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SubjectRe: [PATCH] dcache: better name hash function
Eric Dumazet a écrit :
> unsigned int fold2(unsigned hash)
> {
> return ((unsigned long long)hash * 211) >> 32;
> }
>

I tried this reciprocal thing with 511 and 1023 values and got on a PIII 550 MHz, gcc-3.3.2 :

# ./hashtest 100000 511
jhash_string 0.033123 1.01 234 1.06
fnv32 0.033911 1.02 254 1.38
# ./hashtest 1000000 511
jhash_string 0.331155 1.00 2109 1.10
fnv32 0.359346 1.00 2151 1.65
# ./hashtest 10000000 511
jhash_string 3.383340 1.00 19985 1.03
fnv32 3.849359 1.00 20198 1.53

# ./hashtest 100000 1023
jhash_string 0.033123 1.03 134 1.01
fnv32 0.034260 1.03 142 1.32
# ./hashtest 1000000 1023
jhash_string 0.332329 1.00 1075 1.06
fnv32 0.422035 1.00 1121 1.59
# ./hashtest 10000000 1023
jhash_string 3.417559 1.00 10107 1.01
fnv32 3.747563 1.00 10223 1.35


511 value on 64bit, and 1023 on 32bit arches are nice because
hashsz * sizeof(pointer) <= 4096, wasting space for one pointer only.

Conclusion : jhash and 511/1023 hashsize for netdevices,
no divides, only one multiply for the fold.

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