On Wed, Feb 10, 2021 at 3:25 PM Bill Wendling <morbo@xxxxxxxxxx> wrote:
>
> This hashing function[1] produces better hash table bucket
> distributions. The original hashing function always produced zeros in
> the three least significant bits.
>
> The new hashing funciton gives a modest performance boost.
>
> Original New
> 0:11.41 0:11.38
> 0:11.36 0:11.34
> 0:11.35 0:11.26
> -----------------------
> Avg: 0:11.373 0:11.327
>
> for a performance improvement of 0.4%.
>
> [1] From Numerical Recipes, 3rd Ed. 7.1.4 Random Hashes and Random Bytes
>
Can you please also test with the one libbpf uses internally:
return (val * 11400714819323198485llu) >> (64 - bits);
?
Thanks!
> Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx>
> ---
> hash.h | 25 ++++++++++---------------
> 1 file changed, 10 insertions(+), 15 deletions(-)
>
> diff --git a/hash.h b/hash.h
> index d3aa416..ea201ab 100644
> --- a/hash.h
> +++ b/hash.h
> @@ -33,22 +33,17 @@
>
> static inline uint64_t hash_64(const uint64_t val, const unsigned int bits)
> {
> - uint64_t hash = val;
> + uint64_t hash = val * 0x369DEA0F31A53F85UL + 0x255992D382208B61UL;
>
> - /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
> - uint64_t n = hash;
> - n <<= 18;
> - hash -= n;
> - n <<= 33;
> - hash -= n;
> - n <<= 3;
> - hash += n;
> - n <<= 3;
> - hash -= n;
> - n <<= 4;
> - hash += n;
> - n <<= 2;
> - hash += n;
> + hash ^= hash >> 21;
> + hash ^= hash << 37;
> + hash ^= hash >> 4;
> +
> + hash *= 0x422E19E1D95D2F0DUL;
> +
> + hash ^= hash << 20;
> + hash ^= hash >> 41;
> + hash ^= hash << 5;
>
> /* High bits are more random, so use them. */
> return hash >> (64 - bits);
> --
> 2.30.0.478.g8a0d178c01-goog
>
