The secondary thread of the balancer is meant to retrieve a stream of defragmentation command descriptors from the defragmentation policy, and to be put on pause when there is no new activity to be consumed.

The logic to suspend/resume the action of the thread is currently implemented within the policy, and it could be greatly simplified if it were brought back to the balancer (improving separation of concerns and allowing to create a generic synch infrastructure that could be reused to implement SERVER-64171).