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

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Message-ID: <1428932970.3834.4.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 13 Apr 2015 06:49:30 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	George Dunlap <George.Dunlap@...citrix.com>
Cc:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Wei Liu <wei.liu2@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	netdev <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	edumazet@...gle.com, linux-arm-kernel@...ts.infradead.org,
	Christoffer Dall <christoffer.dall@...aro.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Jonathan Davies <Jonathan.Davies@...rix.com>,
	Felipe Franciosi <felipe.franciosi@...rix.com>,
	Paul Durrant <paul.durrant@...rix.com>
Subject: Re: [Xen-devel] "tcp: refine TSO autosizing" causes performance
 regression on Xen

On Mon, 2015-04-13 at 11:56 +0100, George Dunlap wrote:

> 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?

Sure, please provide a patch, that does not break back pressure.

But just in case, if Xen performance relied on bufferbloat, it might be
very difficult to reach a stable equilibrium : Any small change in stack
or scheduling might introduce a significant difference in 'raw
performance'.



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