Load balancing is a major concern in massively parallel computing. X10 is a partitioned global address space language for scale-out computing and provides a global load balancing (GLB) library that shows high scalability over ten thousand CPU cores. This study proposes a multistage mechanism for GLB to assign execution stages to tasks and introduces a multithread design into GLB to allow efficient data sharing between CPU cores. The system gives high priority to tasks that are assigned to earlier stages and then proceeds with subsequent stage tasks. When a computing node runs out of tasks at the earliest stage, it requests tasks at the earliest stage from other nodes and awaits responses by processing subsequent stage tasks. When the system identifies the task termination at a certain stage, it executes a reduction operation over nodes. Programmers can define their reduction operations to gather or exchange results of completed tasks. This study provides the implementation method of the extended library and evaluates its runtime overhead using the K computer to a maximum of 256 nodes.