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To: Naman Gulati , Joe Damato , Alexander Viro , Christian Brauner , Jan Kara , "David S. 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Checking just for epoll events in a >>> tight loop in the kernel context delivers latency gains to applications >>> that are not interested in napi busypolling with epoll. >>> >>> This patch adds an option to loop just for new events inside >>> ep_busy_loop, guarded by the EPIOCSPARAMS ioctl that controls epoll napi >>> busypolling. >> >> This makes an API change, so I think that linux-api@vger.kernel.org >> needs to be CC'd ? >> >>> A comparison with neper tcp_rr shows that busylooping for events in >>> epoll_wait boosted throughput by ~3-7% and reduced median latency by >>> ~10%. >>> >>> To demonstrate the latency and throughput improvements, a comparison was >>> made of neper tcp_rr running with: >>> 1. (baseline) No busylooping >> >> Is there NAPI-based steering to threads via SO_INCOMING_NAPI_ID in >> this case? More details, please, on locality. If there is no >> NAPI-based flow steering in this case, perhaps the improvements you >> are seeing are a result of both syscall overhead avoidance and data >> locality? >> > > The benchmarks were run with no NAPI steering. > > Regarding syscall overhead, I reproduced the above experiment with > mitigations=off > and found similar results as above. Pointing to the fact that the > above gains are > materialized from more than just avoiding syscall overhead. I suppose the natural follow-up questions are: 1) Where do the gains come from? and 2) Would they materialize with a realistic application? System calls have some overhead even with mitigations=off. In fact I understand on modern CPUs security mitigations are not that expensive to begin with? In a micro-benchmark that does nothing else but bouncing packets back and forth, this overhead might look more significant than in a realistic application? It seems your change does not eliminate any processing from each packet's path, but instead eliminates processing in between packet arrivals? This might lead to a small latency improvement, which might turn into a small throughput improvement in these micro-benchmarks, but that might quickly evaporate when an application has actual work to do in between packet arrivals. It would be good to know a little more about your experiments. You are referring to 5 threads, but does that mean 5 cores were busy on both client and server during the experiment? Which of client or server is the bottleneck? In your baseline experiment, are all 5 server cores busy? How many RX queues are in play and how is interrupt routing configured? Thanks, Martin