Why GD doesn't release memory upon imagedestroy()?

问题

I need a PHP CLI script to do various image transformations via GD functions. The images are quite big so I need to squeeze as much memory as possible. However, imagedestroy() seems not to release memory when asked.

Consider following demo script '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;
    }
    ?>

Let's run it by issuing:

free -m;./test.php;free -m

Example results:

                 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

As you can see, creating one image costs 109-110MB. After creating second one we have double that used up. But destroying first doesn't release memory. All the resources' memory gets released only after both images are destroyed.

Why? Have I overlooked something? What should I do to amend it?

UPDATE: I added setting $res1 to null and then unsetting it altogether. Code:

    $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');

Now result is:

    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)

Also, I added

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

right before imagedestroy($res2); and then things go strange:

    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)

According to 'free' command, the memory gets released only after script ended.

回答1:

I wonder if it's GC... try running gc_collect_cycles() after imagedestroy(). Long shot, but you never know...

来源：https://stackoverflow.com/questions/32320212/why-gd-doesnt-release-memory-upon-imagedestroy

标签

php

memory