netdev - Re: TCP congestion graphs (2.6.19-rc4)

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Date:	Tue, 28 Nov 2006 14:38:39 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...l.org
Cc:	davidwei79@...il.com, rhee@....ncsu.edu, sha2@...u.edu,
	netdev@...r.kernel.org
Subject: Re: TCP congestion graphs (2.6.19-rc4)

From: David Miller <davem@...emloft.net>
Date: Tue, 31 Oct 2006 17:30:28 -0800 (PST)

> From: Stephen Hemminger <shemminger@...l.org>
> Date: Tue, 31 Oct 2006 16:10:07 -0800
> 
> > On Tue, 31 Oct 2006 15:25:16 -0800
> > "Xiaoliang (David) Wei" <davidwei79@...il.com> wrote:
> > 
> > > It seems that the default Vegas alpha parameter in the rc4 is 1...
> > > 
> > > I observed similar situation with the NS2Linux simulator (with 2.6.16
> > > code) and found that if alpha=1, delayed ack will make it broken
> > > (keeping cwnd very low without real congestion)
> > > 
> > > See details at http://www.cs.caltech.edu/%7Eweixl/technical/ns2linux/known_linux/index.html#vegas
> > > 
> > > (Basically alpha==1 means Vegas seeks to see a delay of about 1 packet
> > > worth. With delayed ack, 1 packet worth of delay is common even with
> > > no congestion.)
> > > 
> > > To make Vegas work, I'd suggest to raise alpha to at least 2 or 3.
> > > (and beta has to be at least as large as alpha.)
> > > 
> > > -David
> > > 
> > 
> > I ran with the current default:
> > 	alpha = 1 (scaled 2)
> > 	beta  = 3 (scaled 6)
> > 	gamma = 1 (scaled 2)
> 
> Testing with alpha=2 and beta=4 would be interesting.

Instead of letting this issue rot, I've checked the following into
net-2.6.20

commit cd7f265b9069d8fd66a33d37139821f84ef04f0e
Author: David S. Miller <davem@...set.davemloft.net>
Date:   Tue Nov 28 14:37:38 2006 -0800

    [TCP] Vegas: Increase default alpha to 2 and beta to 4.
    
    This helps Vegas cope better with delayed ACKs, see
    analysis at:
    
    http://www.cs.caltech.edu/%7Eweixl/technical/ns2linux/known_linux/index.html#vegas
    
    Signed-off-by: David S. Miller <davem@...emloft.net>

diff --git a/net/ipv4/tcp_vegas.c b/net/ipv4/tcp_vegas.c
index a3b7aa0..ddc4bcc 100644
--- a/net/ipv4/tcp_vegas.c
+++ b/net/ipv4/tcp_vegas.c
@@ -42,8 +42,8 @@ #include <net/tcp.h>
  * with V_PARAM_SHIFT bits to the right of the binary point.
  */
 #define V_PARAM_SHIFT 1
-static int alpha = 1<<V_PARAM_SHIFT;
-static int beta  = 3<<V_PARAM_SHIFT;
+static int alpha = 2<<V_PARAM_SHIFT;
+static int beta  = 4<<V_PARAM_SHIFT;
 static int gamma = 1<<V_PARAM_SHIFT;
 
 module_param(alpha, int, 0644);
-
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