利用chrome的Timeline观测系统的内存回收逻辑

Jerry Wang

发布于 2019-06-13 20:52:32

2700

发布于 2019-06-13 20:52:32

测试代码如下:

<html>
<script>
var count = 0;
var number = 1000;
for (var i = 0; i < 900; i++) {
  var buggyObject = {
    callAgain: function() {
      var ref = this;
      var val = setTimeout(function() {
        ref.callAgain();
        var newArray = new Array();
        if( count > 3)
         number = 10000;
        for( var a = 0; a < number; a++)
         newArray[a] = a;
        console.log("called");
        count++;
      }, 1000); // end of val declaration
    } // end of call again
  } // end of object declaration
}
buggyObject.callAgain();
</script>
</html>

通过tab timeline观测到在第3900ms时，系统有一个garbage collection的operation, 总共回收内存425k:

garbage collection能够生效的原因是因为在每次timer trigger并且执行完毕后，当前execution environment里的局部变量newArray 引用计数为0，因此可以被回收:

如果将该定义成局部变量的array改成全局变量，则GC不会再发生:

<html>
<script>
var count = 0;
var number = 1000;
var globalArray = [];
for (var i = 0; i < 900; i++) {
  var buggyObject = {
    callAgain: function() {
      var ref = this;
      var val = setTimeout(function() {
        ref.callAgain();
        if( count > 3)
         number = 10000;
        for( var a = 0; a < number; a++)
         globalArray.push(a);
        console.log("called");
        count++;
      }, 1000); // end of val declaration
    } // end of call again
  } // end of object declaration
}
buggyObject.callAgain();
</script>
</html>