核心异步通道上的换能器

Question

If i have 10 buffered subscription channels each with a transducer that takes 5sec to execute. Are the transducers going to be executed concurrent or all on the same thread (assuming multithreaded context)?

Answer 1

it is quite easy to check, logging the thread, the transducer is being executed by:

(def log-chan (chan))

(go-loop []
  (println (<! log-chan))
  (recur))

(def channels
  (repeatedly
    10
    #(chan 10
       (map (fn [item]
              (let [thread (Thread/currentThread)]
                (go (>! log-chan thread)))
              item)))))

(doseq [c channels]
  (go (>! c :item)))

output:

#object[java.lang.Thread 0x77a39fa0 Thread[async-dispatch-27,5,main]]
#object[java.lang.Thread 0x7d5bf4d0 Thread[async-dispatch-31,5,main]]
#object[java.lang.Thread 0x53ecb32b Thread[async-dispatch-29,5,main]]
#object[java.lang.Thread 0x2b74f3ac Thread[async-dispatch-25,5,main]]
#object[java.lang.Thread 0x6eb50f9e Thread[async-dispatch-26,5,main]]
#object[java.lang.Thread 0x30701edb Thread[async-dispatch-30,5,main]]
#object[java.lang.Thread 0x1a370b69 Thread[async-dispatch-36,5,main]]
#object[java.lang.Thread 0x3d9884a2 Thread[async-dispatch-24,5,main]]
#object[java.lang.Thread 0x208941d0 Thread[async-dispatch-23,5,main]]
#object[java.lang.Thread 0x2c77aeb Thread[async-dispatch-28,5,main]]

so you can see, there are different threads.

But it really depends on a go block, inside which you send data to your channel, so if you change this:

(doseq [c channels]
  (go (>! c :item)))

to this:

(go (doseq [c channels]
  (>! c :item)))

you get this output:

#object[java.lang.Thread 0x16856295 Thread[async-dispatch-6,5,main]]
#object[java.lang.Thread 0x16856295 Thread[async-dispatch-6,5,main]]
#object[java.lang.Thread 0x16856295 Thread[async-dispatch-6,5,main]]
#object[java.lang.Thread 0x16856295 Thread[async-dispatch-6,5,main]]
#object[java.lang.Thread 0x16856295 Thread[async-dispatch-6,5,main]]
#object[java.lang.Thread 0x16856295 Thread[async-dispatch-6,5,main]]
#object[java.lang.Thread 0x16856295 Thread[async-dispatch-6,5,main]]
#object[java.lang.Thread 0x16856295 Thread[async-dispatch-6,5,main]]
#object[java.lang.Thread 0x16856295 Thread[async-dispatch-6,5,main]]
#object[java.lang.Thread 0x16856295 Thread[async-dispatch-6,5,main]]

so all channels' transducers would use the same thread

Answer 2

They will be executed in parallel, so on different real threads at the same time. Your machine might not have 10 cores however, so there will be some parking and blocking. You can look up the technical details here in the section 'Parking and Blocking'.