linux-kernel - Re: [PATCH v3 06/10] fs/namei.c: Improve dcache hash function

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Date:	Mon, 30 May 2016 18:27:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	George Spelvin <linux@...encehorizons.net>
Cc:	bfields@...hat.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH v3 06/10] fs/namei.c: Improve dcache hash function

On Mon, May 30, 2016 at 12:06:18PM -0400, George Spelvin wrote:
> Not quite.  The fold_hash() you quote is used only on 64-bit systems,
> which can be assumed to have a reasonable 64-bit multiply.  On 32-bit
> platforms, I avoid using GOLDEN_RATIO_64 at all, since 64x64-bit
> multiplies are so expensive.

Right; as stated performance really isn't a goal here.

> You actually have only 96 bits of input.  The correct prototype is:
> 
> static inline u64 iterate_chain_key(u64 key, u32 idx)

Indeed, although I conveniently ignored that because I didn't think it'd
matter :-)

> If performance mattered, I'd be inclined to use one or two iterations
> of the 32-bit HASH_MIX() function, which is specifically designed
> to add 32 bits to a 64-bit hash value.

Ah, I missed that HASH_MIX() had 64 bit state, so much for being able to
read it seems. Also; should we not move that entire section of
fs/namei.c into linux/hash.h ?

These two primitives seem generally useful.

> A more thorough mixing would be achieved by __jhash_mix().  Basically:
> 
> static inline u64 iterate_chain_key(u64 key, u32 idx)
> {
> 	u32 k0 = key, k1 = key >> 32;
> 
> 	__jhash_mix(idx, k0, k1)	/* Macro that modifies arguments! */
> 
> 	return k0 | (u64)k1 << 32;
> }
> 
> (The order of arguments is chosen to perserve the two "most-hashed" values.)

(I'd never have managed to deduce that property given the information in
jhash.h)

OK, that looks good to me, thanks! I'll go use that then.

> Also, I just had contact from the hppa folks who have brought to my
> attention that it's an example of an out-of-order superscalar CPU that
> *doesn't* have a good integer multiplier.  For general multiplies,
> you have to move values to the FPU and the code is a pain.

Egads, that's horrible, but sounds exactly like the thing you 'like'
given these patches :-) Good luck with that.