On 26.08.2009 16:32 Uhr, Alex Balashov wrote:
Oh, and dynamically instantiating hash tables without having to
enumerate them statically in modparam("htable", "htable", ...) would
be a huge plus. :-)
could be possible to do this way, but the number of slots has to be
initialized at startup. Unlike in other cases, here the hash table is in
shared memory, each slot being protected by a mutex to prevent access
conflicts.
Might be better to create a issue on the tracker collecting these ideas
-- as time allows, somebody may contribute.
Cheers,
Daniel
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:
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 <mico...@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/
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--
Daniel-Constantin Mierla
* http://www.asipto.com/
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