Storing DOMElement consume 4 times more memory in PHP 8.1 than in PHP 8.0

[steffunky]

Description

My application uses DOMElement to work on large XML files and manipulates them extensively. I observed a significant increase in peak memory usage between PHP 8.0 and PHP 8.1 while running the same code base, despite performing the same number of operations (200k DOMElement::setAttribute calls).

Peak memory usage increased from 30MB in PHP 8.0 to 100MB in PHP 8.1, despite similar operations being performed.
The issue persists across multiple PHP minor versions from PHP 7.4.33 to PHP 8.3.6.

I attempted to reproduce the issue in a minimal code snippet and observed that stored DOMElement objects consume increasingly more memory with each PHP version.

https://3v4l.org/Aoi63

Resulted in this output:

With 25 000 DOMElement

PHP Version Range	Memory Usage
8.3.0 - 8.3.6	16 MB
8.1.0 - 8.1.28, 8.2.0 - 8.2.18	14 MB
7.0.0 - 7.0.33, 7.1.0 - 7.1.33, 7.2.0 - 7.2.34, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30	4 MB

With 40 000 DOMElement

PHP Version Range	Memory Usage
8.3.0 - 8.3.6	24 MB
8.2.0 - 8.2.18, 8.1.0 - 8.1.28	22 MB
7.0.0 - 7.0.33, 7.1.0 - 7.1.33, 7.2.0 - 7.2.34, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30	8 MB

But I expected memory usage to remain consistent across PHP versions, especially when performing the same operations.

PHP Version

PHP 8.1.0-8.3.6

Operating System

No response

[nielsdos]

The initial memory increase happened because of #6644.

Here is what happens:
The DOM properties are virtual properties, i.e. they're kind of like "property hooks" but implemented inside the extension+engine. In PHP 8.0 and earlier, the properties weren't declared on the classes, but since that PR I linked above, they are declared. When they became declared, they started occupying memory inside the object's property table. This is unfortunate because that memory is never used because the properties are virtual.

The subsequent memory increase in PHP 8.3 is also an indirect consequence for this: when I added the new properties in PHP 8.3 for DOM classes, the backing zvals started to consume memory.

I previously opened a feature request to make it possible in the engine to declare a property without creating the backing storage in the object's property table: #11644
This will become possible when property hooks land in PHP 8.4.

Unfortunately, this means that it's not possible to fix this issue in stable versions, because it requires modifications to the (public) engine APIs.

[nielsdos]

Based on Ilija's property hooks branch, I wrote a patch to mark the properties as virtual. It's incomplete (I only did DOMNode and DOMElement), but the memory usage is decreased back to what it was: https://gist.github.com/nielsdos/41d4f58e192967fef5c7b1b70fb88aea
Leaving this here for myself in the future after property hooks gets merged.

[steffunky]

Great, many thanks for the quick follow-up!