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large hash tables
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Date: 2008-02-19 (23:01)
From: John Caml <camljohn42@g...>
Subject: large hash tables
I need to load roughly 100 million items into a memory based hash table,
where each key is an integer and each value is an integer and a float. Under
C++ stdlib's map library, this requires 800 MB of memory. Under my naive
porting of my C++ program to Ocaml, only 12 million rows get loaded before I
get the program crashes with the message "Fatal error: exception
Stack_overflow". At the time of the crash, nearly 2 GB of memory are in use.
(My machine -- AMD64 architecture -- has 8 GB of memory, so insufficient
memory is not the issue.)

Two questions:

1. How can I overcome the Stack_overflow error? (I'm already compiling with
ocamlopt rather than ocamlc, which seems to have increased the stack size
from 500 MB to 2 GB.) I'd like to be able to use the full 8 GB on my
2. How can I implment a more efficient hash table? My Ocaml program seems to
require 10x more memory than under C++.

My code appears below. Thank you very much.


exception SplitError

let read_whole_chan chan =
    let movieMajor = Hashtbl.create 777777 in

    let rec loadLines count =
        let line = input_line chan in
        let murList = Pcre.split line in
        match murList with
            | m::u::r::[] ->
                let rFloat = float_of_string r in
                Hashtbl.add movieMajor m (u, rFloat);
                if (count mod 10000) == 0 then Printf.printf "count: %d, m:
%s, u: %s, r: %f \n" count m u rFloat;
                loadLines (count + 1)
            | _ -> raise SplitError

        loadLines 0
        End_of_file -> ()

let read_whole_file filename =
    let chan = open_in filename in
    read_whole_chan chan

let filename = Sys.argv.(1);;

let str = read_whole_file filename;;