r/fold parallelism

core.clj

(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 1000000 (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)))

;; r/fold version, should run in parallel on all cores, but does not
(with-open [reader (io/reader "numbers.csv")]
(doall
 (r/fold
  (/ 1000000 12)
  merge-sums
  sum-vals
  (csv/read-csv reader))))

project.clj

(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})