From c5f5877c043ca471c3a607fa2c864848b19bc49a Mon Sep 17 00:00:00 2001 From: Stephen Hemminger Date: Sun, 25 Mar 2007 20:21:15 -0700 Subject: [PATCH] [TCP] tcp_cubic: faster cube root 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 Signed-off-by: David S. Miller --- net/ipv4/tcp_cubic.c | 16 +++++----------- 1 file changed, 5 insertions(+), 11 deletions(-) diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index 6f08adbda54e..0e6cdfeb207a 100644 --- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -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; }