Re: [patch 2/7] lib/hashmod: Add modulo based hash mechanism

[Eric Dumazet]

On Sat, 2016-04-30 at 15:02 +0200, Thomas Gleixner wrote:

> Yes. So I tested those two:
> 
> u32 hash_64(u64 key)
> {
>        key  = ~key + (key << 18);
>        key ^= key >> 31;
>        key += (key << 2)) + (key << 4);
>        key ^= key >> 11;
>        key += key << 6;
>        key ^= key >> 22;
>        return (u32) key;
> }
> 
> u32 hash_32(u32 key)
> {
>        key = (key + 0x7ed55d16) + (key << 12);
>        key = (key ^ 0xc761c23c) ^ (key >> 19);
>        key = (key + 0x165667b1) + (key <<  5);
>        key = (key + 0xd3a2646c) ^ (key <<  9);
>        key = (key + 0xfd7046c5) + (key <<  3);
>        key = (key ^ 0xb55a4f09) ^ (key >> 16);
>        return key;
> }
> 
> They are really good and the results are similar to the simple modulo prime
> hash. hash64 is slightly faster as the modulo prime as it does not have the
> multiplication.
> 
> I'll send a patch to replace hash_64 and hash_32.
> 
> Text size:
> 	   x86_64	i386	arm
> hash_64	   88		148	128
> hash_32	   88		84	112
> 
> So probably slightly too large to inline.

I use hash_32() in net/sched/sch_fq.c, for all packets sent by Google
servers. (Note that I did _not_ use hash_ptr())

That's gazillions of packets per second, and the current multiply worked
just fine in term of hash spreading.

Are you really going to use something which looks much slower ?

u32 hash_32(u32 key)
{
        key = (key + 0x7ed55d16) + (key << 12);
        key = (key ^ 0xc761c23c) ^ (key >> 19);
        key = (key + 0x165667b1) + (key <<  5);
        key = (key + 0xd3a2646c) ^ (key <<  9);
        key = (key + 0xfd7046c5) + (key <<  3);
        key = (key ^ 0xb55a4f09) ^ (key >> 16);
        return key;
}

Probably having a simple multiple when ARCH_HAS_FAST_MULTIPLIER is
defined might be good enough, eventually by choosing a better
GOLDEN_RATIO_PRIME_32