[TCP] tcp_cubic: faster cube root (c5f5877c) · Commits · 戴 / test

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>

net/ipv4/tcp_cubic.c

+5 −11

Original line number	Diff line number	Diff line
		@@ -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;
		}

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