Hi,
We use mongo replicaSet version 3.4.2 on our production environments. The replicaSet works with 6 nodes , one primary and 5 secondaries. Few weeks ago we have started to get alerts on high cpu consumption on the primary node. At the beginning we thought that we should separate the load between the members so we configured our service to make reads from secondaries and write to the primary.

Unfortunately we are still on the same point. The cpu of the primary node is between 70-100% even on idle times while the secondaries consumes maximum of 20%.

It is important to mention that we creates DB for each one of our customers with average of 9 indexes each - we have 1500DB's like that.

Attached a log of db.currentOp() , mongostat and diagnostic metrics. let me know if more information required.

Thanks,
Roie.