From 74a5293832b3c1f7cb8f86fb9af9ee747138d355 Mon Sep 17 00:00:00 2001 From: Ilya Dryomov Date: Tue, 27 Sep 2016 12:30:09 +0200 Subject: [PATCH] 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 --- net/ceph/crush/mapper.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) diff --git a/net/ceph/crush/mapper.c b/net/ceph/crush/mapper.c index 5fcfb98f309e..511ade95339a 100644 --- a/net/ceph/crush/mapper.c +++ b/net/ceph/crush/mapper.c @@ -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; -- 2.20.1