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[TCP] tcp_cubic: faster cube root

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The Newton-Raphson method is quadratically convergent so
only a small fixed number of steps are necessary.
Therefore it is faster to unroll the loop. Since div64_64 is no longer
inline it won't cause code explosion.

Also fixes a bug that can occur if x^2 was bigger than 32 bits.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Stephen Hemminger 2007-03-25 20:21:15 -07:00 committed by David S. Miller
parent 8570419fb7
commit c5f5877c04
1 changed files with 5 additions and 11 deletions
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16
net/ipv4/tcp_cubic.c
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@ -96,23 +96,17 @@ static void bictcp_init(struct sock *sk)
*/
static u32 cubic_root(u64 a)
{
u32 x, x1;
u32 x;
/* Initial estimate is based on:
* cbrt(x) = exp(log(x) / 3)
*/
x = 1u << (fls64(a)/3);
/*
* Iteration based on:
* 2
* x = ( 2 * x + a / x ) / 3
* k+1 k k
*/
do {
x1 = x;
x = (2 * x + (uint32_t) div64_64(a, x*x)) / 3;
} while (abs(x1 - x) > 1);
/* converges to 32 bits in 3 iterations */
x = (2 * x + (u32)div64_64(a, (u64)x*(u64)x)) / 3;
x = (2 * x + (u32)div64_64(a, (u64)x*(u64)x)) / 3;
x = (2 * x + (u32)div64_64(a, (u64)x*(u64)x)) / 3;
return x;
}