为什么垃圾收集器在JSON.stringify应用后需要花费很多时间来释放内存？

Question

以下是NodeJS代码的示例：

function toMB (byteVal) {
    return (byteVal / 1048576).toFixed(2);
}

console.log('Node memory usage:', toMB(process.memoryUsage()['heapUsed']) + ' MB');

Array(10000000).join('qweqweqweqweqweqweq');

var sec = 0;

setInterval(function(){
    console.log('Node memory usage:',  toMB(process.memoryUsage()['heapUsed']) + ' MB (after', ++sec + ' sec.)');
}, 1000);

我得到了以下结果：

Node memory usage: 2.00 MB
Node memory usage: 183.31 MB (after 1 sec.)
Node memory usage: 183.32 MB (after 2 sec.)
Node memory usage: 183.32 MB (after 3 sec.)
...
Node memory usage: 183.83 MB (after 128 sec.)
Node memory usage: 183.83 MB (after 129 sec.)
Node memory usage: 183.83 MB (after 130 sec.)
Node memory usage: 2.13 MB (after 131 sec.)
Node memory usage: 2.21 MB (after 132 sec.)
Node memory usage: 2.21 MB (after 133 sec.)

垃圾收集器可用内存。 但是如果我使用相同字符串长度的JSON.stringify：

function toMB (byteVal) {
    return (byteVal / 1048576).toFixed(2);
}

console.log('Node memory usage:', toMB(process.memoryUsage()['heapUsed']) + ' MB');

JSON.stringify(Array(10000000).join('qweqweqweqweqweqweq'));

var sec = 0;

setInterval(function(){
    console.log('Node memory usage:',  toMB(process.memoryUsage()['heapUsed']) + ' MB (after', ++sec + ' sec.)');
}, 1000);

我得到了以下结果：

Node memory usage: 2.00 MB
Node memory usage: 364.22 MB (after 1 sec.)
Node memory usage: 364.24 MB (after 2 sec.)
Node memory usage: 364.24 MB (after 3 sec.)
...
Node memory usage: 365.60 MB (after 482 sec.)
Node memory usage: 365.60 MB (after 483 sec.)
Node memory usage: 365.60 MB (after 484 sec.)
Node memory usage: 2.21 MB (after 485 sec.)
Node memory usage: 2.21 MB (after 486 sec.)

为什么时差（485 vs 131）如此之大？