Backend-level expiration / resource deallocation

Can someone please either confirm or refute my understanding: unless I specify the arguments.memcached_expire_time config option, the keys/values never actually expire in the Memcached itself. It's just that Dogpile considers them expired by checking the expiration times, but actual data don't get deallocated from the underlying Memcached storage (which is RAM). This is probably also true for other backends.

I'm actually concerned about resource deallocation by Memcached. And I guess the reason that the current behavior does not lead to (m)any complaints/problems in practice is that Memcached is typically configured with some memory limit, and when it's reached it's just starting to evict old keys.

So, to summarize it: under typical configuration of Dogpile (no arguments.memcached_expire_time specified), cached data is not deleted from Memcached leading to more resource usage than with that config option set to a reasonable timeout.

(Of course, I'm assuming that we're not calling cache_region.delete() explicitly.)

Start by tracing how arguments.memcached_expire_time affects Memcached entries and how cache_region.delete() is handled. Compare expiration and resource usage with and without that option, then establish whether the reported backend-level deallocation behavior is expected and what change or documentation would be needed.