BlockTransferThrottler(hadoop中用于控制处理带宽的util类)

hadoop中用该类来控制对于数据的处理速度 ，主要思想如下：

1、将数据处理分为一个个的周期（period）

2、每个周期内处理的bytes固定 。

3、定义每次处理周期的开始时间 。

4、每次处处理完后调用BlockTransferThrottler.throttle(long numOfBytes)方法，若是还未到周期的结束时间，已经将处理了相应的bytes，则该线程wait 。

代码以及注视如下：

package org.apache.hadoop.hdfs.server.datanode;

/** 
 * a class to throttle the block transfers.
 * This class is thread safe. It can be shared by multiple threads.
 * The parameter bandwidthPerSec specifies the total bandwidth shared by
 * threads.
 * 该类用于控制数据传输的速率
 * 主要是对于每次处理（读/写）分成周期（period），若是该线程的处理速度太快（在这个周期时间还未结束，已经处理了所有的bytes）
 * 则wait该线程一会时间，详见93行
 */
class BlockTransferThrottler {
  private long period;          // period over which bw is imposed       处理周期 
  private long periodExtension; // Max period over which bw accumulates. 处理的最大周期（一般周期的3倍）
  private long bytesPerPeriod; // total number of bytes can be sent in each period  每个时间周期发送的字节数
  private long curPeriodStart; // current period starting time    当前周期开始的时间
  private long curReserve;     // remaining bytes can be sent in the period  剩余可发送的字节数
  private long bytesAlreadyUsed;            //已经发送的字节数

  /** Constructor 
   * @param bandwidthPerSec bandwidth allowed in bytes per second. 
   */
  BlockTransferThrottler(long bandwidthPerSec) {
    this(500, bandwidthPerSec);  // by default throttling period is 500ms 
  }

  /**
   * Constructor
   * @param period in milliseconds. Bandwidth is enforced over this
   *        period.
   * @param bandwidthPerSec bandwidth allowed in bytes per second. 
   */
  BlockTransferThrottler(long period, long bandwidthPerSec) {
    this.curPeriodStart = System.currentTimeMillis();
    this.period = period;
    this.curReserve = this.bytesPerPeriod = bandwidthPerSec*period/1000;
    this.periodExtension = period*3;
  }

  /**
   * @return current throttle bandwidth in bytes per second.
   * 返回处理数据的带宽   bytes per second
   */
  synchronized long getBandwidth() {
    return bytesPerPeriod*1000/period;
  }
  
  /**
   * Sets throttle bandwidth. This takes affect latest by the end of current
   * period.
   * 
   * @param bytesPerSecond 
   */
  synchronized void setBandwidth(long bytesPerSecond) {
    if ( bytesPerSecond <= 0 ) {
      throw new IllegalArgumentException("" + bytesPerSecond);
    }
    bytesPerPeriod = bytesPerSecond*period/1000;
  }
  
  /** Given the numOfBytes sent/received since last time throttle was called,
   * make the current thread sleep if I/O rate is too fast
   * compared to the given bandwidth.
   *
   * @param numOfBytes
   *     number of bytes sent/received since last time throttle was called
   */
  synchronized void throttle(long numOfBytes) {
    if ( numOfBytes <= 0 ) {
      return;
    }

    curReserve -= numOfBytes;
    bytesAlreadyUsed += numOfBytes;
    //一个周期内数据已经发送完
    while (curReserve <= 0) {
      long now = System.currentTimeMillis();
      long curPeriodEnd = curPeriodStart + period;
      //当前时间<（开始时间+周期长度），即提前完成，则wait
      if ( now < curPeriodEnd ) {
        // Wait for next period so that curReserve can be increased.
        try {
          wait( curPeriodEnd - now );
        } catch (InterruptedException ignored) {}
      } else if ( now <  (curPeriodStart + periodExtension)) {
        curPeriodStart = curPeriodEnd;
        curReserve += bytesPerPeriod;
      } else {
        // discard the prev period. Throttler might not have
        // been used for a long time.
        curPeriodStart = now;
        curReserve = bytesPerPeriod - bytesAlreadyUsed;
      }
    }

    bytesAlreadyUsed -= numOfBytes;
  }
}