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

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Date:	Sat, 30 Apr 2016 17:22:47 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Darren Hart <darren@...art.com>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Davidlohr Bueso <dave@...olabs.net>, Chris Mason <clm@...com>,
	Carlos O'Donell <carlos@...hat.com>,
	Torvald Riegel <triegel@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [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:

   https://tglx.de/~tglx/f-ops-h64-t.png

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

Here for comparison the 64bit -> 32 shift version

  https://tglx.de/~tglx/f-ops-wang32-t.png

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

and the modulo prime one:

  https://tglx.de/~tglx/f-ops-mod-t.png

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

Indeed.
 
> 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
today.

Thanks,

	tglx