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

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 is 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)))

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.