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SubjectRe: random table-driven hash benchmarked
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: <>
personal: <> or <>

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