From: Peter Zijlstra <peterz@infradead.org>
To: George Spelvin <linux@sciencehorizons.net>
Cc: bfields@redhat.com, linux-kernel@vger.kernel.org,
torvalds@linux-foundation.org
Subject: Re: [PATCH v3 06/10] fs/namei.c: Improve dcache hash function
Date: Mon, 30 May 2016 18:27:21 +0200 [thread overview]
Message-ID: <20160530162721.GG3193@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20160530160618.22150.qmail@ns.sciencehorizons.net>
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.
next prev parent reply other threads:[~2016-05-30 16:27 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CA+55aFxPSW+84KfQ1N_WmND-wtvgj2zQm8nFPkRcc+gyU=uing@mail.gmail.com>
2016-05-25 7:20 ` [PATCH 00/10] String hash improvements George Spelvin
2016-05-25 8:00 ` Geert Uytterhoeven
2016-05-25 8:11 ` George Spelvin
2016-05-25 8:50 ` Geert Uytterhoeven
2016-05-25 9:07 ` George Spelvin
2016-05-25 16:08 ` Linus Torvalds
2016-05-28 19:57 ` [PATCH v3 " George Spelvin
2016-05-28 19:57 ` [PATCH v3 01/10] Pull out string hash to <linux/stringhash.h> George Spelvin
2016-05-28 19:57 ` [PATCH v3 02/10] fs/namei.c: Add hashlen_string() function George Spelvin
2016-05-28 19:57 ` [PATCH v3 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hashlen_string() George Spelvin
2016-05-28 19:57 ` [PATCH v3 04/10] Change hash_64() return value to 32 bits George Spelvin
2016-05-28 19:57 ` [PATCH v3 05/10] Eliminate bad hash multipliers from hash_32() and hash_64() George Spelvin
2016-05-28 19:57 ` [PATCH v3 06/10] fs/namei.c: Improve dcache hash function George Spelvin
2016-05-30 15:11 ` Peter Zijlstra
2016-05-30 16:06 ` George Spelvin
2016-05-30 16:27 ` Peter Zijlstra [this message]
2016-05-30 18:10 ` George Spelvin
2016-06-02 1:18 ` Linus Torvalds
2016-06-02 2:31 ` George Spelvin
2016-06-02 16:35 ` Linus Torvalds
2016-06-02 18:23 ` George Spelvin
2016-05-28 19:57 ` [PATCH v3 07/10] <linux/hash.h>: Add support for architecture-specific functions George Spelvin
2016-05-29 7:57 ` Geert Uytterhoeven
2016-05-28 19:57 ` [PATCH v3 08/10] m68k: Add <asm/hash.h> George Spelvin
2016-05-28 19:57 ` [PATCH v3 09/10] microblaze: " George Spelvin
2016-05-28 19:57 ` [PATCH v3 10/10] h8300: " George Spelvin
2016-05-28 20:47 ` [PATCH v3 00/10] String hash improvements Linus Torvalds
2016-05-28 20:54 ` George Spelvin
2016-06-02 22:59 ` [PATCH " Fubo Chen
2016-05-26 17:09 ` [PATCH v2 " George Spelvin
2016-05-25 7:21 ` [PATCH 01/10] Pull out string hash to <linux/stringhash.h> George Spelvin
2016-05-25 7:22 ` [PATCH 02/10] fs/namei.c: Add hash_string() function George Spelvin
2016-05-25 7:26 ` [PATCH 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hash_string() George Spelvin
2016-05-25 7:28 ` [PATCH 04/10] Change hash_64() return value to 32 bits George Spelvin
2016-05-25 7:29 ` [PATCH 05/10] Eliminate bad hash multipliers from hash_32() and hash_64() George Spelvin
2016-05-25 7:31 ` [PATCH 06/10] fs/namei.c: Improve dcache hash function George Spelvin
2016-05-25 7:33 ` [PATCH 07/10] <linux/hash.h>: Add support for architecture-specific functions George Spelvin
2016-05-26 17:16 ` [PATCH v2 " George Spelvin
2016-05-25 7:34 ` [PATCH 08/10] m68k: Add <asm/archhash.h> George Spelvin
2016-05-25 7:34 ` George Spelvin
2016-05-25 8:07 ` Geert Uytterhoeven
2016-05-25 8:19 ` George Spelvin
2016-05-25 8:24 ` [PATCH 08v2/10] " George Spelvin
2016-05-25 8:48 ` Geert Uytterhoeven
2016-05-25 8:56 ` [PATCH 08/10] " Philippe De Muyter
2016-05-25 9:14 ` George Spelvin
2016-05-25 9:31 ` Andreas Schwab
2016-05-25 9:51 ` Philippe De Muyter
2016-05-25 13:24 ` Philippe De Muyter
2016-05-25 13:42 ` George Spelvin
2016-05-26 17:19 ` [PATCH v2 08/10] m68k: Add <asm/hash.h> George Spelvin
2016-05-25 7:37 ` [PATCH 09/10] microblaze: Add <asm/archhash.h> George Spelvin
2016-05-26 17:21 ` [PATCH v2 09/10] microblaze: Add <asm/hash.h> George Spelvin
2016-05-25 7:38 ` [PATCH 10/10] h8300: Add <asm/archhash.h> George Spelvin
2016-05-26 17:23 ` [PATCH v2 10/10] h8300: Add <asm/hash.h> George Spelvin
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