multithreading - Parallel doseq for Clojure

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

OK, I think what I want is to have an agent for each loop, with the data sent to the agent using send. The agents triggered using send are run from a thread pool, so the number is limited in some way (it doesn't give the fine-grained control of exactly three threads, but it'll have to do for now).

[Dave Ray explains in comments: to control pool size I'd need to write my own]

(defmacro dopar [seq-expr & body]
  (assert (= 2 (count seq-expr)) "single pair of forms in sequence expression")
  (let [[k v] seq-expr]
    `(apply await
       (for [k# ~v]
         (let [a# (agent k#)]
           (send a# (fn [~k] ~@body))
         a#)))))

which can be used like:

(deftest test-dump
  (dopar [n (range 7 11)]
    (time (do-dump-single "/tmp/single" "a" n 10000000))))

Yay! Works! I rock! (OK, Clojure rocks a little bit too). Related blog post.