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 ques…

Accepted Answer

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.

Python: penalty for sleeping threads

Solution

Problem