import threading
import subprocess

class MyThread(threading.Thread):
    def run(self):
        x = subprocess.Popen(['ls', '-l'],
                             stdout=subprocess.PIPE,
                             stderr=subprocess.PIPE)
        x.communicate()

if __name__ == '__main__':
    threads = []
    for i in range(500):
        t = MyThread()
        threads.append(t)
        t.start()

    for t in threads:
        t.join()