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Re: radix tree implementation for quota ?

On Sun, Nov 28, 2010 at 11:37:12PM +0000, Sad Clouds wrote:
> I've had some issues with a hash table that was power of 2 in size, and
> most of the keys were multiples of 4, or 8 integers, for example.
> However making hash table size a prime number and using a good hash
> function, results in very good key distribution. This works especially
> well when hashing simple intergers, e.g. file descriptors, etc.

If you have issues with a power of 2 hash table size, you do not have a
good hash function in first place. A simple multiplicative hash function
like ((uint64_t)X * random32) >> 32 % size is known to provide good
results for most choices of random32. The suggestion to use a prime
number goes back to using primitive hash functions that don't properly
avoid funnels. With a good hash function, using a non power of 2 size
just tends to be slower.


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