The right way to limit maximum number of threads running at once?

Question

I'd like to create a program that runs multiple light threads, but limits itself to a constant, predefined number of concurrent running tasks, like this (but with no risk of race condition):

import threading  def f(arg):     global running     running += 1     print("Spawned a thread. running=%s, arg=%s" % (running, arg))     for i in range(100000):         pass     running -= 1     print("Done")  running = 0 while True:     if running < 8:         arg = get_task()         threading.Thread(target=f, args=[arg]).start() 

What's the safest/fastest way to implement this?

Answer 1

It sounds like you want to implement the producer/consumer pattern with eight workers. Python has a Queue class for this purpose, and it is thread-safe.

Each worker should call get() on the queue to retrieve a task. This call will block if no tasks are available, causing the worker to go idle until one becomes available. Then the worker should execute the task and finally call task_done() on the queue.

You would put tasks in the queue by calling put() on the queue.

From the main thread, you can call join() on the queue to wait until all pending tasks have been completed.

This approach has the benefit that you are not creating and destroying threads, which is expensive. The worker threads will run continuously, but will be asleep when no tasks are in the queue, using zero CPU time.

(The linked documentation page has an example of this very pattern.)

Answer 2

semaphore is a variable or abstract data type that is used to control access to a common resource by multiple processes in a concurrent system such as a multiprogramming operating system; this can help you here.

threadLimiter = threading.BoundedSemaphore(maximumNumberOfThreads)  class MyThread(threading.Thread):      def run(self):         threadLimiter.acquire()         try:             self.Executemycode()         finally:             threadLimiter.release()      def Executemycode(self):         print(" Hello World!")          # <your code here> 

This way you can easily limit the number of threads that will be executed concurrently during the program execution. Variable, 'maximumNumberOfThreads' can be used to define an upper limit on the maximum value of threads.

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