cluster.schedulingPolicy

调度策略，包括循环计数的 cluster.SCHED_RR，以及由操作系统决定的cluster.SCHED_NONE。 这是一个全局设置，当第一个工作进程被衍生或者调动cluster.setupMaster()时，都将第一时间生效。

除Windows外的所有操作系统中，SCHED_RR都是默认设置。只要libuv可以有效地分发IOCP handle，而不会导致严重的性能冲击的话，Windows系统也会更改为SCHED_RR。

cluster.schedulingPolicy 可以通过设置NODE_CLUSTER_SCHED_POLICY环境变量来实现。这个环境变量的有效值包括"rr" 和 "none"。

The scheduling policy, either cluster.SCHED_RR for round-robin or cluster.SCHED_NONE to leave it to the operating system. This is a global setting and effectively frozen once either the first worker is spawned, or cluster.setupMaster() is called, whichever comes first.

SCHED_RR is the default on all operating systems except Windows. Windows will change to SCHED_RR once libuv is able to effectively distribute IOCP handles without incurring a large performance hit.

cluster.schedulingPolicy can also be set through the NODE_CLUSTER_SCHED_POLICY environment variable. Valid values are 'rr' and 'none'.