On 04/30/2018 09:14 AM, Ben Greear wrote: > On 04/27/2018 08:11 PM, Steven Rostedt wrote: >> >> We'd like this email archived in netdev list, but since netdev is >> notorious for blocking outlook email as spam, it didn't go through. So >> I'm replying here to help get it into the archives. >> >> Thanks! >> >> -- Steve >> >> >> On Fri, 27 Apr 2018 23:05:46 +0000 >> Michael Wenig <mwe...@vmware.com> wrote: >> >>> As part of VMware's performance testing with the Linux 4.15 kernel, >>> we identified CPU cost and throughput regressions when comparing to >>> the Linux 4.14 kernel. The impacted test cases are mostly TCP_STREAM >>> send tests when using small message sizes. The regressions are >>> significant (up 3x) and were tracked down to be a side effect of Eric >>> Dumazat's RB tree changes that went into the Linux 4.15 kernel. >>> Further investigation showed our use of the TCP_NODELAY flag in >>> conjunction with Eric's change caused the regressions to show and >>> simply disabling TCP_NODELAY brought performance back to normal. >>> Eric's change also resulted into significant improvements in our >>> TCP_RR test cases. >>> >>> >>> >>> Based on these results, our theory is that Eric's change made the >>> system overall faster (reduced latency) but as a side effect less >>> aggregation is happening (with TCP_NODELAY) and that results in lower >>> throughput. Previously even though TCP_NODELAY was set, system was >>> slower and we still got some benefit of aggregation. Aggregation >>> helps in better efficiency and higher throughput although it can >>> increase the latency. If you are seeing a regression in your >>> application throughput after this change, using TCP_NODELAY might >>> help bring performance back however that might increase latency. > > I guess you mean _disabling_ TCP_NODELAY instead of _using_ TCP_NODELAY? >
Yeah, I guess auto-corking does not work as intended.
