title: APScheduler opens more threads
body: <p>[Python 3.5.2, APScheduler 3.3.1]</p>
  
  <p>APScheduler starts a number of threads and I would like to know why.</p>
  
  <p>This is the code I'm executing (in PyCharm, where I can also graph the threads):</p>
  
  <pre><code>from apscheduler.schedulers.background import BackgroundScheduler
  import time
  
  def process_to_execute():
      time.sleep(0.5)
  
  scheduler = BackgroundScheduler()
  scheduler.add_job(process_to_execute, 'cron', second="*/1")
  scheduler.start()
  
  while True:
      time.sleep(1)
  </code></pre>
  
  <p>The threads graph is:
  <a href="https://i.stack.imgur.com/7vkiZ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/7vkiZ.png" alt="threads_graph"></a></p>
  
  <p>Why is APScheduler creating so many threads when one would be enough?</p>
  
  <p>Thread-7 is created at 1.6 sec. and its task finishes at 2.1 sec.
  A new task is executed at 2.6 sec: instead of using immediately Thread-7, Thread-8 is created but the task is executed in Thread-7, leaving thread-8 empty... Is there a reason for this? </p>
  
  <p>The number of scheduler's threads is limited to 10. </p>