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

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Date:	Thu, 28 Apr 2016 19:25:04 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Thomas Gleixner <tglx@...utronix.de>
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 Thu, Apr 28, 2016 at 4:26 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>
> I'll try to dig up some time to analyze the hash_long failure unless someone
> familiar with the problem is beating me to it.

I'm not sure you need to spend time analyzing failure: if you get bad
hashing with hash_long(), then we know that is a bad hash without
having to really try to figure out why.

It's the hashes that _look_ like they might be good hashes, but
there's not a lot of analysis behind it, that I would worry about. The
simple prime modulus _should_ be fine, but at the same time I kind of
suspect we can do better. Especially since it has two multiplications.

Looking around, there's

    http://burtleburtle.net/bob/hash/integer.html

and that 32-bit "full avalanche" hash in six shifts looks like it
could be better. You wouldn't want to inline it, but the point of a
full avalanche bit mixing _should_ be that you could avoid the whole
"upper bits" part, and it should work independently of the target set
size.

So if that hash works better, it would be a pretty good replacement
option for hash_int().

There is also

    https://gist.github.com/badboy/6267743

that has a 64 bit to 32 bit hash function that might be useful for
"hash_long()".

Most of the people who worry about hashes tend to have strings to
hash, not just a single word like a pointer, but there's clearly
people around who have tried to search for good hashes that really
spread out the bits.

                Linus