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SubjectRe: [PATCH] dcache: better name hash function
On Tue, 27 Oct 2009 03:45:50 +0100
Eric Dumazet <eric.dumazet@gmail.com> wrote:

> 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.
>
> Thanks

long on i386 is 32 bits so it is 32 bit multiply. There is also an optimization
that uses shift and adds.




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