From 22119240b1c8f64eebb6ffb368c927b2f8be4136 Mon Sep 17 00:00:00 2001 From: Stephen Hemminger Date: Wed, 25 Oct 2006 23:04:12 -0700 Subject: [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" . 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 Signed-off-by: David S. Miller --- net/ipv4/tcp_cubic.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index a60ef38d75c68..6ad184802266b 100644 --- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -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 ) -- cgit 1.2.3-korg