Re: random table-driven hash benchmarked

Chuck Lever (cel@monkey.org)
Sat, 24 Apr 1999 21:54:57 -0400 (EDT)

On Sun, 25 Apr 1999, Peter Steiner wrote:
> Here is an improved version of the multiplicative hash. It includes
> some statistics (alt-SysRq-m). The size of the hash buffer can be
> adjusted by changeing HASHBITS in the range of 10-15. On my system with
> 64MB RAM a hash table size of 4096 entries is big enough
> (#define HASHBITS 12). It's tested on 32 bit x86 processors, but I
> tried to make it working on 64 bit systems too.

> -#define _hashfn(dev,block) (((unsigned)(HASHDEV(dev)^block)) & bh_hash_mask)
> +#define _hashfn(dev,block) ( \
> + ( (((unsigned) (block)* 0x9E3779B1 )>>(32-HASHBITS))^HASHDEV(dev) ) & bh_hash_mask)

in real world tests, 31 - HASHBITS works better. but for the buffer cache
in specific, i have found that 11 is the best shift value. however, the
best shift value can vary depending on the size of the hash table.

- Chuck Lever

--
corporate:	<chuckl@netscape.com>
personal:	<chucklever@netscape.net> or <cel@monkey.org>


The Linux Scalability project:
	http://www.citi.umich.edu/projects/linux-scalability/




-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.rutgers.edu
Please read the FAQ at http://www.tux.org/lkml/