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Subject: Re: [OMPI devel] RFC: improve the hash function used by opal_hash_table_t
From: George Bosilca (bosilca_at_[hidden])
Date: 2013-06-11 18:10:42


The one-at-the-time version computes on chars, if the performance of the hash function is a critical element in the equation then you will be better off avoiding its usage. I would suggest going with Murmur (http://en.wikipedia.org/wiki/MurmurHash) instead, which is faster and perform well in random distribution. Another interesting features is that there are specialized derivative for strings based key, a feature that might prove helpful with the MCA parameters and the MPI_T stuff.

  George.

On Jun 11, 2013, at 23:32 , Nathan Hjelm <hjelmn_at_[hidden]> wrote:

> What: Implement a better hash function in opal_hash_table_t. The function is a simple one-at-a-time Jenkin's hash (see http://en.wikipedia.org/wiki/Jenkins_hash_function) and has good collision rates and isn't overly complex or slow.
>
> Why: I am preparing an update to the MCA variable system (adding performance variables) which will include a hash-based lookup function (in preperation for the inevitable MPI_T_cvar/pvar/category lookup functions-- MPI 3.0 errata). The current hash function is not very good so now seems like a good time to update it.
>
> When: Will push this to trunk on Thursday if there are no objections.
>
> Patch below
>
> Index: opal/class/opal_hash_table.c
> ===================================================================
> --- opal/class/opal_hash_table.c (revision 28609)
> +++ opal/class/opal_hash_table.c (working copy)
> @@ -356,14 +356,20 @@
> static inline uint32_t opal_hash_value(size_t mask, const void *key,
> size_t keysize)
> {
> - size_t h, i;
> - const unsigned char *p;
> -
> - h = 0;
> - p = (const unsigned char *)key;
> - for (i = 0; i < keysize; i++, p++)
> - h = HASH_MULTIPLIER*h + *p;
> - return (uint32_t)(h & mask);
> + const unsigned char *p = (const unsigned char *) key;
> + uint32_t h = 0, i;
> +
> + for (i = 0 ; i < keysize ; ++i, ++p) {
> + h += *p;
> + h += h << 10;
> + h ^= h >> 6;
> + }
> +
> + h += h << 3;
> + h ^= h >> 11;
> + h += h << 15;
> +
> + return h & (uint32_t) mask;
> }
>
> int opal_hash_table_get_value_ptr(opal_hash_table_t* ht, const void* key,
>
>
>
>
> -Nathan
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