Concurrent Prime Sieve in Clojure

December 2015 · 3 minute read

While watching the Go Concurrency Patterns talk by Rob Pike, I came across a Concurrent Prime Sieve implementation in Go which Rob had claimed to be beautiful concurrent code. Curious enough, I checked out the example and decided on implementing it in Clojure.

Let’s begin by creating an infinite sequence generator(starting from 2):

(defn num-generator [out]
    (loop [n 2]
      (>!! out n)
      (recur (inc n)))))

The go block ensures that the body is run on a seperate thread. I used the blocking put instead of the parking one to only generate a number when required by the program. Had I used the parking put >! instead, we would’ve encountered the OutOfMemory exception while executing the function.

Let’s test this generator:

(def ichan (chan))
; Blocks at the put statement, waiting for a read
(num-generator ichan)

(<!! ichan)  ; => 2
(<!! ichan)  ; => 3
(<!! ichan)  ; => 4

Not the best way to test of course, but it seems to work as expected.

Next, let’s write a filter function that takes an input channel, an output channel and a value, and filters out the non-prime numbers from the input channel before sending them to the output channel. This is what the “prime sieve” refers to.

(defn filter-prime [in out val]
    (loop [n 0]
      (let [i (<!! in)]
        (when (not (= (mod i val) 0))
          (>!! out i)))
      (recur (inc n)))))

Before testing filter-prime, let’s tie the above two functions together with the user input using fetch-prime:

(defn fetch-prime [count]
  (let [ch (chan)]
    (num-generator ch)
    (loop [n count
           c ch]
      (when (> n 0)
        (let [p (<!! c)
              ch1 (chan)]
          ;; Just printing the prime nos for now
          (println "Prime => " p)
          (filter-prime c ch1 p)
          (recur (dec n) ch1))))
    (close! ch)))

And, finally test to see if it works:

(fetch-prime 6)
; Prime => 2
; Prime => 3
; Prime => 5
; Prime => 7
; Prime => 11
; Prime => 13

The whole process can be visualised as a bunch of sieves lined up on top of each other vertically with the numbers from the generator passing through them. Essentially, each sieve is a filter-prime with the last prime val passed as it’s parameter. At any pass, if the modulus value is zero, the number (retrieved from the generator) in that pass is discarded.

Though this took me a bit of head banging, in the end, I got a much better picture of the concurreny model using go blocks in Clojure. The way in which different threads communicate with each other as a Daisy Chain filter to remove the non-primes makes this a neat example for demonstrating concurrency.

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