"tcp: refine TSO autosizing" causes performance regression on Xen

[Eric Dumazet]

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 at 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.

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 ?