0 wa, 0.0 hi, 3.5 si, 0.0 st
> > %Cpu26 : 0.0 us, 0.3 sy, 0.0 ni, 98.0 id, 0.0 wa, 0.0 hi, 1.7 si, 0.0 st
> > %Cpu27 : 0.0 us, 0.0 sy, 0.0 ni, 98.3 id, 0.0 wa, 0.0 hi, 1.7 si, 0.0 st
> > %Cpu28 : 2.0 us, 0.3 sy, 0.0 ni, 93.0 id, 0.0 wa, 0.0 hi, 4.7 si, 0.0 st
> >
> > SMC-R 2 thread
> > %Cpu21 : 4.3 us, 18.1 sy, 0.0 ni, 77.6 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu22 : 2.7 us, 20.6 sy, 0.0 ni, 76.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu23 : 4.7 us, 28.7 sy, 0.0 ni, 66.6 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu24 : 0.7 us, 2.3 sy, 0.0 ni, 17.3 id, 0.0 wa, 0.0 hi, 79.7 si, 0.0 st
> > %Cpu25 : 7.7 us, 23.6 sy, 0.0 ni, 68.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu26 : 3.7 us, 8.8 sy, 0.0 ni, 87.5 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu27 : 0.0 us, 0.7 sy, 0.0 ni, 99.3 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu28 : 1.3 us, 8.6 sy, 0.0 ni, 90.1 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> >
> > TCP 4 thread
> > %Cpu21 : 10.0 us, 55.3 sy, 0.0 ni, 34.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu22 : 8.7 us, 50.5 sy, 0.0 ni, 40.8 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu23 : 11.7 us, 63.7 sy, 0.0 ni, 24.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu24 : 3.1 us, 13.9 sy, 0.0 ni, 75.6 id, 0.0 wa, 0.0 hi, 7.5 si, 0.0 st
> > %Cpu25 : 9.3 us, 30.9 sy, 0.0 ni, 49.8 id, 0.0 wa, 0.0 hi, 10.0 si, 0.0 st
> > %Cpu26 : 8.5 us, 28.3 sy, 0.0 ni, 56.3 id, 0.0 wa, 0.0 hi, 6.8 si, 0.0 st
> > %Cpu27 : 4.3 us, 21.4 sy, 0.0 ni, 64.9 id, 0.0 wa, 0.0 hi, 9.4 si, 0.0 st
> > %Cpu28 : 12.4 us, 48.3 sy, 0.0 ni, 30.5 id, 0.0 wa, 0.0 hi, 8.7 si, 0.0 st
> >
> > SMC-R 4 thread
> > %Cpu21 : 6.1 us, 21.4 sy, 0.0 ni, 72.5 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu22 : 5.9 us, 21.8 sy, 0.0 ni, 72.3 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu23 : 6.5 us, 28.1 sy, 0.0 ni, 65.4 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu24 : 4.1 us, 9.3 sy, 0.0 ni, 5.5 id, 0.0 wa, 0.0 hi, 81.0 si, 0.0 st
> > %Cpu25 : 3.7 us, 8.4 sy, 0.0 ni, 87.9 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu26 : 3.3 us, 10.9 sy, 0.0 ni, 85.8 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu27 : 4.7 us, 11.3 sy, 0.0 ni, 84.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu28 : 1.0 us, 4.3 sy, 0.0 ni, 94.6 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> >
> > TCP 8 thread
> > %Cpu21 : 14.7 us, 63.2 sy, 0.0 ni, 22.1 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu22 : 14.6 us, 61.1 sy, 0.0 ni, 24.3 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu23 : 12.9 us, 66.9 sy, 0.0 ni, 20.2 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu24 : 15.4 us, 52.1 sy, 0.0 ni, 20.3 id, 0.0 wa, 0.0 hi, 12.2 si, 0.0 st
> > %Cpu25 : 11.2 us, 52.7 sy, 0.0 ni, 19.7 id, 0.0 wa, 0.0 hi, 16.3 si, 0.0 st
> > %Cpu26 : 14.3 us, 54.3 sy, 0.0 ni, 20.8 id, 0.0 wa, 0.0 hi, 10.6 si, 0.0 st
> > %Cpu27 : 12.1 us, 52.8 sy, 0.0 ni, 21.4 id, 0.0 wa, 0.0 hi, 13.8 si, 0.0 st
> > %Cpu28 : 14.7 us, 49.1 sy, 0.0 ni, 21.2 id, 0.0 wa, 0.0 hi, 15.0 si, 0.0 st
> >
> > SMC-R 8 thread
> > %Cpu21 : 6.3 us, 20.4 sy, 0.0 ni, 73.3 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu22 : 8.3 us, 18.3 sy, 0.0 ni, 73.4 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu23 : 5.1 us, 23.3 sy, 0.0 ni, 71.6 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu24 : 1.3 us, 3.4 sy, 0.0 ni, 1.0 id, 0.0 wa, 0.0 hi, 94.3 si, 0.0 st
> > %Cpu25 : 6.3 us, 15.6 sy, 0.0 ni, 78.1 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu26 : 6.5 us, 12.7 sy, 0.0 ni, 80.8 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu27 : 7.4 us, 13.5 sy, 0.0 ni, 79.1 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> > %Cpu28 : 5.8 us, 13.3 sy, 0.0 ni, 80.9 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> >
> >
> > It looks like SMC-R only uses one core to do softirq work, I presume this is the rx/tx tasklet, right?
>
> Yep, it only used one CQ (one CPU core) to handle data (tasklet), which
> is solved in [1].
>
> > > Secondly, it seems that there is lots of connections in this test.
> > > If it takes too much time to establish a connection, or the number of
> > > final connections does not reach the specified value, the result will be
> > > greatly affected. Could you please give out more details about the
> > > connections numbers during benchmark?
> >
> > In our environment, client always use 50*8=400 connections.
>
> 400 connections is not too much. We found some regressions when the
> number of connections reaches the scale of thousands.
>
> >
> > > We have noticed SMC has some limitations in multiple threads and many
> > > connections. This benchmark happens to be basically in line with this
> > > scenario. In general, there are some aspects in brief:
> > > 1. control path (connection setup and dismiss) is not as fast as TCP;
> > > 2. data path (lock contention, CQ spreading, etc.) needs further improvement;
> >
> > SMC-R control path setup time slower than TCP is reasonable and tolerable.
>
> Connection setup is the one of hardest part to solve. If this is okay, I
> think SMC should suitable for your scenario.
>
> >
> > > About CPU limitation, SMC use one CQ and one core to handle data
> > > transmission, which cannot spread workload over multiple cores. There is
> > > is an early temporary solution [1], which also need to improve (new CQ
> > > API, WR refactor). With this early solution, it shows several times the
> > > performance improvement.
> > >
> > > About the improvement of connection setup, you can see [2] for more
> > > details, which is still a proposal now, and we are working on it now.
> > > This show considerable performance boost.
> > >
> > > [1] https://lore.kernel.org/all/20220126130140.66316-1-tonylu@linux.alibaba.com/
> > > [2] https://lore.kernel.org/all/1653375127-130233-1-git-send-email-alibuda@linux.alibaba.com/
> > >
> > > Thanks,
> > > Tony LU
> > >
> >
> > We just noticed the CQ per device as well. Actually we tried creating more CQs, multiple rx tasklets,
> > but nothing seems to work. Maybe we got it wrong somewhere...Now We plan to try [1] first.
>
> The key point of this patch [1] is to spread CQ vector to different
> cores. It can solve single core issue of tasklet (si high in some CPU
> core).
>
> Looking forward for your feedback, thanks.
>
> > Thank you very much for your reply!
> >
> > [1] https://lore.kernel.org/all/20220126130140.66316-1-tonylu@linux.alibaba.com/
> >
> > Regards,
> > Yacan
> >
> >
>
> [1] https://lore.kernel.org/all/20220126130140.66316-1-tonylu@linux.alibaba.com/
>
> Cheers,
> Tony Lu
Hi Tony,
This patch proved absolutely useful in our environment!
ops/sec 1 thread 2 threads 4 threads 8 threads
w/o patch 236k 365k 382k 394k
w/ patch 289k 726k 1061k 1243k
Ratio +22% +98% +177% +215%
Thank you very much.
Regards,
Yacan
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