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 <[EMAIL PROTECTED]>
---
net/ipv4/tcp_cubic.c | 16 +++++-----------
1 file changed, 5 insertions(+), 11 deletions(-)
--- net-2.6.22.orig/net/ipv4/tcp_cubic.c 2007-03-06 12:24:34.000000000
-0800
+++ net-2.6.22/net/ipv4/tcp_cubic.c 2007-03-06 14:43:37.000000000 -0800
@@ -96,23 +96,17 @@
*/
static u32 cubic_root(u64 a)
{
- u32 x, x1;
+ u64 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 + div64_64(a, x*x)) / 3;
+ x = (2 * x + div64_64(a, x*x)) / 3;
+ x = (2 * x + div64_64(a, x*x)) / 3;
return x;
}
-
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