Clojure core.async, any way to control number of threads in that (go…) thread pool?

Question

By default (go..) will use twice the number of cores + 42 threads for the thread pool. Is there any way I can set the number of threads, or number of CPUs that the code can

Answer 1

The current accepted answer was valid up to this commit so basically now you have two cases:

• If you want to just change the max number of threads in the pool, you pass the number as Java property clojure.core.async.pool-size (it defaults to 8)

• If you want to replace the ExecutorService, you use the same trick of alter-var-root but targeting the new implementation (there a protocol to implement):

  (ns your-app.threadpool
    (:require [clojure.core.async.impl.protocols :as protocols]
              [clojure.core.async.impl.concurrent :as conc]
              [clojure.core.async.impl.exec.threadpool :as tp])
    (:import java.util.concurrent.Executors))
  
  (defonce my-executor
    (let [executor-svc (Executors/newFixedThreadPool
                        1
                        (conc/counted-thread-factory "async-dispatch-%d" true))]
      (reify protocols/Executor
         (protocols/exec [this r]
           (.execute executor-svc ^Runnable r)))))
  
  (alter-var-root #'clojure.core.async.impl.dispatch/executor
                  (constantly (delay my-executor)))
  

Answer 2

In the current Clojure version of core.async, the thread pool executor is located in the clojure.core.async.impl.dispatch namespace. You can alter the executor var and supply a custom thread pool ExecutorService.

(ns sandbox
  (:require [clojure.core.async.impl.concurrent :as conc]
            [clojure.core.async.impl.exec.threadpool :as tp]
            [clojure.core.async :as async]))

(defonce my-executor
  (java.util.concurrent.Executors/newFixedThreadPool
   1
   (conc/counted-thread-factory "my-async-dispatch-%d" true)))

(alter-var-root #'clojure.core.async.impl.dispatch/executor
                (constantly (delay (tp/thread-pool-executor my-executor))))

(async/go
 (println 
  (Thread/currentThread))) ;=> #<Thread Thread[my-async-dispatch-1,5,main]>

Note: Core.async is still in alpha, so, hopefully, this will change in the future.