netdev - Re: TCP rx window autotuning harmful at LAN context

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Message-ID: <1e41a3230903110834k7fd0e48clfb01e9ec5a189f1@mail.gmail.com>
Date:	Wed, 11 Mar 2009 08:34:23 -0700
From:	John Heffner <johnwheffner@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	David Miller <davem@...emloft.net>, md@....sk,
	netdev@...r.kernel.org
Subject: Re: TCP rx window autotuning harmful at LAN context

On Wed, Mar 11, 2009 at 8:01 AM, Andi Kleen <andi@...stfloor.org> wrote:
> On Wed, Mar 11, 2009 at 06:30:58AM -0700, David Miller wrote:
>> From: Andi Kleen <andi@...stfloor.org>
>> Date: Wed, 11 Mar 2009 11:03:35 +0100
>>
>> > Perhaps this points to the default buffer sizing heuristics to
>> > be too aggressive for >= 1GB?
>>
>> It's necessary Andi, you can't fill a connection on a trans-
>> continental connection without at least a 4MB receive buffer.
>
> Seems pretty arbitary to me. It's the value for a given bandwidth*latency
> product, but why not half or twice the bandwidth? I don't think
> that number is written in stone like you claim.

It is of course just a number, though not exactly arbitrary -- it's
approximately the required value for transcontinental 100 Mbps paths.
Choosing the value is a matter of engineering trade-offs, and seemed
like a reasonable cap at this time.

Any cap so much lower that it would give a small bound for LAN
latencies would bring us back to the bad old days where you couldn't
get anything more than 10 Mbps on the wide area.

  -John
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