[Kamailio-Users] What happens with filled htables?

[Daniel-Constantin Mierla]

On 26.08.2009 17:14 Uhr, Alex Balashov wrote:
> Daniel-Constantin Mierla wrote:
>
>> just to be sure everyone got it right regarding the size attribute in 
>> htable parameter ... the number of slots for hash table is 2^size (2 
>> power size). So, size=4 means that the hash table has 16 slots.
>>
>> Reason for that is the speed in getting the slot number - modulo 
>> function is very fast in such cases, the slot number being actually a 
>> bitwise operation: hash_value & (2^size - 1)
>> --- 2^size is cached at startup.
>
> Thanks, that was a much-needed clarification omitted from my explanation.
>
> I have always been told that table sizes that are prime numbers result 
> in better distribution of factors due to the way modulo works, 
> although I am not a mathematician and don't have a ready understanding 
> of why. Is this true?  Is there some reason you did not take this 
> approach?  Is hash_value & 2(^size - 1) equally fast?
>
not a mathematician myself and havent tried the approach with prime 
numbers. The fair distribution is (attempted to be) ensured by the hash 
functions. They exist for long time in ser/openser, iirc, Andrei 
Pelinescu implemented them (with some improvements in SR) and he tested 
to get a fair distribution for strings, SIP addresses (being used in 
location table as well).

I did tests as well, mainly for location and under heavy load the (max - 
min) was in average less than 10% of max, which is good.

Cheers,
Daniel

-- 
Daniel-Constantin Mierla
* http://www.asipto.com/