linux-kernel - Re: [PATCH] dcache: better name hash function

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Date:	Tue, 27 Oct 2009 03:45:50 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stephen Hemminger <shemminger@...tta.com>,
	Al Viro <viro@...iv.linux.org.uk>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Octavian Purdila <opurdila@...acom.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dcache: better name hash function

Stephen Hemminger <shemminger@...tta.com>, Al Viro a écrit :
> --- a/include/linux/dcache.h	2009-10-26 14:58:45.220347300 -0700
> +++ b/include/linux/dcache.h	2009-10-26 15:12:15.004160122 -0700
> @@ -45,15 +45,28 @@ struct dentry_stat_t {
>  };
>  extern struct dentry_stat_t dentry_stat;
>  
> -/* Name hashing routines. Initial hash value */
> -/* Hash courtesy of the R5 hash in reiserfs modulo sign bits */
> -#define init_name_hash()		0
> +/*
> + * Fowler / Noll / Vo (FNV) Hash
> + * see: http://www.isthe.com/chongo/tech/comp/fnv/
> + */
> +#ifdef CONFIG_64BIT
> +#define FNV_PRIME  1099511628211ull
> +#define FNV1_INIT  14695981039346656037ull
> +#else
> +#define FNV_PRIME  16777619u
> +#define FNV1_INIT  2166136261u
> +#endif
> +
> +#define init_name_hash()	FNV1_INIT
>  
> -/* partial hash update function. Assume roughly 4 bits per character */
> +/* partial hash update function. */
>  static inline unsigned long
> -partial_name_hash(unsigned long c, unsigned long prevhash)
> +partial_name_hash(unsigned char c, unsigned long prevhash)
>  {
> -	return (prevhash + (c << 4) + (c >> 4)) * 11;
> +	prevhash ^= c;
> +	prevhash *= FNV_PRIME;
> +
> +	return prevhash;
>  }
>  
>  /*

OK, but thats strlen(name) X (long,long) multiplies.

I suspect you tested on recent x86_64 cpu.

Some arches might have slow multiplies, no ?

jhash() (and others) are optimized by compiler to use basic and fast operations.
jhash operates on blocs of 12 chars per round, so it might be a pretty good choice once
out-of-line (because its pretty large and full_name_hash() is now used by
a lot of call sites)

Please provide your test program source, so that other can test on various arches.

Thanks
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