netdev - Re: "tcp: refine TSO autosizing" causes performance regression on Xen

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Date:	Thu, 09 Apr 2015 10:07:05 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	xen-devel@...ts.xensource.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	christoffer.dall@...aro.org,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Wei Liu <wei.liu2@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>, edumazet@...gle.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: "tcp: refine TSO autosizing" causes performance regression on
 Xen

On Thu, 2015-04-09 at 17:36 +0100, Stefano Stabellini wrote:

> 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%

Cool.

> 
> echo 1048576 > /proc/sys/net/ipv4/tcp_limit_output_bytes
> fills the gap entirely, same performance as before "refine TSO
> autosizing"


Sure, this basically disables TCP Small Queue and select the opposite :

Favor single flow throughput and huge latencies (bufferbloat)


> 
> 
> 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?

I guess this is a reasonable choice.

Note that /proc/sys/net/ipv4/tcp_limit_output_bytes is already
documented.



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