Queue.get_nowait() sometimes bogusly returns Empty

[Aahz]

In article <mailman.1026837865.32291.python-list at python.org>,
Geoffrey Talvola  <gtalvola at nameconnector.com> wrote:
>
>In case you're wondering, this is in Webware's WebKit application server
>where there is a pool of N threads, and a queue of (non-thread-safe) servlet
>instances acting as a cache.  We only want to create servlet instances when
>all existing instances are in-use.  When a given thread needs a servlet, it
>basically does:
>
>	try:
>		instance = cache.get_nowait()
>	except Queue.Empty:
>		instance = new_instance()
>	# use the instance...
>	cache.put(instance)
>
>The problem is that every once in a while get_nowait() returns Empty when
>the queue isn't actually empty, so the size of the cache grows slowly and
>unboundedly over time.  But I'd prefer for the cache to contain at most N
>instances where N is the number of threads.

You've got several options.  Simplest is to change the exception so that
it only creates new instances when there are fewer than N instances.  

What I'd consider doing, though, is make a data queue instead of a
servlet cache queue, such that all the servlet threads grab their data
from that data queue.  Then you have a separate monitor thread that
periodically checks the data queue and creates new servlet threads when
it hits "too many" items.  Conversely, if the data queue stays empty,
you delete servlet instances (by passing quit tokens in the data queue
would be simplest).
-- 
Aahz (aahz at pythoncraft.com)           <*>         http://www.pythoncraft.com/

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