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    SubjectRe: more on hash functions
    On 8 Apr 1999, Andi Kleen wrote:
    > > the buffer hash function i used is this:
    > >
    > > #define _hashfn(dev,block) \
    > > ((((unsigned long)(block) * (40499UL * 65543UL)) >> 17) & bh_hash_mask)
    > >
    > > this is the same number of instructions as the old function, except that
    > > one of the instructions is IMUL. i haven't looked up how expensive this
    > > might be. however, i believe a fixed expense is better than a higher
    > > loops/lookup average, since IMUL with a constant multiplier doesn't incur
    > > any memory overhead once it's in the instruction cache, whereas an extra
    > > iteration of find_buffer might be another 2 memory references, either or
    > > both of which might be cache misses.
    >
    > This is correct for the x86 picture, but breaks e.g. on earlier sparcs
    > or some m68ks where multiplication is very slow. Remember Linux is not x86
    > only.

    yes, i'm well aware of the other archs. :)

    i think "breaks" is a little strong, though. perhaps it is slower on
    older hardware. can you say *how* slow an integer multiplication by a
    constant is on older SPARC CPUs or on, say, a 486, compared to memory
    references on these machines? that's the real tradeoff here, and i think
    it would be good to measure the difference under load before dismissing
    the idea.

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

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


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