Why is r/fold not parallelized across all available cpus?

Sun, 19 Jul 2020 11:14:29 +0000

I would have expected the below to be parallelized across all available cpus (in this case 12) but it only runs on one. What am I doing wrong?

(ns csv2summap.core
  (:require [clojure.data.csv :as csv]
            [clojure.java.io :as io]
            [clojure.core.reducers :as r])))

(with-open [writer (io/writer "numbers.csv")]
  (csv/write-csv
   writer
   (take 10000000
         (repeatedly #(vector (char (+ 65 (rand-int 26))) (rand-int 1000))))))

(defn sum-vals
([] {})
([m [k v]]
 (update m k (fnil + 0) (Integer/parseInt v))))

(defn merge-sums
([] {})
([& m] (apply merge-with + m)))

(time
(with-open [reader (io/reader "numbers.csv")]
  (doall
   (r/fold
    (/ 10000000 12)
    merge-sums
    sum-vals
    (csv/read-csv reader)))))

(def n-cpu (.availableProcessors (Runtime/getRuntime)))
=>12

(ns csv2summap.core
  (:require [clojure.data.csv :as csv]
            [clojure.java.io :as io]
            [clojure.core.reducers :as r])))

(defproject csv2summap "0.1.0-SNAPSHOT"
  :description "FIXME: write description"
  :url "http://example.com/FIXME"
  :license {:name "EPL-2.0 OR GPL-2.0-or-later WITH Classpath-exception-2.0"
            :url "https://www.eclipse.org/legal/epl-2.0/"}
  :dependencies [[org.clojure/clojure "1.10.1"]
                 [org.clojure/data.csv "1.0.0"]]
  :repl-options {:init-ns csv2summap.core})

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Why is r/fold not parallelized across all available cpus?