We maintain an unbounded cache of WiredTiger sessions. And each of these sessions has a cursor cache.

Because we asynchronously release sessions into the cache without a ticket, we can accumulate more sessions than the execution control system can limit.

The current limit is just a 5 minute idle timeout.

The size of these idle sessions can affect WiredTiger performance in some algorithms. We should impose a limit on the number of cached sessions and also expose metrics about the size of these caches.

There is some discussion here in SERVER-84449.