From 798950e055e04cbea0464fb83cab406ee8d35154 Mon Sep 17 00:00:00 2001 From: John Brooks Date: Wed, 14 Jan 2015 17:24:02 -0700 Subject: [PATCH] Use a better hash function for libsepol's avtab This function, based on murmurhash3, has much better distribution than the original. Using the current default of 4096 buckets, there are many fewer collisions: Before: 2893000 entries and 4096/4096 buckets used, longest chain length 1649 After: 2732000 entries and 4096/4096 buckets used, longest chain length 764 The difference becomes much more significant when buckets are increased. A naive attempt to expand the current function to larger outputs doesn't yield any significant improvement; so this function is a prerequisite for increasing the bucket size. Signed-off-by: John Brooks --- libsepol/src/avtab.c | 39 ++++++++++++++++++++++++++++++++++++--- 1 file changed, 36 insertions(+), 3 deletions(-) diff --git a/libsepol/src/avtab.c b/libsepol/src/avtab.c index ea947cbe..86e782c3 100644 --- a/libsepol/src/avtab.c +++ b/libsepol/src/avtab.c @@ -49,10 +49,43 @@ #include "debug.h" #include "private.h" -static inline int avtab_hash(struct avtab_key *keyp, uint16_t mask) +/* Based on MurmurHash3, written by Austin Appleby and placed in the + * public domain. + */ +static inline int avtab_hash(struct avtab_key *keyp, uint32_t mask) { - return ((keyp->target_class + (keyp->target_type << 2) + - (keyp->source_type << 9)) & mask); + static const uint32_t c1 = 0xcc9e2d51; + static const uint32_t c2 = 0x1b873593; + static const uint32_t r1 = 15; + static const uint32_t r2 = 13; + static const uint32_t m = 5; + static const uint32_t n = 0xe6546b64; + + uint32_t hash = 0; + +#define mix(input) { \ + uint32_t v = input; \ + v *= c1; \ + v = (v << r1) | (v >> (32 - r1)); \ + v *= c2; \ + hash ^= v; \ + hash = (hash << r2) | (hash >> (32 - r2)); \ + hash = hash * m + n; \ +} + + mix(keyp->target_class); + mix(keyp->target_type); + mix(keyp->source_type); + +#undef mix + + hash ^= hash >> 16; + hash *= 0x85ebca6b; + hash ^= hash >> 13; + hash *= 0xc2b2ae35; + hash ^= hash >> 16; + + return hash & mask; } static avtab_ptr_t