The following changes since commit 08a2cbf64720d9371ea4649b1bdc00257916a326:
Update RBD documentation (2016-05-02 08:25:16 -0600)
are available in the git repository at:
git://git.kernel.dk/fio.git master
for you to fetch changes up to 2078c1369f993b4d760d3fe1f90cb6ffa4389fe5:
hash: import Linux sparse hash fix (2016-05-03 13:56:47 -0600)
----------------------------------------------------------------
Jens Axboe (1):
hash: import Linux sparse hash fix
hash.h | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)
---
Diff of recent changes:
diff --git a/hash.h b/hash.h
index 02b0614..1d7608b 100644
--- a/hash.h
+++ b/hash.h
@@ -28,13 +28,29 @@
#error Define GOLDEN_RATIO_PRIME for your wordsize.
#endif
-#define GR_PRIME_64 0x9e37fffffffc0001ULL
+/*
+ * The above primes are actively bad for hashing, since they are
+ * too sparse. The 32-bit one is mostly ok, the 64-bit one causes
+ * real problems. Besides, the "prime" part is pointless for the
+ * multiplicative hash.
+ *
+ * Although a random odd number will do, it turns out that the golden
+ * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
+ * properties.
+ *
+ * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
+ * (See Knuth vol 3, section 6.4, exercise 9.)
+ */
+#define GOLDEN_RATIO_32 0x61C88647
+#define GOLDEN_RATIO_64 0x61C8864680B583EBull
static inline unsigned long __hash_long(unsigned long val)
{
unsigned long hash = val;
#if BITS_PER_LONG == 64
+ hash *= GOLDEN_RATIO_64;
+#else
/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
unsigned long n = hash;
n <<= 18;
@@ -49,9 +65,6 @@ static inline unsigned long __hash_long(unsigned long val)
hash += n;
n <<= 2;
hash += n;
-#else
- /* On some cpus multiply is faster, on others gcc will do shifts */
- hash *= GOLDEN_RATIO_PRIME;
#endif
return hash;
@@ -65,7 +78,7 @@ static inline unsigned long hash_long(unsigned long val, unsigned int bits)
static inline uint64_t __hash_u64(uint64_t val)
{
- return val * GR_PRIME_64;
+ return val * GOLDEN_RATIO_64;
}
static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
@@ -77,7 +90,7 @@ static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
* Bob Jenkins jhash
*/
-#define JHASH_INITVAL GOLDEN_RATIO_PRIME
+#define JHASH_INITVAL GOLDEN_RATIO_32
static inline uint32_t rol32(uint32_t word, uint32_t shift)
{
--
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