Message-ID: <1428932970.3834.4.camel@edumazet-glaptop2.roam.corp.google.com>
Subject: Re: [Xen-devel] "tcp: refine TSO autosizing" causes performance
 regression on Xen
From: Eric Dumazet <eric.dumazet@gmail.com>
To: George Dunlap <George.Dunlap@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
        "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
        Wei Liu <wei.liu2@citrix.com>, Ian Campbell <Ian.Campbell@citrix.com>,
        netdev <netdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        edumazet@google.com, linux-arm-kernel@lists.infradead.org,
        Christoffer Dall <christoffer.dall@linaro.org>,
        David Vrabel <david.vrabel@citrix.com>,
        Jonathan Davies <Jonathan.Davies@citrix.com>,
        Felipe Franciosi <felipe.franciosi@citrix.com>,
        Paul Durrant <paul.durrant@citrix.com>
Date: Mon, 13 Apr 2015 06:49:30 -0700
In-Reply-To: <CAFLBxZaVjFHh4UBnksGZS4waBr4jLdO8aJegyKvsU1-TvVt2Dg@mail.gmail.com>
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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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