Bug#206416: dpkg package hash table insufficient
On Wed, Aug 20, 2003 at 09:09:50PM +0200, Falk Hueffner wrote:
> > Attached is a patch which resizes the hashtable to 8191 chains of
> > buckets. This is the largest prime less than 8192 and offers an
> > improvement (over common things like dpkg -p or dpkg -l) of between
> > 20 and 30 percent on the runtime. An option would be to reduce the
> > table to only 4093 chains, but that reduces the gains to between 20
> > and 25 percent.
> There is no reason to make the number of buckets a prime number unless
> either your hash function really sucks or you use double hashing. In
> fact, it is a bad idea to do so, since modulo powers of two is a lot
> faster than general modulo.
Let's have a look...
(a) Practical Algorithms for Programmers, Binstock & Rex
(b) The Art of Computer Programming Vol 3. "Sorting and Searching", Knuth
(c) Introduction to algorithms, Cormen, Leiserson & Rivest
and opens them up*
(a) Page 90, Section 'Performance Issues' in the Hashing chapter
Bullet point two:
o Be sure that the number of slots in each hash table is a prime number
(a) goes on to say:
Common sense tells us that the maximum dispersal of remainders occurs
when we divide by a prime number. Practice bears out this assumption.
The worst possible division is to divide by a multiple of 2 since this
serves only to mask off bits in the number being divided.
(b) Page 516
... such considerations suggest we choose m to be a prime number ...
(c) Page 228 "The division method"
When using the division method, we usually avoid certain values of m.
For example, m should not be a power of 2...
...good values for m are primes, not too close to exact powers of 2.
So, ultimately, I agree that 8191 isn't the ideal number, perhaps we
want to drop a few back from there and consider 8161 instead.
Daniel Silverstone http://www.digital-scurf.org/
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