Please be very, very careful if you are using memcache for sessions. There is this bug: MDL-45375
Well, it is not really a bug, more a limitation of the memcache API. However, the up-shot is that if you are storing sessions in a particular memcache store, then you must not store any MUC caches in the same memcache store.
If you do, then any time that Moodle purges any memcache cache, every user will lose their session. And, it seems that Moodle does purge various caches in various places from time-to-time.
They are working on a warning for the admin notifications screen: MDL-45724.
Also, because of this bug, I just switched on server back from memcache sessions to DB sessions. Before doing that, I did a quick load-test, and I was surprised to find how similar the performance was between memcache sessions and DB sessions. Now this was on a very well-tuned Postgres database, but still food for thought.
(All the OU's busy sites are still on memcache sessions. In terms of Moodle scalability, the database is a single bottle-neck that is hard to scale. Hence, on a large a busy site, it does make sense to get the session storage off the DB server. However, if you are moving it to memcache storage, it must be a separate set of memcache storage, not shared with your caches.)