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?
Thanks,
Ben
--
Ben Greear <gree...@candelatech.com>
Candela Technologies Inc http://www.candelatech.com
