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SubjectRe: [patch 2/7] lib/hashmod: Add modulo based hash mechanism
On Fri, 29 Apr 2016, Linus Torvalds wrote:
> Picking a new value almost at random (I say "almost", because I just
> started with that 32-bit multiplicand value that mostly works and
> shifted it up by 32 bits and then randomly added a few more bits to
> avoid long ranges of ones and zeroes), I picked
> #define GOLDEN_RATIO_PRIME_64 0x9e3700310c100d01UL
> and it is *much* better in my test harness.
> Of course, things like that depend on what patterns you test, But I
> did have a "range of strides and hash sizes" I tried. So just for fun:
> try changing GOLDEN_RATIO_PRIME_64 to that value, and see if the
> absolutely _horrid_ page-aligned case goes away for you?

It solves that horrid case:

It's faster than the shifts based version but the degradation with
hyperthreading is slightly worse.

Here for comparison the 64bit -> 32 shift version

FYI, that works way better than the existing shift machinery in hash_64

and the modulo prime one:

> It really looks like those multiplication numbers were very very badly picked.


> Still, that number doesn't do very well if the hash is small (say, 8
> bits).

I'm still waiting for the other test to complete. Will send numbers later



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