[TCP] cubic: precompute constants
Revised version of patch to pre-compute values for TCP cubic. * d32,d64 replaced with descriptive names * cube_factor replaces srtt[scaled by count] / HZ * ((1 << (10+2*BICTCP_HZ)) / bic_scale) * beta_scale replaces 8*(BICTCP_BETA_SCALE+beta)/3/(BICTCP_BETA_SCALE-beta); Signed-off-by: Stephen Hemminger <shemminger@osdl.org> Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Stephen Hemminger
David S. Miller
Родитель
90933fc8ba
Коммит
89b3d9aaf4
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@ -16,7 +16,7 @@
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#include <linux/mm.h>
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#include <linux/module.h>
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#include <net/tcp.h>
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#include <asm/div64.h>
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#define BICTCP_BETA_SCALE 1024 /* Scale factor beta calculation
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* max_cwnd = snd_cwnd * beta
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@ -34,15 +34,20 @@ static int initial_ssthresh = 100;
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static int bic_scale = 41;
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static int tcp_friendliness = 1;
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static u32 cube_rtt_scale;
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static u32 beta_scale;
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static u64 cube_factor;
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/* Note parameters that are used for precomputing scale factors are read-only */
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module_param(fast_convergence, int, 0644);
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MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
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module_param(max_increment, int, 0644);
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MODULE_PARM_DESC(max_increment, "Limit on increment allowed during binary search");
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module_param(beta, int, 0644);
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module_param(beta, int, 0444);
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MODULE_PARM_DESC(beta, "beta for multiplicative increase");
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module_param(initial_ssthresh, int, 0644);
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MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
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module_param(bic_scale, int, 0644);
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module_param(bic_scale, int, 0444);
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MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
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module_param(tcp_friendliness, int, 0644);
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MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
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@ -151,65 +156,13 @@ static u32 cubic_root(u64 x)
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return (u32)end;
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}
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static inline u32 bictcp_K(u32 dist, u32 srtt)
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{
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u64 d64;
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u32 d32;
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u32 count;
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u32 result;
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/* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
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so K = cubic_root( (wmax-cwnd)*rtt/c )
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the unit of K is bictcp_HZ=2^10, not HZ
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c = bic_scale >> 10
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rtt = (tp->srtt >> 3 ) / HZ
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the following code has been designed and tested for
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cwnd < 1 million packets
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RTT < 100 seconds
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HZ < 1,000,00 (corresponding to 10 nano-second)
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*/
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/* 1/c * 2^2*bictcp_HZ */
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d32 = (1 << (10+2*BICTCP_HZ)) / bic_scale;
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d64 = (__u64)d32;
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/* srtt * 2^count / HZ
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1) to get a better accuracy of the following d32,
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the larger the "count", the better the accuracy
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2) and avoid overflow of the following d64
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the larger the "count", the high possibility of overflow
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3) so find a "count" between bictcp_hz-3 and bictcp_hz
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"count" may be less than bictcp_HZ,
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then d64 becomes 0. that is OK
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*/
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d32 = srtt;
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count = 0;
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while (((d32 & 0x80000000)==0) && (count < BICTCP_HZ)){
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d32 = d32 << 1;
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count++;
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}
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d32 = d32 / HZ;
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/* (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ) */
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d64 = (d64 * dist * d32) >> (count+3-BICTCP_HZ);
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/* cubic root */
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d64 = cubic_root(d64);
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result = (u32)d64;
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return result;
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}
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/*
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* Compute congestion window to use.
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*/
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static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
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{
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u64 d64;
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u32 d32, t, srtt, bic_target, min_cnt, max_cnt;
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u64 offs;
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u32 delta, t, bic_target, min_cnt, max_cnt;
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ca->ack_cnt++; /* count the number of ACKs */
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@ -220,8 +173,6 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
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ca->last_cwnd = cwnd;
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ca->last_time = tcp_time_stamp;
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srtt = (HZ << 3)/10; /* use real time-based growth function */
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if (ca->epoch_start == 0) {
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ca->epoch_start = tcp_time_stamp; /* record the beginning of an epoch */
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ca->ack_cnt = 1; /* start counting */
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@ -231,7 +182,11 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
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ca->bic_K = 0;
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ca->bic_origin_point = cwnd;
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} else {
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ca->bic_K = bictcp_K(ca->last_max_cwnd-cwnd, srtt);
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/* Compute new K based on
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* (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
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*/
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ca->bic_K = cubic_root(cube_factor
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* (ca->last_max_cwnd - cwnd));
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ca->bic_origin_point = ca->last_max_cwnd;
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}
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}
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@ -239,9 +194,9 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
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/* cubic function - calc*/
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/* calculate c * time^3 / rtt,
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* while considering overflow in calculation of time^3
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* (so time^3 is done by using d64)
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* (so time^3 is done by using 64 bit)
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* and without the support of division of 64bit numbers
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* (so all divisions are done by using d32)
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* (so all divisions are done by using 32 bit)
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* also NOTE the unit of those veriables
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* time = (t - K) / 2^bictcp_HZ
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* c = bic_scale >> 10
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@ -255,18 +210,16 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
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<< BICTCP_HZ) / HZ;
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if (t < ca->bic_K) /* t - K */
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d32 = ca->bic_K - t;
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offs = ca->bic_K - t;
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else
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d32 = t - ca->bic_K;
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offs = t - ca->bic_K;
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d64 = (u64)d32;
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d32 = (bic_scale << 3) * HZ / srtt; /* 1024*c/rtt */
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d64 = (d32 * d64 * d64 * d64) >> (10+3*BICTCP_HZ); /* c/rtt * (t-K)^3 */
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d32 = (u32)d64;
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/* c/rtt * (t-K)^3 */
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delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
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if (t < ca->bic_K) /* below origin*/
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bic_target = ca->bic_origin_point - d32;
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bic_target = ca->bic_origin_point - delta;
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else /* above origin*/
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bic_target = ca->bic_origin_point + d32;
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bic_target = ca->bic_origin_point + delta;
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/* cubic function - calc bictcp_cnt*/
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if (bic_target > cwnd) {
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@ -288,16 +241,16 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
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/* TCP Friendly */
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if (tcp_friendliness) {
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u32 scale = 8*(BICTCP_BETA_SCALE+beta)/3/(BICTCP_BETA_SCALE-beta);
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d32 = (cwnd * scale) >> 3;
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while (ca->ack_cnt > d32) { /* update tcp cwnd */
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ca->ack_cnt -= d32;
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u32 scale = beta_scale;
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delta = (cwnd * scale) >> 3;
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while (ca->ack_cnt > delta) { /* update tcp cwnd */
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ca->ack_cnt -= delta;
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ca->tcp_cwnd++;
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}
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if (ca->tcp_cwnd > cwnd){ /* if bic is slower than tcp */
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d32 = ca->tcp_cwnd - cwnd;
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max_cnt = cwnd / d32;
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delta = ca->tcp_cwnd - cwnd;
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max_cnt = cwnd / delta;
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if (ca->cnt > max_cnt)
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ca->cnt = max_cnt;
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}
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@ -428,6 +381,34 @@ static struct tcp_congestion_ops cubictcp = {
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static int __init cubictcp_register(void)
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{
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BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
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/* Precompute a bunch of the scaling factors that are used per-packet
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* based on SRTT of 100ms
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*/
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beta_scale = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta);
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cube_rtt_scale = (bic_scale << 3) / 10; /* 1024*c/rtt */
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/* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
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* so K = cubic_root( (wmax-cwnd)*rtt/c )
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* the unit of K is bictcp_HZ=2^10, not HZ
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*
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* c = bic_scale >> 10
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* rtt = 100ms
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*
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* the following code has been designed and tested for
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* cwnd < 1 million packets
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* RTT < 100 seconds
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* HZ < 1,000,00 (corresponding to 10 nano-second)
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*/
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/* 1/c * 2^2*bictcp_HZ * srtt */
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cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
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/* divide by bic_scale and by constant Srtt (100ms) */
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do_div(cube_factor, bic_scale * 10);
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return tcp_register_congestion_control(&cubictcp);
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}
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