Clojure STM example

trans_stm.clj

(def accounts
  (into {} (for [i (range 1000001)]
             [i (ref 1000)])))

(time
 (let [fs (dotimes [offset 10]
            (future
              (doseq [i (shuffle (range (* offset 100000)
                                        (* (inc offset) 100000)))]
                (dosync
                 (alter (accounts i) + 100)
                 (alter (accounts (inc i)) - 100)))))]
   (doseq [f fs]
     @f)))

Lazily create accounts like this:

(def lazy-accounts (atom {}))

(defn create-account-ref [accounts account-id]
  (if (accounts account-id) accounts
   #_else (assoc accounts account-id (ref 1000))))

(defn account-ref [account-id]
  (locking lazy-accounts ; otherwise n threads could create n-1 refs that are immediately trashed (wasting CPU on expensive query to put into ref)
    ((swap! lazy-accounts create-account-ref account-id) account-id)))

; Time it
(time
 (let [fs (for [offset (range 10)]
            (future
              (doseq [i (shuffle (range (* offset 100000)
                                        (* (inc offset) 100000)))]
                (dosync
                 (alter (account-ref i) + 100)
                 (alter (account-ref (inc i)) - 100)))))]
   (doseq [f fs]
     @f)))

"Elapsed time: 1614.612589 msecs"