HDFS源码-5 FairCallQueue

1.加权轮询调度
WeightedRoundRobinMultiplexer

/**
 * Determines which queue to start reading from, occasionally drawing from
 * low-priority queues in order to prevent starvation. Given the pull pattern
 * [9, 4, 1] for 3 queues:
 *
 * The cycle is (a minimum of) 9+4+1=14 reads.
 * Queue 0 is read (at least) 9 times
 * Queue 1 is read (at least) 4 times
 * Queue 2 is read (at least) 1 time
 * Repeat
 *
 * There may be more reads than the minimum due to race conditions. This is
 * allowed by design for performance reasons.
 */
 /**
  *确定从哪个队列开始读取，偶尔从中抽取
  *低优先级队列，以防止饥饿。 鉴于拉动模式
  * [9,4,1]为3个队列：
 *
  *周期是（最少）9 + 4 + 1 = 14次读数。
  *队列0被读取（至少）9次
  *队列1被读取（至少）4次
  *队列2被读取（至少）1次
  *重复
 *
  *由于竞争条件，可能会有比最小值更多的读数。 这是
  *出于性能原因，设计允许。
 */