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

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Date:	Wed, 11 Mar 2009 09:29:20 +0100
From:	Marian Ďurkovič <md@....sk>
To:	David Miller <davem@...emloft.net>
Cc:	johnwheffner@...il.com, netdev@...r.kernel.org
Subject: Re: TCP rx window autotuning harmful at LAN context

On Tue, Mar 10, 2009 at 09:18:16AM -0700, David Miller wrote:
> There are both global system-wide and socket local limits to how much
> memory can be consumed by TCP receive data.  If things get beyond the
> configured limits, we back off.  You could modify those if you
> personally wish.
> 
> It's really good that you brought up this issue.
> 
> And it's really good that you've explained your own personal
> workaround for this issue.

Beg your pardon - "personal" ?! Is our university the only place where
people use Linux on workstations with 100 Mbps ethernet connection?
Isn't the stock kernel supposed to work decently for them - or should
they all become TCP experts and fiddle with various parameters in order
not to cause harm to other applications or the whole LAN just by starting
a single bulk transfer?

For the last time: setting TCP window to BDP is well-known and generally
accepted practice. Autotuning does NOT respect it, and for 100 Mpbs 
connections at LAN context it might set the rx window somewhere between
100*BDP and 300*BDP. Since the BDP formula obviously applies also in 
reverse direction, i.e.

delay=window/bandwith

setting insanely huge window results in insanely increased LAN latencies
(upto buffer limits). Is this really something noone cares about ?! 
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