diff options
| author | Stephen Hemminger <shemminger@linux-foundation.org> | 2007-03-25 20:21:15 -0700 |
|---|---|---|
| committer | David S. Miller <davem@sunset.davemloft.net> | 2007-04-25 22:23:47 -0700 |
| commit | c5f5877c043ca471c3a607fa2c864848b19bc49a (patch) | |
| tree | b28d6cf22a0fc96f86a5c07a1b64cd8a2d3c7668 | |
| parent | 8570419fb7be0af84085ac8f13307392a748482c (diff) | |
[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 <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
| -rw-r--r-- | net/ipv4/tcp_cubic.c | 16 |
1 files changed, 5 insertions, 11 deletions
diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index 6f08adbda54..0e6cdfeb207 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; } |