RateLimiter的 SmoothBursty（非warmup预热）及SmoothWarmingUp（预热，冷启动）

SmoothBursty

主要思想

记录 1秒内的微秒数/permitsPerSencond = 时间间隔interval,每一个interval可获得一个令牌
根据允许使用多少秒内的令牌参数，计算出maxPermits
setRate时初始化下次interval时间，及storedPermits

acquire时，计算当前nowMicros，如果大于下次interval时间时间，则更新storedPermits和下次interval时间，计算storedPermits能否满足此次acquire，如果能，则需要等待的时间为0，如果不能，则计算还需要多少微秒等待，并在非同步块外执行sleep操作

如果其他线程已经刷新了nextFreeTicketMicros，会如下情况acquire是无timeout的

Thread 1: acquire 11 -> storedPermits不能满足要求 -> waitTime = (acquire - stored) * stableIntervalMicros -> nextFreeTicketMicros += waitMicros ----->  out lock sleep
Thread 2: acquire 2 -> nowMicros < nextFreeTicketMicros , stored = 0,被线程1消耗完了 -> freshPermits = requiredPermits - storedPermitsToSpend 即 = requiredPermits -> waitTime = freshPermits * stableIntervalMicros
-> nextFreeTicketMicros += waitTime,此时的nextFreeTicketMicros包含了Thread1需要等待的时间 -------> out lock sleep a longer time

tryAquire(num,timeout)逻辑

timeoutMicros = timeout.toMicros
lock()
nowMicros = ...
canAcquire = nextFreeTicketMicros <= nowMicros + timeoutMicros
if(!canAcquire){
 return false;
}
else{
  microsToWait = ...
} 
unlock()
sleep(microsToWait)
return true;

SmoothWarmingUp

主要思想和SmoothBursty相似，由于带预热过程，刚开始由于availablePermitsAboveThreshold>0.0,速率会较慢，如果持续获取令牌，则会使availablePermitsAboveThreshold=0，速率变快

• 从0->thresholdPermits，生成一个令牌的时间：stableIntervalMicros

• 从thresholdPermits-> maxPermits ,生成一个令牌的时间：stableIntervalMicros + permits * slope;

  @Override
  final long reserveEarliestAvailable(int requiredPermits, long nowMicros) {
  resync(nowMicros);
  long returnValue = nextFreeTicketMicros;
  //当前需要且尽最大可能消费的
  double storedPermitsToSpend = min(requiredPermits, this.storedPermits);
  //新鲜permits个数，这些个数是一定会产生等待的，除了0
  double freshPermits = requiredPermits - storedPermitsToSpend;
  //计算需要wait的总时间
  long waitMicros =
  //非busty类型的storedPermitsToWaitTime直接返回0
  storedPermitsToWaitTime(this.storedPermits, storedPermitsToSpend)
  + (long) (freshPermits * stableIntervalMicros);
  //下次有票时间
  this.nextFreeTicketMicros = LongMath.saturatedAdd(nextFreeTicketMicros, waitMicros);
  this.storedPermits -= storedPermitsToSpend;
  return returnValue;
  }

   //已知permitsToTake <= storedPermits
   @Override
   long storedPermitsToWaitTime(double storedPermits, double permitsToTake) {
     //减去预热需要保留的permits，剩下的可消耗的数量
     double availablePermitsAboveThreshold = storedPermits - thresholdPermits;
     long micros = 0;
     // measuring the integral on the right part of the function (the climbing line)
     //如果有剩余可用的令牌
     if (availablePermitsAboveThreshold > 0.0) {
      //剩余可用的和需要获取的个数取小值
   	double permitsAboveThresholdToTake = min(availablePermitsAboveThreshold, permitsToTake);
   	// TODO(cpovirk): Figure out a good name for this variable.
   	//用可消耗的数量 + （可消耗的数量 - 实际消耗的数量）permitsToTime
   	//在预热阶段从thresholdPermits到maxPermits的耗时并非是stableIntervalMicros * n
   	//会耗费更多的时间，其计算规则不同，所以才需要把permitsAboveThresholdToTake从permitsToTake减去
   	//length 可能作为一个经验值，相当于补充permitsAboveThresholdToTake个令牌需要的平均时间值*2
                   //剩余可用的-实际需要且最大能消耗的令牌,得到最终剩余的令牌个数，可能是0
   	double length = permitsToTime(availablePermitsAboveThreshold)
   	    + permitsToTime(availablePermitsAboveThreshold - permitsAboveThresholdToTake);
   	//这里确实不好理解，从语义环境来说，它是从 thresholdPermits 到 maxPertmis 过程中
   	//生成 permitsAboveThresholdToTake 个令牌需要耗费的时间
   	//并且带coldFactor的构造函数不是public,SmoothWarmingUp也是private-package的
   	micros = (long) (permitsAboveThresholdToTake * length / 2.0);
   	//从permitsToTake中减去保留预热需留下个数后最终消耗的个数，这部分个数由于是提前存在的、富余的
   	//因此不需要计算到wait时间
   	permitsToTake -= permitsAboveThresholdToTake;
     }
     // measuring the integral on the left part of the function (the horizontal line)
     //如果没有剩余可用令牌，走的是stableIntervalMicros * n
     micros += (stableIntervalMicros * permitsToTake);
     return micros;
   }	
  

length/2可以理解为下图

	//permits值越小，需要的时间就越少，值越大，需要的时间就越大
	private double permitsToTime(double permits) {
	  //double coldIntervalMicros = stableIntervalMicros * coldFactor;
	  // thresholdPermits = 0.5 * warmupPeriodMicros / stableIntervalMicros;
      //maxPermits =
      thresholdPermits + 2.0 * warmupPeriodMicros / (stableIntervalMicros + coldIntervalMicros);
      //slope带比率的时间，可以理解为增长因子
	  //slope =  (coldIntervalMicros - stableIntervalMicros) / (maxPermits - thresholdPermits)
	  //return表示成这样更易于理解 stableIntervalMicros + (coldIntervalMicros - stableIntervalMicros) * (permits/(maxPermits - thresholdPermits))
	  return stableIntervalMicros + permits * slope;
	}