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[TCP] cubic: scaling error 

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>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Stephen Hemminger 2006-10-25 23:04:12 -07:00 committed by David S. Miller
parent 291b58d663
commit 22119240b1
1 changed files with 3 additions and 3 deletions
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6
net/ipv4/tcp_cubic.c
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@ -190,7 +190,7 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
*/
/* 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 @@ static inline void measure_delay(struct sock *sk)
(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 @@ static int __init cubictcp_register(void)
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 )