PHP memory leak

[adamziel]

The issue below was reported by @sejas. It negatively impacts user experience of Playground and wp-now. Let's make sure:

• PHP has ample memory limit available. Confirm php.ini defaults to, say, 1GB memory.
• php_wasm.c doesn't contribute to the memory leak problem. PHP itself suffers from one, see pm.max_requests. Is php_wasm.c making it worse?
• PHP runtime is automatically rotated when an out of memory error is detected AND more than 10 requests were handled to avoid an "infinite rotate trap". This could rely on the error logging API implemented by @bgrgicak.

A few memory leaks were already patched in this repo, find old PRs for more context.

pm.max_requests int The number of requests each child process should execute before respawning. This can be useful to work around memory leaks in 3rd party libraries. For endless request processing specify '0'. Equivalent to PHP_FCGI_MAX_REQUESTS. Default value: 0.

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What @sejas reported:

I've created a PHP function to make it much easier to benchmark the memory usage: I'm currently adding it to the index.php, but you can also try the plugin in Studio or wp-now.

• You can also try on Playground WEB. https://playground.wordpress.net/#{%22landingPage%22:%22/%22,%22steps%22:[{%22step%22:%22installPlugin%22,%22pluginZipFile%22:{%22resource%22:%22url%22,%22url%22:%22https://raw.githubusercontent.com/sejas/playground-plugin-use-all-system-memory/main/playground-plugin-use-all-system-memory.zip%22}}]}

function useAllMemory() {
    echo "Initial memory usage: " . (memory_get_usage()/(1024*1024)) . ' MB <br>';
    // ini_set('memory_limit', '1024MB'); // The php limit seems to be 128MB but it doesn't affect the results.
    $data = '';

    while (true) {
        $data .= str_repeat('a', 1024 * 1024); // Increase string size by 1MB in each iteration
        echo "* " . (memory_get_usage()/(1024*1024)) . ' MB <br>';
    }
}
useAllMemory();
die();

Here are my results:

Observe that the site screenshot displays almost 60MB of maximum memory. The next (2nd) page load displays 29MB The third page load 4.4MB and then 2.4 MB and 1.3 MB

Screenshot:

• I'll replicate same workaround as https://github.com/WordPress/playground-tools/pull/152, but that doesn't fix the issue, only mitigates it's effect.

cc @brandonpayton

[brandonpayton]

@adamziel I haven't finished testing yet, but updating wasm_memory_storage to zero allocated memory looks like it might solve the issue. Apparently many mmap() implementations zero memory for anonymous mmap. And PHP may depend on this behavior.

I'm currently rebuilding all php-wasm versions for a real PR, but you can check out https://github.com/WordPress/wordpress-playground/pull/1220/commits/c304fecfdda29149dd489c1afa7a8978c8486b16 under #1220 for the potential fix. The unit tests are passing there, but the e2e tests are not. I'm not yet sure if the failures are related to the memory leak fix.

[brandonpayton]

We merged a potential fix in #1229. Let's see how it goes and close this if all is indeed well.

[adamziel]

@brandonpayton with #1229 merged, the original reproduction scenario still triggers the out of memory error in a browser. I can see the node.js test is passing, that's weird!

[brandonpayton]

@adamziel the original reproduction scenario always triggers an out of memory error because it uses an infinite loop that concatenates strings. Is this what you are testing with?

    while (true) {
        $data .= str_repeat('a', 1024 * 1024); // Increase string size by 1MB in each iteration
        echo "* " . (memory_get_usage()/(1024*1024)) . ' MB <br>';
    }

I wish we could see the output prior to the fatal, but now, the error is just printed to a console with no output shown. I will see if we can change that back so we actually see output prior to error.

[brandonpayton]

I wish we could see the output prior to the fatal, but now, the error is just printed to a console with no output shown. I will see if we can change that back so we actually see output prior to error.

It looks like that behavior probably has to do with this PR: https://github.com/WordPress/wordpress-playground/pull/1137

The main PHP script output used to print in the browser, but now, when the script exits with an error, the error is printed to the console with no partial response shown in the content pane. For at least the web interface, it seems like showing a partial response to the user is more helpful than showing nothing. Planning to file a bug for this. cc @bgrgicak

[brandonpayton]

@adamziel, it turns out I am able to reproduce this failure as well with the original repo and the web version. It fails after making a 50MB string, even though there should be much more memory available. Digging into this...

[brandonpayton]

@adamziel, I think what we are now seeing is expected. When I test in the browser, the PHP memory limit is 128M, and the "Allowed memory size exhausted" happens when the attempted allocation would surpass the allowed memory size.

From the end of the test:

* memory_get_usage(true): 74.0625 MB

Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 57671704 bytes) in /wordpress/index.php on line 14

The limit is 128M, and the new allocation would add ~55MB to about 75MB making about 130MB, which exceeds the configured limit and triggers the error.

Full log: https://gist.github.com/brandonpayton/d4239a0da6828647ed73770da95db043

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Related to the memory limit, do we expect it to be 128M, and if so, would it make sense to choose a higher default?

[adamziel]

What, whaaaat, you're right! I increased the memory_limit and I was able to get that loop to allocate 200M – yay! Exemplary work on this one @brandonpayton! 🎉 Let's close and discuss the larger memory limit separately – we might want to also consider lowering the overall HEAP size, I think it's ~2GB now?

[adamziel]

Let's follow up in these:

• https://github.com/WordPress/wordpress-playground/pull/1232

• 1233

[sejas]

@brandonpayton You are awesome!! I think this was one of the hardest problems to solve. ❤️

I tested it on NodeJS and I can see each request allocates always the maximum memory.

    $data = '';
    $i = 0;
    while ($i++ < 50) {
        $data .= str_repeat('a', 1024 * 1024); // Increase string size by 1MB in each iteration until a maximum of 50MB
        echo "* " . (memory_get_usage()/(1024*1024)) . ' MB <br>';
    }

Thank you!!

[brandonpayton]

Thank you, @sejas!