26 Aug
2009
26 Aug
'09
4:32 p.m.
Oh, and dynamically instantiating hash tables without having to
enumerate them statically in modparam("htable", "htable", ...) would
be
a huge plus. :-)
Alex Balashov wrote:
Hey Daniel,
Another feature idea:
Have a way to serialise a hash table and pass it around between elements
condensed into a header value. It can then be deserialised and
resurrected into a hash table on the other side.
I have a setup I run that involves 3 Kamailio proxies that perform
different parts of the logic and have to constantly use 5-6 custom
headers to shuttle data around. It is very annoying and messy to manage
all this.
What do you think?
-- Alex
Daniel-Constantin Mierla wrote:
--
Alex Balashov - Principal
Evariste Systems
Web : http://www.evaristesys.com/
Tel : (+1) (678) 954-0670
Direct : (+1) (678) 954-0671
comprehensive description ... for more, see
wikipedia:
http://en.wikipedia.org/wiki/Hash_table
Cheers,
Daniel
On 26.08.2009 15:43 Uhr, Alex Balashov wrote:
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(a)gmail.com>om>:
>
> 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/
>
>
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