Re: [Xen-devel] "tcp: refine TSO autosizing" causes performance regression on Xen

[Jonathan Davies]

On 13/04/15 11:56, George Dunlap wrote:
> On Thu, Apr 9, 2015 at 5:36 PM, Stefano Stabellini
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > 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@xxxxxxxxxx>
> > > > Date:   Sun Dec 7 12:22:18 2014 -0800
> > > >
> > > >      tcp: refine TSO autosizing
>
> [snip]

I recently discussed this issue on netdev in the following thread:

https://www.marc.info/?l=linux-netdev&m=142738853820517

> > > 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.
>
> [snip]
>
> > > 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?
>
> Is the problem perhaps that netback/netfront delays TX completion?
> Would it be better to see if that can be addressed properly, so that
> the original purpose of the patch (fighting bufferbloat) can be
> achieved while not degrading performance for Xen?  Or at least, so
> that people get decent perfomance out of the box without having to
> tweak TCP parameters?

I agree; reducing the completion latency should be the ultimate goal. 
However, that won't be easy, so we need a work-around in the short term. 
I don't like the idea of relying on documenting the recommendation to 
change tcp_limit_output_bytes; too many people won't read this advice 
and will expect the out-of-the-box defaults to be reasonable.

Following Eric's pointers to where a similar problem had been 
experienced in wifi drivers, I came up with two proof-of-concept patches 
that gave a similar performance gain without any changes to sysctl 
parameters or core tcp/ip code. See 
https://www.marc.info/?l=linux-netdev&m=142746161307283.

I haven't yet received any feedback from the xen-netfront maintainers 
about whether those ideas could be reasonably adopted.

Jonathan
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