netdev - Re: TCP fast retransmit

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Date:	Tue, 13 Dec 2011 15:59:17 +0100
From:	Carsten Wolff <carsten@...ffcarsten.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"Esztermann, Ansgar" <Ansgar.Esztermann@...-bpc.mpg.de>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: TCP fast retransmit

On Tuesday 13 December 2011, you wrote:
> Le mardi 13 décembre 2011 à 15:05 +0100, Esztermann, Ansgar a écrit :
> > On Dec 9, 2011, at 17:31 , Eric Dumazet wrote:
> > >>> It seems you have a lot of packet reorders.
> > >>> 
> > >>> Are you using multipath or some channel bonding ?
> > >> 
> > >> Not that I am aware of (i.e. not on our end). However, the connection
> > >> will probably routed through a firewall. I will have to check if it is
> > >> configured to avoid reordering.
> > > 
> > > Is it a linux based firewall ?
> > 
> > No, it's Cisco.
> 
> OK
> 
> You could increase /proc/sys/net/ipv4/tcp_reordering from 3 to 16
> 
> (Not sure it will change anything, since its kinda dynamic for a tcp
> session)

It's dynamic, but the sysctl changes the dynamic a lot, because its the 
minimum value of the dupack threshold. Be sure not to set this too high, or 
you will risk a lot of RTOs, which should kill performance even more 
thoroughly.

Carsten
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