When a HighRAMJob turns up at the head of the queue, the current implementation of support for HighRAMJobs in the Capacity Scheduler has problem in that the scheduler stops assigning tasks to all TaskTrackers in the cluster until a HighRAMJob finds a suitable TaskTrackers for all its tasks.
This causes a severe utilization problem since effectively no new tasks are allowed to run until the HighRAMJob (at the head of the queue) gets slots.