Commit 22119240 authored by Stephen Hemminger's avatar Stephen Hemminger Committed by David S. Miller
Browse files

[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: default avatarStephen Hemminger <shemminger@osdl.org>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 291b58d6

net/ipv4/tcp_cubic.c

+3 −3
Original line number Diff line number Diff line
@@ -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 )

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