介绍在多道程序系统中，一个作业从提交到执行，通常都要经历多级调度，如高级调度、低级调度、中级调度以及I/O调度等。而系统的运行性能，如吞吐量的大小、周转时间的长短、响应的及时性等，在很大程度上都取决于调度，因而调度便成了多道系统的关键。-introduced in the multi-program systems, operating from a submission to the Executive, normally you have to go through multi-level scheduling, such as higher scheduling, dispatching junior, intermediate Scheduling and I / O scheduling. While the operating system capabilities, such as throughput size, the length of turnaround time, the timeliness of response, will depend largely on scheduling, which has become more scheduling system to the key.