[Kamailio-Users] What happens with filled htables?

[Alex Balashov]

The size of a hash table only indicates how many buckets it has, not how 
many entries it can hold.  Entries can hash to the same bucket ID;  this 
will result in collisions, which hash table implementations can manage.
Various algorithms dealing with collision scenarios account allow the 
buckets to be oversubscribed, if necessary.

Typical collision management strategies are either to hang a linked list 
off a bucket containing all entries beyond the head that also mapped to 
that bucket, or some sort of mathematical approach that computes a 
different bucket to use at deterministic relation to the one that has 
collided.

Larger table sizes will - given a decent and appropriate hash algorithm 
- cause fewer (if any) collisions since the factors and/or divisors 
involved in common hash functions are more numerous.  This is desirable 
because a collision-ridden table takes longer to search, undermining its 
usefulness as a data structure of O(1) search complexity.

For example, if a list of collided keys (and value pointers) is hung off 
a bucket, that list is searched linearly once the hash computation is 
run and the first value encountered is not found to be the one sought.

So, a very small table size will cause your table to degenerate into a 
few parallel linear structures, and that's assuming a perfect uniform 
distribution from the hash function and variance in keys.  Larger table 
sizes eliminate - or mitigate - this problem.

catalina oancea wrote:

> What do you mean with:
> "A hash table is filled when no more shm is available, it is better
> not to get there since not much will work at that time." ?
> 
> In the docs I understood that the size parameter decides the number of entries:
> "size - number specifying the size of hash table. The number of
> entries in the table is 2^size "
> 
> 
> 2009/8/26 Daniel-Constantin Mierla <miconda at gmail.com>:
>>
>> On 26.08.2009 14:38 Uhr, Alex Balashov wrote:
>>> Daniel-Constantin Mierla wrote:
>>>> Hi,
>>>>
>>>> On 26.08.2009 14:26 Uhr, catalina oancea wrote:
>>>>> Hello
>>>>>
>>>>> I use the htable module a lot but the only problem, when I add a new
>>>>> entry in a htable, is when I will delete it. My question is: if the
>>>>> hash table is completely filled and I try to add a new value to it, do
>>>>> I get an error or is an old value automatically deleted to be able to
>>>>> write my new value? If an old value was automatically deleted whenever
>>>>> a new value is added, I wouldn't have to bother deleting the values I
>>>>> no longer need.
>>>>>
>>>> the are deleted only if you have auto-expire set for htable -- see readme
>>>> for defining htables.
>>>>
>>>> A hash table is filled when no more shm is available, it is better not to
>>>> get there since not much will work at that time.
>>> There is no way to manually delete a key->value in a bucket?
>> it is (was there from first day):
>>
>> $sht(a=>x) = null;
>>
>> I have been talking about auto-delete.
>>
>> There are options to delete by regular expression matching against key or
>> value, see:
>> http://kamailio.org/docs/modules/1.5.x/htable.html#id2491912
>>
>> Cheers,
>> Daniel
>>
>> --
>> Daniel-Constantin Mierla
>> * http://www.asipto.com/
>>
>>
>> _______________________________________________
>> Kamailio (OpenSER) - Users mailing list
>> Users at lists.kamailio.org
>> http://lists.kamailio.org/cgi-bin/mailman/listinfo/users
>> http://lists.openser-project.org/cgi-bin/mailman/listinfo/users
>>


-- 
Alex Balashov - Principal
Evariste Systems
Web     : http://www.evaristesys.com/
Tel     : (+1) (678) 954-0670
Direct  : (+1) (678) 954-0671