Re: [eclipse-clp-users] dictionary hash collisions

From: Kish Shen <kisshen_at_...5...>
Date: Thu, 15 Oct 2009 18:10:59 +0100
Joachim Schimpf wrote:
> Stephan Schiffel wrote:
>> Hi all,
>>
>> I have a rather large program with a lot of predicates and other symbols which 
>> fill the dictionary:
>>
>> dictionary_entries:     20666
>> dict_hash_usage:        6816 / 8192
>> dict_hash_collisions:   7917 / 6816
>>
>> I worry about the last line and the negative performance impact these 
>> collisions may have. What happens if there are a lot of hash collisions in 
>> the dictionary? Can this have a negative influence on the performance of a 
>> program?
> 
> You have here on average 3 (20666/6813) entries per collision list, which
> should not be a problem (of course, some can be longer).
> 
> The other point is that not many operations need to do a hash lookup
> in the dictionary.  The system usually passes pointers to the entries
> directly.  Only operations that make new atoms or functors, e.g. read
> predicates, atom_string/2 etc do name lookups.  The one operation that
> may be critical is functor/3, which could be slowed down by long
> collision chains.
> 
> -- Joachim
> 
The last line is more alarming than it actually is, because of an error 
in the documentation for statistics/2 -- which incorrectly states that 
the 7917 in the dict_hash_collisions line is the length of the longest 
list in a hash-table slot, but it is not, and is supposed to be the 
total number of collisions, although Joachim have noticed a problem with 
it and the count is incorrect, so it is pretty non-informative right now.

Cheers,

Kish

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Received on Thu Oct 15 2009 - 17:11:11 CEST

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