ON PARALLELISM IN GAJURE

7 OCTOBER, 2009 — CHARLOTTESVILLE, VA

Recently, I read about some experiments in parallel genetic programming, and I decided to run my own with Gajure. Since I wrote Gajure in Clojure, I added parallelism without difficulty. All I needed was to change out a few of my map functions with pmap, its parallel counterpart. This took literally a few seconds — for instance, the core selection function roulette-select changed by two characters:

(defn roulette-select
  "Select num individuals from pop, with an individual's
   fitness porportional to selection likelihood."
  [pop fit-fn num]
  (let [pop-fits (pmap fit-fn pop)
        inc-fits (iterate (fn [[pfit idx]]
                          [(+ (nth pop-fits
                           (+ idx 1)) pfit) (+ idx 1)])
                          [(first pop-fits) 0])
        max-fitness (apply + pop-fits)
        pick-one (fn [num] (second (first (drop-while
                                         #(< (first %) num)
                                         inc-fits))))]
    (pmap (fn [x] (nth pop (pick-one (rand-int max-fitness))))
               (range num))))

A genetic algorithm seems a perfect target for parallelism. Within a given generation, we can independently run fitness computations and mutation operations for each population member. But performance gains did not accrue. At least, not on my dual-core machine.

So what happened? The running time to evolve “helloworld” more than doubled. What previously took 1450ms takes 3162ms with the parallel implementation. Probably, the overhead involved in creating parallel mappings outweighs any other benefits, given that I only have two processors [1]. I except the triviality of the problem space also contributes. In an ideal use-case, each fitness evaluation would do more work. I’ve committed the pmap changes to github under a separate branch.

1 Would this change if threads were more lightweight?