Commit 7e58886b authored by Stephen Hemminger's avatar Stephen Hemminger Committed by David S. Miller
Browse files

[TCP]: cubic optimization 



Use willy's work in optimizing cube root by having table for small values.

Signed-off-by: default avatarStephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 22b9a0a3

net/ipv4/tcp_cubic.c

+39 −11
Original line number Diff line number Diff line
@@ -91,23 +91,51 @@ static void bictcp_init(struct sock *sk) 
		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;

}



/*

 * calculate the cubic root of x using Newton-Raphson

/* calculate the cubic root of x using a table lookup followed by one

 * Newton-Raphson iteration.

 * Avg err ~= 0.195%

 */

static u32 cubic_root(u64 a)

{

	u32 x;



	/* Initial estimate is based on:

	 * cbrt(x) = exp(log(x) / 3)

	u32 x, b, shift;

	/*

	 * cbrt(x) MSB values for x MSB values in [0..63].

	 * Precomputed then refined by hand - Willy Tarreau

	 *

	 * For x in [0..63],

	 *   v = cbrt(x << 18) - 1

	 *   cbrt(x) = (v[x] + 10) >> 6

	 */

	x = 1u << (fls64(a)/3);

	static const u8 v[] = {

		/* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,

		/* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,

		/* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,

		/* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,

		/* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,

		/* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,

		/* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,

		/* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,

	};



	/* 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;

	b = fls64(a);

	if (b < 7) {

		/* a in [0..63] */

		return ((u32)v[(u32)a] + 35) >> 6;

	}



	b = ((b * 84) >> 8) - 1;

	shift = (a >> (b * 3));



	x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;



	/*

	 * Newton-Raphson iteration

	 *                         2

	 * x    = ( 2 * x  +  a / x  ) / 3

	 *  k+1          k         k

	 */

	x = (2 * x + (u32)div64_64(a, (u64)x * (u64)(x - 1)));

	x = ((x * 341) >> 10);

	return x;

}