When do expired cached items get deleted from the cache in Memcache

Memcached uses a lazy expiration, which means it uses no extra cpu expiring items. When an item is requested (a get request) it checks the expiration time to see if the item is still valid before returning it to the client.

Similarly when adding a new item to the cache, if the cache is full, it will look at for expired items to replace before replacing the least used items in the cache.