Commit 74a52938 authored Sep 27, 2016 by Ilya Dryomov

crush: don't normalize input of crush_ln iteratively



Use __builtin_clz() supported by GCC and Clang to figure out
how many bits we should shift instead of shifting by a bit
in a loop until the value gets normalized. Improves performance
of this function by up to 3x in worst-case scenario and overall
straw2 performance by ~10%.

Reflects ceph.git commit 110de33ca497d94fc4737e5154d3fe781fa84a0a.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

parent 464691bd

net/ceph/crush/mapper.c

+9 −3

Original line number	Diff line number	Diff line
		@@ -253,9 +253,15 @@ static __u64 crush_ln(unsigned int xin)

		/* normalize input */
		iexpon = 15;
		while (!(x & 0x18000)) {
		x <<= 1;
		iexpon--;

		/*
		* figure out number of bits we need to shift and
		* do it in one step instead of iteratively
		*/
		if (!(x & 0x18000)) {
		int bits = __builtin_clz(x & 0x1FFFF) - 16;
		x <<= bits;
		iexpon = 15 - bits;
		}

		index1 = (x >> 8) << 1;