I think as long as we are updating this we might as well add a performant hash function. I see Murmur2 is 32-bit and has a public domain implementation: https://code.google.com/p/smhasher/source/browse/trunk/MurmurHash2.cpp
Since Murmur2 hashes in 4-byte blocks I expect it probably would perform better than either a nieve crc32 or Jenkins. Though a hardware accelerated crc32 (if available) would probably work great as well. Both are good suggestions that I will look into. I will also look into the string variant of Murmur though that would require another set of opal_hash_table functions for string keys (I was using the _ptr variety). -Nathan On Wed, Jun 12, 2013 at 12:10:42AM +0200, George Bosilca wrote: > 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 <hje...@lanl.gov> 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 > > _______________________________________________ > > devel mailing list > > de...@open-mpi.org > > http://www.open-mpi.org/mailman/listinfo.cgi/devel > > > _______________________________________________ > devel mailing list > de...@open-mpi.org > http://www.open-mpi.org/mailman/listinfo.cgi/devel
