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

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Date:	Wed, 11 Mar 2009 11:02:31 +0200
From:	"Rémi Denis-Courmont" 
	<remi.denis-courmont@...ia.com>
To:	ext Marian Ďurkovič <md@....sk>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: TCP rx window autotuning harmful at LAN context

On Monday 09 March 2009 13:25:21 ext Marian Ďurkovič wrote:
>   The behaviour could be descibed as "spiraling death" syndrome. While
> TCP with constant and decently sized rx window natively reduces
> transmission rate when RTT increases, autotuning performs exactly the
> opposite - as a response to increased RTT it increases the rx window size
> (which in turn again increases RTT...) As this happens again and again, the
> result is complete waste of all available buffers at sending host or at the
> bottleneck point, resulting in upto 267 msec (!) latency in LAN context
> (with 100 Mbps ethernet connection, default txqueuelen=1000, MTU=1500 and
> sky2 driver). Needles to say that this means the LAN is almost unusable.

This is very likely a stupid question, but anyway...

Is this with all applications, or only some pathological ones (one of which we 
both wrote code for, alright) with abnormally large send buffers?

-- 
Rémi Denis-Courmont
Maemo Software, Nokia Devices R&D

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