From: Stephen Hemminger Date: Thu, 22 Dec 2005 03:32:08 +0000 (-0800) Subject: [TCP] cubic: precompute constants X-Git-Tag: v2.6.16-rc1~474^2~22^2~9^2~38 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=89b3d9aaf46791177c5a5fa07a3ed38a035b5ef5;p=linux-2.6 [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 Signed-off-by: David S. Miller --- diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index bb5dc4bfb6..44fd40891a 100644 --- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -16,7 +16,7 @@ #include #include #include - +#include #define BICTCP_BETA_SCALE 1024 /* Scale factor beta calculation * max_cwnd = snd_cwnd * beta @@ -34,15 +34,20 @@ static int initial_ssthresh = 100; static int bic_scale = 41; static int tcp_friendliness = 1; +static u32 cube_rtt_scale; +static u32 beta_scale; +static u64 cube_factor; + +/* Note parameters that are used for precomputing scale factors are read-only */ module_param(fast_convergence, int, 0644); MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence"); module_param(max_increment, int, 0644); MODULE_PARM_DESC(max_increment, "Limit on increment allowed during binary search"); -module_param(beta, int, 0644); +module_param(beta, int, 0444); MODULE_PARM_DESC(beta, "beta for multiplicative increase"); module_param(initial_ssthresh, int, 0644); MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold"); -module_param(bic_scale, int, 0644); +module_param(bic_scale, int, 0444); MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)"); module_param(tcp_friendliness, int, 0644); MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness"); @@ -151,65 +156,13 @@ static u32 cubic_root(u64 x) return (u32)end; } -static inline u32 bictcp_K(u32 dist, u32 srtt) -{ - u64 d64; - u32 d32; - u32 count; - u32 result; - - /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3 - so K = cubic_root( (wmax-cwnd)*rtt/c ) - the unit of K is bictcp_HZ=2^10, not HZ - - c = bic_scale >> 10 - rtt = (tp->srtt >> 3 ) / HZ - - the following code has been designed and tested for - cwnd < 1 million packets - RTT < 100 seconds - HZ < 1,000,00 (corresponding to 10 nano-second) - - */ - - /* 1/c * 2^2*bictcp_HZ */ - d32 = (1 << (10+2*BICTCP_HZ)) / bic_scale; - d64 = (__u64)d32; - - /* srtt * 2^count / HZ - 1) to get a better accuracy of the following d32, - the larger the "count", the better the accuracy - 2) and avoid overflow of the following d64 - the larger the "count", the high possibility of overflow - 3) so find a "count" between bictcp_hz-3 and bictcp_hz - "count" may be less than bictcp_HZ, - then d64 becomes 0. that is OK - */ - d32 = srtt; - count = 0; - while (((d32 & 0x80000000)==0) && (count < BICTCP_HZ)){ - d32 = d32 << 1; - count++; - } - d32 = d32 / HZ; - - /* (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ) */ - d64 = (d64 * dist * d32) >> (count+3-BICTCP_HZ); - - /* cubic root */ - d64 = cubic_root(d64); - - result = (u32)d64; - return result; -} - /* * Compute congestion window to use. */ static inline void bictcp_update(struct bictcp *ca, u32 cwnd) { - u64 d64; - u32 d32, t, srtt, bic_target, min_cnt, max_cnt; + u64 offs; + u32 delta, t, bic_target, min_cnt, max_cnt; ca->ack_cnt++; /* count the number of ACKs */ @@ -220,8 +173,6 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd) ca->last_cwnd = cwnd; ca->last_time = tcp_time_stamp; - srtt = (HZ << 3)/10; /* use real time-based growth function */ - if (ca->epoch_start == 0) { ca->epoch_start = tcp_time_stamp; /* record the beginning of an epoch */ ca->ack_cnt = 1; /* start counting */ @@ -231,7 +182,11 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd) ca->bic_K = 0; ca->bic_origin_point = cwnd; } else { - ca->bic_K = bictcp_K(ca->last_max_cwnd-cwnd, srtt); + /* Compute new K based on + * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ) + */ + ca->bic_K = cubic_root(cube_factor + * (ca->last_max_cwnd - cwnd)); ca->bic_origin_point = ca->last_max_cwnd; } } @@ -239,9 +194,9 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd) /* cubic function - calc*/ /* calculate c * time^3 / rtt, * while considering overflow in calculation of time^3 - * (so time^3 is done by using d64) + * (so time^3 is done by using 64 bit) * and without the support of division of 64bit numbers - * (so all divisions are done by using d32) + * (so all divisions are done by using 32 bit) * also NOTE the unit of those veriables * time = (t - K) / 2^bictcp_HZ * c = bic_scale >> 10 @@ -255,18 +210,16 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd) << BICTCP_HZ) / HZ; if (t < ca->bic_K) /* t - K */ - d32 = ca->bic_K - t; + offs = ca->bic_K - t; else - d32 = t - ca->bic_K; + offs = t - ca->bic_K; - d64 = (u64)d32; - d32 = (bic_scale << 3) * HZ / srtt; /* 1024*c/rtt */ - d64 = (d32 * d64 * d64 * d64) >> (10+3*BICTCP_HZ); /* c/rtt * (t-K)^3 */ - d32 = (u32)d64; + /* c/rtt * (t-K)^3 */ + delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ); if (t < ca->bic_K) /* below origin*/ - bic_target = ca->bic_origin_point - d32; + bic_target = ca->bic_origin_point - delta; else /* above origin*/ - bic_target = ca->bic_origin_point + d32; + bic_target = ca->bic_origin_point + delta; /* cubic function - calc bictcp_cnt*/ if (bic_target > cwnd) { @@ -288,16 +241,16 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd) /* TCP Friendly */ if (tcp_friendliness) { - u32 scale = 8*(BICTCP_BETA_SCALE+beta)/3/(BICTCP_BETA_SCALE-beta); - d32 = (cwnd * scale) >> 3; - while (ca->ack_cnt > d32) { /* update tcp cwnd */ - ca->ack_cnt -= d32; + u32 scale = beta_scale; + delta = (cwnd * scale) >> 3; + while (ca->ack_cnt > delta) { /* update tcp cwnd */ + ca->ack_cnt -= delta; ca->tcp_cwnd++; } if (ca->tcp_cwnd > cwnd){ /* if bic is slower than tcp */ - d32 = ca->tcp_cwnd - cwnd; - max_cnt = cwnd / d32; + delta = ca->tcp_cwnd - cwnd; + max_cnt = cwnd / delta; if (ca->cnt > max_cnt) ca->cnt = max_cnt; } @@ -428,6 +381,34 @@ static struct tcp_congestion_ops cubictcp = { static int __init cubictcp_register(void) { BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE); + + /* Precompute a bunch of the scaling factors that are used per-packet + * based on SRTT of 100ms + */ + + beta_scale = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta); + + cube_rtt_scale = (bic_scale << 3) / 10; /* 1024*c/rtt */ + + /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3 + * so K = cubic_root( (wmax-cwnd)*rtt/c ) + * the unit of K is bictcp_HZ=2^10, not HZ + * + * c = bic_scale >> 10 + * rtt = 100ms + * + * the following code has been designed and tested for + * cwnd < 1 million packets + * RTT < 100 seconds + * HZ < 1,000,00 (corresponding to 10 nano-second) + */ + + /* 1/c * 2^2*bictcp_HZ * srtt */ + cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */ + + /* divide by bic_scale and by constant Srtt (100ms) */ + do_div(cube_factor, bic_scale * 10); + return tcp_register_congestion_control(&cubictcp); }