为什么GD在imagedestroy()时不释放内存

我需要一个PHP CLI脚本通过GD函数进行各种图像转换。这些图像非常大，所以我需要尽可能多地压缩内存。然而，imagedestroy()在被请求时似乎不释放内存。

考虑以下演示脚本'test.php':

    #!/usr/bin/php5
    <?php
    $memory = [0=> memoryCheck()];
    $res1 = imagecreatetruecolor(8192, 4096);
    memoryReport('Res1 created');
    $res2 = imagecreatetruecolor(8192, 4096);
    memoryReport('Res2 created');
    imagedestroy($res1);
    memoryReport('Res1 destroyed');
    imagedestroy($res2);
    memoryReport('Res2 destroyed');
    // memory reporting functions follow:
    function memoryCheck()
    {
        return (int) trim(substr(shell_exec('free -b'), 166, 11));
    }
    function format($value)
    {
        $val = 'abs($value);
        $unit=array('B','kiB','MiB','GiB','TiB','PiB');
        return @round($val/pow(1024,($i=floor((($val==0)?0:log($val,1024))))),2).' '.$unit[$i];
    }
    function memoryReport($msg)
    {
        global $memory;
        $start = $memory[0];
        $prev = end($memory);
        $now = memoryCheck();
        echo sprintf("%s: %s (%s)'n", 
                        $msg, 
                        format($now-$start), 
                        (($diff=$now-$prev) <0) ? '-'. format($diff) : '+'. format($diff)
                    );
        $memory[] = $now;
    }
    ?>

让我们通过发出:

./test.php;

结果示例:

                 total       used       free     shared    buffers     cached
    Mem:          7890       7072        818        561        218       2497
    -/+ buffers/cache:       4355       3534
    Swap:         8299          0       8299
    Res1 created: 109.76 MiB (+109.76 MiB)
    Res2 created: 218.77 MiB (+109.01 MiB)
    Res1 destroyed: 218.9 MiB (+140 kiB)
    Res2 destroyed: 888 kiB (-218.04 MiB)
                 total       used       free     shared    buffers     cached
    Mem:          7890       7072        817        561        218       2498
    -/+ buffers/cache:       4356       3534
    Swap:         8299          0       8299

可以看到，创建一个图像需要109-110MB。在创建第二个后，我们用掉了两倍。但是先销毁并不能释放记忆。所有资源的内存只有在两个镜像都被销毁后才会被释放。

为什么?我是不是忽略了什么?我该怎么修改呢?

更新:我将$res1设置为null，然后将其全部取消设置。代码:

    $memory = [0=> memoryCheck()];
    $res1 = imagecreatetruecolor(8192, 4096);
    memoryReport('Res1 created');
    $res2 = imagecreatetruecolor(8192, 4096);
    memoryReport('Res2 created');
    imagedestroy($res1);
    memoryReport('Res1 destroyed');
    $res1 = null;
    memoryReport('Res1 is null');
    unset($res1);
    memoryReport('Res1 is unset');
    imagedestroy($res2);
    memoryReport('Res2 destroyed');

现在的结果是:

    Res1 created: 109.48 MiB (+109.48 MiB)
    Res2 created: 219.33 MiB (+109.85 MiB)
    Res1 destroyed: 219.5 MiB (+168 kiB)
    Res1 is null: 220.15 MiB (+668 kiB)
    Res1 is unset: 220.38 MiB (+232 kiB)
    Res2 destroyed: 2 MiB (-218.36 MiB)

我还添加了

    gc_collect_cycles();
    memoryReport('GC collect');

imagedestroy($res2);然后事情变得奇怪了:

    Res1 created: 109.59 MiB (+109.59 MiB)
    Res2 created: 219.08 MiB (+109.5 MiB)
    Res1 destroyed: 219.21 MiB (+132 kiB)
    Res1 is null: 219.36 MiB (+148 kiB)
    Res1 is unset: 219.75 MiB (+408 kiB)
    GC collect: 220.57 MiB (+836 kiB)
    Res2 destroyed: 220.46 MiB (-108 kiB)

根据'free'命令，只有在脚本结束后才会释放内存