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[TCP]: TCP Compound quad root function

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The original code did a 64 bit divide directly, which won't work on
32 bit platforms.  Rather than doing a 64 bit square root twice,
just implement a 4th root function in one pass using Newton's method.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Stephen Hemminger 2006-06-05 17:29:39 -07:00 committed by David S. Miller
parent f890f92104
commit a4ed258495
1 changed files with 65 additions and 23 deletions
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88
net/ipv4/tcp_compound.c
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@ -52,8 +52,6 @@
#define TCP_COMPOUND_ALPHA 3U
#define TCP_COMPOUND_BETA 1U
#define TCP_COMPOUND_KAPPA_POW 3
#define TCP_COMPOUND_KAPPA_NSQRT 2
#define TCP_COMPOUND_GAMMA 30
#define TCP_COMPOUND_ZETA 1
@ -156,6 +154,58 @@ static void tcp_compound_state(struct sock *sk, u8 ca_state)
vegas_disable(sk);
}
/* 64bit divisor, dividend and result. dynamic precision */
static inline u64 div64_64(u64 dividend, u64 divisor)
{
u32 d = divisor;
if (divisor > 0xffffffffULL) {
unsigned int shift = fls(divisor >> 32);
d = divisor >> shift;
dividend >>= shift;
}
/* avoid 64 bit division if possible */
if (dividend >> 32)
do_div(dividend, d);
else
dividend = (u32) dividend / d;
return dividend;
}
/* calculate the quartic root of "a" using Newton-Raphson */
static u32 qroot(u64 a)
{
u32 x, x1;
/* Initial estimate is based on:
* qrt(x) = exp(log(x) / 4)
*/
x = 1u << (fls64(a) >> 2);
/*
* Iteration based on:
* 3
* x = ( 3 * x + a / x ) / 4
* k+1 k k
*/
do {
u64 x3 = x;
x1 = x;
x3 *= x;
x3 *= x;
x = (3 * x + (u32) div64_64(a, x3)) / 4;
} while (abs(x1 - x) > 1);
return x;
}
/*
* If the connection is idle and we are restarting,
* then we don't want to do any Vegas calculations
@ -304,29 +354,21 @@ static void tcp_compound_cong_avoid(struct sock *sk, u32 ack,
dwnd = vegas->dwnd;
if (diff < (TCP_COMPOUND_GAMMA << V_PARAM_SHIFT)) {
u32 i, j, x, x2;
u64 v;
u32 x;
v = 1;
for (i = 0; i < TCP_COMPOUND_KAPPA_POW; i++)
v *= old_wnd;
for (i = 0; i < TCP_COMPOUND_KAPPA_NSQRT; i++) {
x = 1;
for (j = 0; j < 200; j++) {
x2 = (x + v / x) / 2;
if (x2 == x || !x2)
break;
x = x2;
}
v = x;
}
x = (u32) v >> TCP_COMPOUND_ALPHA;
/*
* The TCP Compound paper describes the choice
* of "k" determines the agressiveness,
* ie. slope of the response function.
*
* For same value as HSTCP would be 0.8
* but for computaional reasons, both the
* original authors and this implementation
* use 0.75.
*/
v = old_wnd;
x = qroot(v * v * v) >> TCP_COMPOUND_ALPHA;
if (x > 1)
dwnd = x - 1;
else