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

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Date:	Wed, 11 Mar 2009 15:56:29 +0100
From:	Marian Ďurkovič <md@....sk>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: TCP rx window autotuning harmful at LAN context

On Wed, Mar 11, 2009 at 04:01:49PM +0100, Andi Kleen 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.

Besides being arbitrary, it's also incorrect. The defaults at
tcp.c are setting both tcp_wmem and tcp_rmem to 4 MB ignoring
the fact, that it results in 4MB send buffer but only 3 MB 
receive buffer due to other defaults (tcp_adv_win_scale=2).
 
Indeed, 3MB*(1538/1448)/100Mbps is equal to 267.3 msec
- i.e. exactly the latency we're seeing.

   With kind regards,

         M.



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