Unserialized objects use significantly more memory than ones created with the normal constructor

Description

Consider the following code:

class SomeClass {
    public $a = 1;
    public $b = 2;
    public $c = 3;
    public $d = 4;
    public $e = 5;
    public $f = 6;
}

$s1 = serialize(new SomeClass());

function new_object() {
    return new SomeClass();
}

function unserialize_object() {
    global $s1;
    return unserialize($s1);
}


class OtherClass {
    public $a = 1;
    public $b = 2;
    public $c = 3;
    public $d = 4;
    public $e = 5;
    public $f = 6;
    
    public function __unserialize(array $attrs) {
        foreach ($attrs as $name => $value) 
            $this->$name = $value;
    }
}

$s2 = serialize(new OtherClass());

function new_object_other() {
    return new OtherClass();
}

function unserialize_object_other() {
    global $s2;
    return unserialize($s2);
}


foreach ([
    'new_object', 'unserialize_object',
    'new_object_other', 'unserialize_object_other',
] as $f) {
    $arr = [];
    for ($i = 0; $i < 50000; ++$i) {
        $arr[] = $f();
    }

    echo $f, " ", memory_get_usage() >> 20, "\n";
}

SomeClass contains some data. new_object() will create an instance, while unserialize_object() will unserialize an instance of exactly the same state (see $s1). When an array of 50k elements is created with one or another, the output for memory usage is as follows:

new_object 10
unserialize_object 43

I.e. the 50k instances created with the constructor use 10M memory, while the unserialized ones 43M. It seems that some data structures that are used while unserializing are not freed. However, when unsetting $arr to destroy the instances, memory consumption will drop to 1M, as expected.

Now consider OtherClass which has the __unserialize method. Nothing fancy, it just sets the attributes as the default handler would do. new_object_other() and unserialize_object_other() replicate the same logic as explained above, but for the new class. Output in this case is

new_object_other 10
unserialize_object_other 10

That is, memory consumption is the same regardless of how the objects are created.

Surplus memory usage grows with the number of attributes.

Tested with 8.1.2 and 8.2.0. Can also be seen on https://3v4l.org/ .

PHP Version

8.2.0

Operating System

No response

Beginne damit, das bereitgestellte PHP-Skript auszuführen und den Speicherverbrauch von memory_get_usage() für new_object(), unserialize_object() und die __unserialize()-Variante bei 50.000 Objekten zu vergleichen. Verfolge die an der Reproduktion beteiligten serialize()/unserialize()-Pfade. Als erledigt gilt die Aufgabe, wenn die entsprechenden standardmäßig deserialisierten Objekte nicht mehr wesentlich mehr Speicher behalten als normal konstruierte Objekte.