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

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Message-ID: <alpine.DEB.2.02.1504151846110.7690@kaball.uk.xensource.com>
Date:	Wed, 15 Apr 2015 18:58:07 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	George Dunlap <george.dunlap@...citrix.com>,
	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, 15 Apr 2015, Eric Dumazet wrote:
> 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.

What actions do you have in mind exactly?  It would be great if you
could suggest how to move forward from here, beside documentation.

I don't think we can really expect every user that spawns a new VM in
the cloud to manually echo blah >
/proc/sys/net/ipv4/tcp_limit_output_bytes to an init script.  I cannot
imagine that would work well.
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