Question

I haven’t used multithreading in Clojure at all so am unsure where to start.

I have a doseq whose body can run in parallel. What I’d like is for there always to be 3 threads running (leaving 1 core free) that evaluate the body in parallel until the range is exhausted. There’s no shared state, nothing complicated – the equivalent of Python’s multiprocessing would be just fine.

So something like:

(dopar 3 [i (range 100)]
  ; repeated 100 times in 3 parallel threads...
  ...)

Where should I start looking? Is there a command for this? A standard package? A good reference?

So far I have found pmap, and could use that (how do I restrict to 3 at a time? looks like it uses 32 at a time – no, source says 2 + number of processors), but it seems like this is a basic primitive that should already exist somewhere.

clarification: I really would like to control the number of threads. I have processes that are long-running and use a fair amount of memory, so creating a large number and hoping things work out OK isn’t a good approach (example which uses a significant chunk available mem).

update: Starting to write a macro that does this, and I need a semaphore (or a mutex, or an atom i can wait on). Do semaphores exist in Clojure? Or should I use a ThreadPoolExecutor? It seems odd to have to pull so much in from Java – I thought parallel programming in Clojure was supposed to be easy… Maybe I am thinking about this completely the wrong way? Hmmm. Agents?

Answer 1

pmap will actually work fine in most circumstances – it uses a thread pool with a sensible number of threads for your machine. I wouldn’t bother trying to create your own mechanisms to control the number of threads unless you have real benchmark evidence that the defaults are causing a problem.

Having said that, if you really want to limit to a maximum of three threads, an easy approach is to just use pmap on 3 subsets of the range:

(defn split-equally [num coll] 
  "Split a collection into a vector of (as close as possible) equally sized parts"
  (loop [num num 
         parts []
         coll coll
         c (count coll)]
    (if (<= num 0)
      parts
      (let [t (quot (+ c num -1) num)]
        (recur (dec num) (conj parts (take t coll)) (drop t coll) (- c t)))))) 

(defmacro dopar [thread-count [sym coll] & body]
 `(doall (pmap 
    (fn [vals#]
      (doseq [~sym vals#]
        ~@body))  
    (split-equally ~thread-count ~coll))))

Note the use of doall, which is needed to force evaluation of the pmap (which is lazy).