Python: penalty for sleeping threads

Question

this question relates to performance penalities that may or may not arise from having a large number of sleeping python threads on a webserver.

Background: I am implementing an online shop using django/satchmo. A requirement is for delayed payment. The customer can reserve a product and allow a third party to pay for it at a later date (via a random and unique URL).

To handle unreserving an item I am creating a thread which will sleep for the reservation time and then remove the reservation/mark the product as sold when it awakes. It looks like this:

#Reserves a product when it is placed in the cart
def reserve_cart_product(product):
  log.debug("Reserving %s" % product.name)
  product.active = False
  product.featured = False
  product.save()
  from threading import Timer
  Timer(CART_RESERVE_TIME, check_reservation, (product,)).start()

I am using the same technique when culling the unique URLs after they have expired, only the Timer sleeps for much longer (typically 5 days).

So, my question to you SO is as follows:

Is having a large numnber of sleeping threads going to seriously effect performance? Are there better techniques for scheduling a one off event sometime in the future. I would like to keep this in python if possible; no calling at or cron via sys.

The site isn't exactly high traffic; a (generous) upper limit on products ordered per week would be around 100. Combined with cart reservation, this could mean there are 100+ sleeping threads at any one time. Will I regret scheduling tasks in this manner?

Thanks

Answer 1

I see no reason why this shouldn't work. The underlying code for Timer (in threading.py) simply uses time.sleep. Once it's been waiting for awhile, it basically runs a loop with time.sleep(0.05) This should result in CPU usage of basically 0%, even with hundreds of threads. Here's a simple example, where I noticed 0% cpu usage for the python process:

import threading

def nothing():
    pass

def testThreads():
    timers = [threading.Timer(10.0, nothing) for _ in xrange(881)]
    print "Starting threads."
    map(threading.Thread.start, timers)
    print "Joining threads."
    map(threading.Thread.join, timers)
    print "Done."

if __name__ == "__main__":
    testThreads()

The real issue is that you may not be able to actually start too many threads. On my 64-bit 4GB system, I can only start 881 threads before I get an error. If you really will only have a few hundred, though, I can't imagine it won't work.