I indendet to see how to use fold to utilize pralell processing just to see that its prallel without the use of fold and pmap :/

all-paralell.clj

(ns reducers.core
  ;(:require [clojure.core.reducers :as r])
  )

(def N (iterate inc 0))

(def finish 6e6)

; this is just for testing if a random map function will stop the parelleltiy
(defn only-even [x]
  (if (even? x)
    x
    0))


(time (reduce + (map only-even (take finish N)))) ; "Elapsed time: 50830.559 msecs" 999999000000

Craig noted that the CPU usage I'm seeing might be from the GC collecting all the created wrapper objects.

pmap is only fast if the amount of work for each map run is bigger that the overhead. In this case its not true. One would normally use chunking before the pmap. However I have not done this and thus can not give a more in depth answer.

This may be a bit better for comparison,
(time (->> (range 6e6) vec (filter even?) (reduce +))) ;; "Elapsed time: 1894.056685 msecs" 8999997000000
(time (->> (range 6e6) vec (r/filter even?) (r/fold +))) ;; "Elapsed time: 1697.588728 msecs" 8999997000000