On Mon, 30 Apr 2018 09:14:04 -0700 Ben Greear <gree...@candelatech.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? Yes, thank you for catching that. -- Steve
