Re: [Caml-list] Hashtbl performance

[Fabrice Le Fessant <Fabrice.Le_fessant@inria.fr>]

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I think it is a well known bug of the hash function:

let list = ref [];;
for i = 1 to 30 do
  list := i :: !list;
  Printf.printf "%d " (Hashtbl.hash !list)
done;;

1 65601 8392706 797307010 578301955 797307010 8392706 65601 1 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

As you can see, long lists all have the same hash value. It is due to
the fact that the hash function does a depth-first traversal of the
datastructure, starting with the last field, instead of a breadth-first
traversal, and, as it only visits 10 nodes, they are always the same
ones for all lists of length > 10.

Thus, you should consider using your own hash function, probably only
considering the ints in the list and its length. Then, use the functor
in the Hashtbl module.

--Fabrice

On 07/27/2011 11:07 PM, Toby Kelsey wrote:
> I have written two versions of a small program, one using an (int list) data structure and the other an ((int*int) list); and I tested using Hashtbl, Set and (Jean-Christophe Filliatre's) Trie to cache these elements in each version.
> The relative run times of the programs turns out to be:
> 
>                  Hashtbl  Set  Trie
> (int list)         1      2.1  2.0
> ((int*int) list)  34.7    2.6  2.1
> 
> 
> There is a slight inherent speed difference between the 2 versions but the major effect is the cache type (in fact the caching difference must be larger than these ratios due to non-cache computations). I expected Hashtbl might be a bit slower than more specialised data structures, but the large speed difference with different data structures was unexpected. Presumably the slow-down is due to excessive hash collisions.
> 
> I had expected that the generic Hashtbl would be written to give adequate speed for all types of data and when I look at OCaml code on the web Hashtbl is usually used, so it seems most OCaml programmers believe the standard Hashtbl is a reasonable choice for most data types as well.
> 
> I haven't tested the Batteries or OCaml-Core hash tables so these may be more consistent, but if not, my question is can you predict how well Hashtbl will work for different types of data and so what to use it with, or it is just not reliable enough for general-purpose caching/memoization?
> 
> If anyone wants to look at the code, the (int list) Hashtbl version is at <http://rosettacode.org/wiki/Sokoban#OCaml> and the other versions are in <http://pastebin.com/hGn1AL9L> temporarily. Apart from the caching and the int/int pair changes they should be identical.
> 
> Toby
> 

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