Messages in this thread | | | Date | Thu, 9 Apr 2015 17:36:47 +0100 | From | Stefano Stabellini <> | Subject | Re: "tcp: refine TSO autosizing" causes performance regression on Xen |
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On Thu, 9 Apr 2015, Eric Dumazet wrote: > On Thu, 2015-04-09 at 16:46 +0100, Stefano Stabellini wrote: > > Hi all, > > > > I found a performance regression when running netperf -t TCP_MAERTS from > > an external host to a Xen VM on ARM64: v3.19 and v4.0-rc4 running in the > > virtual machine are 30% slower than v3.18. > > > > Through bisection I found that the perf regression is caused by the > > prensence of the following commit in the guest kernel: > > > > > > commit 605ad7f184b60cfaacbc038aa6c55ee68dee3c89 > > Author: Eric Dumazet <edumazet@google.com> > > Date: Sun Dec 7 12:22:18 2014 -0800 > > > > tcp: refine TSO autosizing > > > > > > A simple revert would fix the issue. > > > > Does anybody have any ideas on what could be the cause of the problem? > > Suggestions on what to do to fix it? > > You sent this to lkml while networking discussions are on netdev. > > This topic had been discussed on netdev multiple times.
Sorry, and many thanks for the quick reply!
> This commit restored original TCP Small Queue behavior, which is the > first step to fight bufferbloat. > > Some network drivers are known to be problematic because of a delayed TX > completion. > > So far this commit did not impact max single flow throughput on 40Gb > mlx4 NIC. (ie : line rate is possible) > > Try to tweak /proc/sys/net/ipv4/tcp_limit_output_bytes to see if it > makes a difference ?
A very big difference:
echo 262144 > /proc/sys/net/ipv4/tcp_limit_output_bytes brings us much closer to the original performance, the slowdown is just 8%
echo 1048576 > /proc/sys/net/ipv4/tcp_limit_output_bytes fills the gap entirely, same performance as before "refine TSO autosizing"
What would be the next step for here? Should I just document this as an important performance tweaking step for Xen, or is there something else we can do?
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