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

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Message-ID: <552F7936.9070205@eu.citrix.com>
Date:	Thu, 16 Apr 2015 09:56:22 +0100
From:	George Dunlap <george.dunlap@...citrix.com>
To:	Eric Dumazet <eric.dumazet@...il.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 04/15/2015 07:19 PM, Eric Dumazet wrote:
> On Wed, 2015-04-15 at 19:04 +0100, George Dunlap wrote:
> 
>> Maybe you should stop wasting all of our time and just tell us what
>> you're thinking.
> 
> I think you make me wasting my time.
> 
> I already gave all the hints in prior discussions.

Right, and I suggested these two options:

"Obviously one solution would be to allow the drivers themselves to set
the tcp_limit_output_bytes, but that seems like a maintenance
nightmare.

"Another simple solution would be to allow drivers to indicate whether
they have a high transmit latency, and have the kernel use a higher
value by default when that's the case." [1]

Neither of which you commented on.  Instead you pointed me to a comment
that only partially described what the limitations were. (I.e., it
described the "two packets or 1ms", but not how they related, nor how
they related to the "max of 2 64k packets outstanding" of the default
tcp_limit_output_bytes setting.)

 -George


[1]
http://marc.info/?i=<CAFLBxZYt7-v29ysm=f+5QMOw64_QhESjzj98udba+1cS-PfObA@...l.gmail.com>
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