diff options
Diffstat (limited to 'net/dccp/ccids/lib/tfrc_equation.c')
| -rw-r--r-- | net/dccp/ccids/lib/tfrc_equation.c | 96 |
1 files changed, 51 insertions, 45 deletions
diff --git a/net/dccp/ccids/lib/tfrc_equation.c b/net/dccp/ccids/lib/tfrc_equation.c index ddac2c511e2..88ef98285be 100644 --- a/net/dccp/ccids/lib/tfrc_equation.c +++ b/net/dccp/ccids/lib/tfrc_equation.c @@ -1,6 +1,4 @@ /* - * net/dccp/ccids/lib/tfrc_equation.c - * * Copyright (c) 2005 The University of Waikato, Hamilton, New Zealand. * Copyright (c) 2005 Ian McDonald <ian.mcdonald@jandi.co.nz> * Copyright (c) 2005 Arnaldo Carvalho de Melo <acme@conectiva.com.br> @@ -13,7 +11,6 @@ */ #include <linux/module.h> -#include <asm/div64.h> #include "../../dccp.h" #include "tfrc.h" @@ -27,7 +24,7 @@ The following two-column lookup table implements a part of the TCP throughput equation from [RFC 3448, sec. 3.1]: - s + s X_calc = -------------------------------------------------------------- R * sqrt(2*b*p/3) + (3 * t_RTO * sqrt(3*b*p/8) * (p + 32*p^3)) @@ -36,7 +33,7 @@ s is the packet size in bytes R is the round trip time in seconds p is the loss event rate, between 0 and 1.0, of the number of loss - events as a fraction of the number of packets transmitted + events as a fraction of the number of packets transmitted t_RTO is the TCP retransmission timeout value in seconds b is the number of packets acknowledged by a single TCP ACK @@ -48,9 +45,9 @@ which we can break down into: - s + s X_calc = --------- - R * f(p) + R * f(p) where f(p) is given for 0 < p <= 1 by: @@ -63,7 +60,7 @@ * the return result f(p) The lookup table therefore actually tabulates the following function g(q): - g(q) = 1000000 * f(q/1000000) + g(q) = 1000000 * f(q/1000000) Hence, when p <= 1, q must be less than or equal to 1000000. To achieve finer granularity for the practically more relevant case of small values of p (up to @@ -80,10 +77,10 @@ } With the given configuration, we have, with M = TFRC_CALC_X_ARRSIZE-1, - lookup[0][0] = g(1000000/(M+1)) = 1000000 * f(0.2%) - lookup[M][0] = g(1000000) = 1000000 * f(100%) - lookup[0][1] = g(TFRC_SMALLEST_P) = 1000000 * f(0.01%) - lookup[M][1] = g(TFRC_CALC_X_SPLIT) = 1000000 * f(5%) + lookup[0][0] = g(1000000/(M+1)) = 1000000 * f(0.2%) + lookup[M][0] = g(1000000) = 1000000 * f(100%) + lookup[0][1] = g(TFRC_SMALLEST_P) = 1000000 * f(0.01%) + lookup[M][1] = g(TFRC_CALC_X_SPLIT) = 1000000 * f(5%) In summary, the two columns represent f(p) for the following ranges: * The first column is for 0.002 <= p <= 1.0 @@ -611,20 +608,17 @@ static inline u32 tfrc_binsearch(u32 fval, u8 small) /** * tfrc_calc_x - Calculate the send rate as per section 3.1 of RFC3448 + * @s: packet size in bytes + * @R: RTT scaled by 1000000 (i.e., microseconds) + * @p: loss ratio estimate scaled by 1000000 * - * @s: packet size in bytes - * @R: RTT scaled by 1000000 (i.e., microseconds) - * @p: loss ratio estimate scaled by 1000000 - * Returns X_calc in bytes per second (not scaled). - * - * Note: DO NOT alter this code unless you run test cases against it, - * as the code has been optimized to stop underflow/overflow. + * Returns X_calc in bytes per second (not scaled). */ u32 tfrc_calc_x(u16 s, u32 R, u32 p) { - int index; + u16 index; u32 f; - u64 tmp1, tmp2; + u64 result; /* check against invalid parameters and divide-by-zero */ BUG_ON(p > 1000000); /* p must not exceed 100% */ @@ -632,41 +626,41 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p) if (R == 0) { /* possible divide by zero */ DCCP_CRIT("WARNING: RTT is 0, returning maximum X_calc."); return ~0U; - } + } - if (p <= TFRC_CALC_X_SPLIT) { /* 0.0000 < p <= 0.05 */ + if (p <= TFRC_CALC_X_SPLIT) { /* 0.0000 < p <= 0.05 */ if (p < TFRC_SMALLEST_P) { /* 0.0000 < p < 0.0001 */ DCCP_WARN("Value of p (%d) below resolution. " "Substituting %d\n", p, TFRC_SMALLEST_P); index = 0; - } else /* 0.0001 <= p <= 0.05 */ + } else /* 0.0001 <= p <= 0.05 */ index = p/TFRC_SMALLEST_P - 1; - f = tfrc_calc_x_lookup[index][1]; + f = tfrc_calc_x_lookup[index][1]; - } else { /* 0.05 < p <= 1.00 */ + } else { /* 0.05 < p <= 1.00 */ index = p/(1000000/TFRC_CALC_X_ARRSIZE) - 1; f = tfrc_calc_x_lookup[index][0]; } - /* The following computes X = s/(R*f(p)) in bytes per second. Since f(p) - * and R are both scaled by 1000000, we need to multiply by 1000000^2. - * ==> DO NOT alter this unless you test against overflow on 32 bit */ - tmp1 = ((u64)s * 100000000); - tmp2 = ((u64)R * (u64)f); - do_div(tmp2, 10000); - do_div(tmp1, tmp2); - - return (u32)tmp1; + /* + * Compute X = s/(R*f(p)) in bytes per second. + * Since f(p) and R are both scaled by 1000000, we need to multiply by + * 1000000^2. To avoid overflow, the result is computed in two stages. + * This works under almost all reasonable operational conditions, for a + * wide range of parameters. Yet, should some strange combination of + * parameters result in overflow, the use of scaled_div32 will catch + * this and return UINT_MAX - which is a logically adequate consequence. + */ + result = scaled_div(s, R); + return scaled_div32(result, f); } -EXPORT_SYMBOL_GPL(tfrc_calc_x); - -/* +/** * tfrc_calc_x_reverse_lookup - try to find p given f(p) - * * @fvalue: function value to match, scaled by 1000000 + * * Returns closest match for p, also scaled by 1000000 */ u32 tfrc_calc_x_reverse_lookup(u32 fvalue) @@ -678,22 +672,34 @@ u32 tfrc_calc_x_reverse_lookup(u32 fvalue) /* Error cases. */ if (fvalue < tfrc_calc_x_lookup[0][1]) { - DCCP_WARN("fvalue %d smaller than resolution\n", fvalue); - return tfrc_calc_x_lookup[0][1]; + DCCP_WARN("fvalue %u smaller than resolution\n", fvalue); + return TFRC_SMALLEST_P; } if (fvalue > tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][0]) { - DCCP_WARN("fvalue %d exceeds bounds!\n", fvalue); + DCCP_WARN("fvalue %u exceeds bounds!\n", fvalue); return 1000000; } if (fvalue <= tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][1]) { index = tfrc_binsearch(fvalue, 1); return (index + 1) * TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE; - } - + } + /* else ... it must be in the coarse-grained column */ index = tfrc_binsearch(fvalue, 0); return (index + 1) * 1000000 / TFRC_CALC_X_ARRSIZE; } -EXPORT_SYMBOL_GPL(tfrc_calc_x_reverse_lookup); +/** + * tfrc_invert_loss_event_rate - Compute p so that 10^6 corresponds to 100% + * When @loss_event_rate is large, there is a chance that p is truncated to 0. + * To avoid re-entering slow-start in that case, we set p = TFRC_SMALLEST_P > 0. + */ +u32 tfrc_invert_loss_event_rate(u32 loss_event_rate) +{ + if (loss_event_rate == UINT_MAX) /* see RFC 4342, 8.5 */ + return 0; + if (unlikely(loss_event_rate == 0)) /* map 1/0 into 100% */ + return 1000000; + return max_t(u32, scaled_div(1, loss_event_rate), TFRC_SMALLEST_P); +} |