Messages in this thread | | | Date | Thu, 6 May 1999 11:12:28 -0400 (EDT) | From | Chuck Lever <> | Subject | Re: Hash functions (was Re: 2.2.6_andrea2.bz2) |
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On 6 May 1999, Harvey J. Stein wrote: > > I believe the hash function used by Chuck Lever uses the latter approach > > (with a multiplication by a prime number). This also saves you from > > the burden of having to find the next prime number. Also a multiplication > > by a big prime constant is usually faster than finding a (non-constant) > > modulus. > > That can't be right. An even times a prime is still even, so you'll > still miss alternate buckets if your table size is a power of 2. More > precisely, if a hash_fcn mod 2^m suffers from poor distribution, then > hash_fcn * prime mod 2^m will have the same poor distribution - 2 > objects that went into the same bucket under hash_fcn will still be in > the same bucket under hash_fcn * prime because primes are invertible > mod 2^m.
that's one reason why you don't use the LSB of the multiplication result. instead, it is right-shifted so you get the middle bits, which are much more likely to be "random."
please, take a look at
http://www.citi.umich.edu/projects/linux-scalability/reports/hash.html
- Chuck Lever -- corporate: <chuckl@netscape.com> personal: <chucklever@netscape.net> or <cel@monkey.org>
The Linux Scalability project: http://www.citi.umich.edu/projects/linux-scalability/
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