DFS Scalability: When the namenode is restarted it consumes 80% CPU

When the namenode is restarted, the datanodes register and each block is inserted into neededReplication. When the namenode exists, safemode it sees starts processing neededReplication. It picks up a block from neededReplication, sees that it has already has the required number of replicas, and continues to the next block in neededReplication. The blocks remain in neededReplication permanentlyhe namenode worker thread to scans this huge list of blocks once every 3 seconds. This consumes plenty of CPU on the namenode.