The current implementation of WiredTiger maintains enough version history to support reads to the oldest active transaction in the system. When operating as part of a distributed database, it will be useful to keep more history to help establish common points in time among nodes.

MongoDB will maintain a sliding window of history by having the calculated oldest_timestamp lag the current calculation by a sliding amount. That work will happen in SERVER-31767.

What that project needs is a new method that queries how much pressure there is on the cache due to required history constraints. It may be that some existing statistics can be repurposed to answer the question "is history causing cache pressure?".