Re: Hash functions (was Re: 2.2.6_andrea2.bz2)

[Peter Steiner <p.steiner@t-online.de>]

>BTW: 2**32*(sqrt(5)-1)/2 is 2654435770 and not 2654425957UL that you are
>using in your hashfn. So were does 2654425957UL came from?

2654425957UL was an early multiplier (40499 * 65543). Kind of quick way
to get an almost prime near the golden ratio. Later it changed to
2654435761 which is a real prime (read Chucks page, section "A Little
Theory" for more information).

The golden ratio isn't really necessary. Instead of using the 'correct'
shift value:

   (hash * 0x9E3779B1) >> (32 - HASH_BITS)

he uses a general:

  (hash * 0x9E3779B1) >> SHIFT

where SHIFT can be (32 - HASH_BITS) but may be varied.

- --

Why using (32 - HASH_BITS) is 'correct' (in theory):

The golden ratio method uses no integer multiplier but an irrational
one (m), with  m = (sqrt(5) - 1) / 2 = 0.618033988

Calculation of the hash index goes

  h: hash table size
  i: index into the hash table
  x: position in the hash buffer x = [0;1)
  i: index into the hash table
  k: hash key

  x = frac(m * k)
  i = int(x * h)

Doing that in integer an integer representation of m is needed. However
there obviously is no integer representation for an irrational number,
so it has to be rounded.

  X: integer representation of x
  M: integer representation of m

For 32 bit systems:

  M = m * 2^32 = 2654435770

Getting X is easy since 'fract' is automatically done by overflowing
the range of an integer:

  X = M * k

To get i:

  i = int( x * h )
    = int( X * h / 2^32 )

using h = 2^HASH_BITS:

  i = int( X * 2^HASH_BITS / 2^32 )
    = int( X / 2^(32 - HASH_BITS) )
    = X >> (32 - HASH_BITS)

So we get:

  i = (M * k) >> (32 - HASH_BITS)

- --

At this point it should be clear, why M doesn't need to be a prime. It
represents a number between 0 and 1 which obviously never is a prime at
all.

However, according to Chuck's benchmarks SHIFT = (32 - HASH_BITS) does
not necessarily give the best results. It's often better to to use a
SHIFT smaller than (32 - HASH_BITS).

I really don't understand why. For example the hashfn for SHIFT=11
(best SHIFT for the buffer cache) could be written as ('normalized'):

 i = (hash * 0x1BBCD880 ) >> (32 - HASH_BITS)

This is not even near the golden ratio nor in one of Knuth's
recommended intervals...

Peter
-- 
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