计算每秒字节数（平滑方式） [英] Calculating bytes per second (the smooth way)

问题描述

我正在寻找一种解决方案来计算重复调用的函数（下文）的每秒传输的字节数。由于它的不准确性，我不要 想简单地将发送的字节除以所用的总时间：它导致在运行几分钟后无法显示快速的速度变化。

I am looking for a solution to calculate the transmitted bytes per second of a repeatedly invoked function (below). Due to its inaccuracy, I do not want to simply divide the transmitted bytes by the elapsed overall time: it resulted in the inability to display rapid speed changes after running for a few minutes.

预设（大约每50ms调用一次）：

The preset (invoked approximately every 50ms):

function uploadProgress(loaded, total){
    var bps = ?;
    $('#elem').html(bps+' bytes per second');
};

• 如何获取（仅）最后一秒的平均字节数
  
• 有什么其他的计算非闪烁但精确的bps值的做法可用吗？ c> / li>
  
  • How to obtain the average bytes per second for (only) the last n seconds and is it a good idea?
  • What other practices for calculating a non-flickering but precise bps value are available?

  推荐答案

  你的第一个想法不错，它被称为移动平均线，并提供您定期调用更新功能，您只需要保留一个队列（一个 FIFO缓冲区）：

  Your first idea is not bad, it's called a moving average, and providing you call your update function in regular intervals you only need to keep a queue (a FIFO buffer) of a constant length:

  var WINDOW_SIZE = 10;
  var queue = [];
  
  function updateQueue(newValue) {
      // fifo with a fixed length
      queue.push(newValue);
      if (queue.length > WINDOW_SIZE)
          queue.shift();
  }
  
  function getAverageValue() {
  
      // if the queue has less than 10 items, decide if you want to calculate
      // the average anyway, or return an invalid value to indicate "insufficient data"
  
      if (queue.length < WINDOW_SIZE) {
  
          // you probably don't want to throw if the queue is empty, 
          // but at least consider returning an 'invalid' value in order to
          // display something like "calculating..."
  
          return null;
      }
  
      // calculate the average value
      var sum = 0;
      for (var i = 0; i < queue.length; i++) {
          sum += queue[i];
      }
      return sum / queue.length;
  }
  
  // calculate the speed and call `updateQueue` every second or so
  var updateTimer = setInterval(..., 1000);
  

  避免计算速度突然变化的更简单的方法是使用，rel =nofollow noreferrer> http://en.wikipedia.org/wiki/Low-pass_filter#Discrete-time_realization PT1滤波器的简单离散近似将是：

  An even simpler way to avoid sudden changes in calculated speed would be to use a low-pass filter. A simple discrete approximation of the PT1 filter would be:

  其中 u [k] 是输入（或实际值）在样本 k ， y [k] 是输出（或过滤值 ）在样本 k 和 T 是时间常数（较大 T 意味着 y 将跟随 u 更慢）。

  Where u[k] is the input (or actual value) at sample k, y[k] is the output (or filtered value) at sample k, and T is the time constant (larger T means that y will follow u more slowly).

  这将被翻译成如下：

  var speed = null;
  var TIME_CONSTANT = 5;
  
  function updateSpeed(newValue) {    
      if (speed === null) {
          speed = newValue;
      } else {
          speed += (newValue - speed) / TIME_CONSTANT;
      }
  }
  
  function getFilteredValue() {
      return speed;
  }
  

  这两种解决方案都会给出类似的结果（至少您的目的），而后一个似乎有点简单（并且需要更少的内存）。

  Both solutions will give similar results (for your purpose at least), and the latter one seems a bit simpler (and needs less memory).

  此外，我不会更新快速的值。过滤只能以50ms的刷新率将闪烁变为摆动。我不认为任何人都希望上传速度以每秒多次刷新率（甚至几秒钟）显示。

  Also, I wouldn't update the value that fast. Filtering will only turn "flickering" into "swinging" at a refresh rate of 50ms. I don't think anybody expects to have an upload speed shown at a refresh rate of more than once per second (or even a couple of seconds).

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