In our production setup, we see a rather large (5-6GB) RAM footprint for the config servers. This seems an unreasonable amount of RAM used by the config server. The DB servers in the cluster use approximately 10GB of RAM.

We have 2 shards, both consisting of a 3-member replica set. For this cluster we have 3 config servers setup in a replica set. Furthermore, we have approximately 15 mongos processes running on various machines.

I can't imagine that this level of memory usage is by design or intended. It is unclear to me at the moment what is causing the size of the footprint, and what will cause it to increase. This makes it impossible to determine how this will scale with future changes to the data and setup.

I've attached the (sanitized w.r.t. host names) output of serverStatus on both the primary and a secondary config server. My question is whether or not this behaviour is expected. If so, what causes it to scale? If not, what is causing it?