PHP 8.1 Failed to set memory limit to ... bytes (Current memory usage is ... bytes)

[brainfoolong]

Description

Hi!

We recently upgraded to PHP 8.1 and now we sporadically get this kind of new errors:

Failed to set memory limit to 536870912 bytes (Current memory usage is 870318080 bytes)

The problem is, the memory usage cannot be that high at that moment, because we use ini_set on the very start of the script, where almost nothing happened.

Furthermore, we can call the same page 100x times, the error does not appear, but sometime, it happens. It is impossible to reproduce. It definitely only is on PHP 8.1 - With PHP 8.0 we never had this kind of issues.

I guess that the system has sometimes wrong internal memory usage stats, maybe from another thread on the same system?

Is there something we can do about it, without just error suppressing or try/catch?

We are on Windows Server, with Apache 2.4.52 and PHP 8.1.2 TSx64.

As said above, unfortunately i have no reproducible example, but the code that throw the error is this line
ini_set("memory_limit", "512M");

Edit: Have now a reproducible script at #8116 (comment)

PHP Version

PHP 8.1.2 TSx64

Operating System

Windows Server 2019

[iluuu1994]

Not sure if it helps, but you could try setting memory_limit in your php.ini so you'll get an error when you actually allocate the memory. But yeah, if ini_set is virtually the first call that sounds strange, although I know anything about the peculiarities of Windows + Apache. What does var_dump(memory_get_usage()); when you put it at the very top of index.php?

[brainfoolong]

Thanks for feedback. I cannot change values in php.ini, we don't want all scripts to take 512M of memory. Also i cannot add a dump, because this happen in production in very very rare circumstances, no luck to reproduce it. The dump would bomb our logs, as it's a very big application where every request, no just the buggy one, runs through a single entry point of index.php. Production has a lot more requests, so maybe it's an indicator that concurrent requests on Apache will cause memory usage stats to glitch through. I don't think the number PHP has at this point is correct.

It is really hard to nail it. Out of 5000 requests we had it currently happen only a few times.

I would understand if at that point, the system memory is probably too low to allocate this memory, but the error says that it is already using over 800M when trying to set 512M. Which is pretty much impossible, because the same script run even when it runs fine at 99.9% of time, it doesn't crash because it never hit the 512M limit. I am pretty sure that this was already so in PHP 8.0 but from what i've read in the old bug-tracker, before 8.1 this error was silently ignored...

[brainfoolong]

Furthermore, the funny thing is, the initial memory limit from php.ini is just 128M before trying to set 512M at runtime, so the script, if really has 800M of memory used should have stopped long before.

[cmb69]

I think this behavior is caused by fixing https://bugs.php.net/bug.php?id=81585 (PHP 8.0.15 and 8.1.2); there has been a follow up fix that should be in PHP 8.0.17 and 8.1.4. Unfortunately, there are no snapshot builds for Windows currently, so you either have to build yourself, or wait for PHP 8.1.4RC1 (expected on 2022-03-03) to verify.

[brainfoolong]

@cmb69 Looks somewhat similar yes but isn't there a other problem with this? Because the initial limit is 128M and the error shows a usage of 800M, the script should have terminated a way before we even can try to set a new memory limit that is 512M? For me this sounds to different levels of memory checks or at least a missing check when the actual limit if 128M reaches. ini_set seems to then force to do a proper memory_check which causes the error.

TLDR: When having 128M initially, trying to set it to 512M and the errors says it already has 800M at this point -> can this be a correct error message?

[brainfoolong]

Thanks to https://bugs.php.net/bug.php?id=81585 an the posted test script, i've created a testscript that can now be reproduced. The error is similar, but not quite the same, i guess?

Just call it 2 times in your browser, the first time, it just runs fine, second time, it fails.

Output first call:

Limit initial: 256M
finished successfully

Output second call:

Limit initial: 256M
error - Failed to set memory limit to 536870912 bytes (Current memory usage is 578813952 bytes)

Hope this helps
test.php.txt

[cmb69]

The script works as expected (at least with the built-in Webserver) with current PHP-8.1 head. I still think the mentioned follow up fix is responsible for that.

[brainfoolong]

Ok, i will test when a 8.1.4 windows build is available.