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

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Date:	Wed, 15 Apr 2015 10:41:28 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	George Dunlap <george.dunlap@...citrix.com>
Cc:	Jonathan Davies <Jonathan.Davies@...rix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Wei Liu <wei.liu2@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	netdev <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Paul Durrant <paul.durrant@...rix.com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	Felipe Franciosi <felipe.franciosi@...rix.com>,
	linux-arm-kernel@...ts.infradead.org,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [Xen-devel] "tcp: refine TSO autosizing" causes performance
 regression on Xen

On Wed, 2015-04-15 at 18:23 +0100, George Dunlap wrote:

> Which means that max(2*skb->truesize, sk->sk_pacing_rate >>10) is
> *already* larger for Xen; that calculation mentioned in the comment is
> *already* doing the right thing.

Sigh.

1ms of traffic at 40Gbit is 5 MBytes

The reason for the cap to /proc/sys/net/ipv4/tcp_limit_output_bytes is
to provide the limitation of ~2 TSO packets, which _also_ is documented.

Without this limitation, 5 MBytes could translate to : Fill the queue,
do not limit.

If a particular driver needs to extend the limit, fine, document it and
take actions.



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