Messages in this thread | | | Date | Tue, 27 Oct 2009 03:45:50 +0100 | From | Eric Dumazet <> | Subject | Re: [PATCH] dcache: better name hash function |
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Stephen Hemminger <shemminger@vyatta.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.
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