From: Stephen Hemminger <shemminger@osdl.org>
To: "David S. Miller" <davem@davemloft.net>
Cc: Douglas Leith <doug.leith@nuim.ie>, netdev@vger.kernel.org
Subject: [PATCH] tcp: cubic scaling error
Date: Wed, 25 Oct 2006 10:52:29 -0700 [thread overview]
Message-ID: <20061025105229.3b93aa17@freekitty> (raw)
Doug Leith observed a discrepancy between the version of CUBIC described
in the papers and the version in 2.6.18. A math error related to scaling
causes Cubic to grow too slowly.
Patch is from "Sangtae Ha" <sha2@ncsu.edu>. I validated that
it does fix the problems.
See the following to show behavior over 500ms 100 Mbit link.
Sender (2.6.19-rc3) --- Bridge (2.6.18-rt7) ------- Receiver (2.6.19-rc3)
1G [netem] 100M
http://developer.osdl.org/shemminger/tcp/2.6.19-rc3/cubic-orig.png
http://developer.osdl.org/shemminger/tcp/2.6.19-rc3/cubic-fix.png
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
--- a/net/ipv4/tcp_cubic.c 2006-09-25 16:25:43.000000000 -0700
+++ b/net/ipv4/tcp_cubic.c 2006-10-25 09:17:28.000000000 -0700
@@ -190,7 +190,7 @@
*/
/* change the unit from HZ to bictcp_HZ */
- t = ((tcp_time_stamp + ca->delay_min - ca->epoch_start)
+ t = ((tcp_time_stamp + (ca->delay_min>>3) - ca->epoch_start)
<< BICTCP_HZ) / HZ;
if (t < ca->bic_K) /* t - K */
@@ -259,7 +259,7 @@
(s32)(tcp_time_stamp - ca->epoch_start) < HZ)
return;
- delay = tcp_time_stamp - tp->rx_opt.rcv_tsecr;
+ delay = (tcp_time_stamp - tp->rx_opt.rcv_tsecr)<<3;
if (delay == 0)
delay = 1;
@@ -366,7 +366,7 @@
beta_scale = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta);
- cube_rtt_scale = (bic_scale << 3) / 10; /* 1024*c/rtt */
+ cube_rtt_scale = (bic_scale * 10); /* 1024*c/rtt */
/* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
* so K = cubic_root( (wmax-cwnd)*rtt/c )
next reply other threads:[~2006-10-25 23:02 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-10-25 17:52 Stephen Hemminger [this message]
2006-10-26 6:04 ` [PATCH] tcp: cubic scaling error David Miller
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