一度に実行する非同期処理の数を制限して、何回かに分けて処理する

var Pile = function() {
   this.pile = [];
   this.concurrency = 0;
   this.done = null;
   this.max_concurrency = 10;
}
Pile.prototype = {
  add: function(callback) {
   this.pile.push(callback);
  },
  run: function(done, max_concurrency) {
      this.done = done || this.done;
      this.max_concurrency = max_concurrency || this.max_concurrency;
      var target = this.pile.length;
      var that = this;
      var next = function() {
         that.concurrency--;
         (--target == 0 ? that.done() : that.run());
      };
      while(this.concurrency < this.max_concurrency && this.pile.length > 0) {
         this.concurrency++;
         var callback = this.pile.shift();
         callback(next);
      }
   }
};


pilex = new Pile();

var counter = 0;

for(var i = 0; i < 50; i++) {
   pilex.add( function test(next) { //実行する非同期処理を登録
      setTimeout( function() {
         counter++;
        echo(counter +" Hello world");
         next();
      }, 1500);
     }
   );
}

pilex.run(function() {  //登録した処理を実行
    echo("Done "+counter);
},5);  //一度に実行する非同期処理の制限数