Performance penalty for builtin/extension functions due to interrupt checks

[dktapps]

Description

I was profiling an extension that's intended to act as a faster replacement for the pack() and unpack() functions. The functions in the extension are very fast - just a few instructions each.

During profiling I discovered, much to my surprise, that the overhead of the DO_FCALL opcode far exceeded the time actually spent in my function, and that 2/3rds of the overhead was coming from ZEND_VM_FCALL_INTERRUPT_CHECK.

Doing some digging I found that this interrupt check runs after every single internal function call. This means that my extension's functions are being overshadowed by interrupt checks on every call which are >5x slower than the actual function itself.

The net result overall is less visible (a 5-10% performance degradation in my synthetic test script), but I have to ask if there's a better way to do this. Why is PHP checking for interrupts after every internal function call? I get it for stuff like network and I/O functions which could take a long time, but not every builtin function is like that. For the fast ones, this is an unavoidable and (probably) unnecessary performance penalty.

I don't know much about how the interrupt system works, but my feeling is that there has to be a better way to implement it than having every internal function call pay such a penalty. Even if there was a way to avoid this interrupt check with some ZEND_ACC flag that would tell the engine a function is "fast" and not to do the interrupt check?

[nielsdos]

This interupt check was added without much dicussion way back (before my time) IIRC
See also Dmitry's comment on a related pr #14627 (comment)

[mvorisek]

With all interrupt checks removed, what is the performance improvement on the performance benchmarks?

[dktapps]

Zend/bench.php doesn't show a significant difference in most tests, but I think largely because it doesn't really do much internal function calling. I didn't run the other benchmarks because my env doesn't have the right extensions.

FWIW I don't think killing the interrupt checks completely is the right move, but I do think it could probably be polled less often.

[mvorisek]

I mean to remove them temporary to verify the impact on realworld userland app. If you know how to remove them, then you can push the php-src code to GH and GHA will output the benchmark numbers.

[arnaud-lb]

Interrupts are checked quite often, including before entering userland function (via ZEND_VM_ENTER_EX()), after returning from internal functions (via explicit check), in smart branches, loops, and some jumps (via ZEND_VM_SET_OPCODE()), etc. Most of these (including internal calls) were added in 5c68836.

As @mvorisek has suggested, it would be interesting to verify the impact of interrupts by disabling them temporarily. If they are indeed the source of the overhead, a possible workaround would be to declare the function as frameless, as frameless calls do not check interrupts. This may be beneficial on its own, too.

[dktapps]

If they are indeed the source of the overhead, a possible workaround would be to declare the function as frameless, as frameless calls do not check interrupts. This may be beneficial on its own, too.

I had looked into this, but the documentation on frameless functions is extremely sparse. Also, as far as I know, they have to be declared differently from regular functions, which would be a pain in my case as I still need to support PHP <8.4.

Benchmark results are as follows. The results look a bit underwhelming, but I will say that I don't think the instruction counts paint a full picture though.

PHP 8.4 base

https://github.com/php/php-src/actions/runs/17728778760/job/50375221322

{
    "branch": "PHP-8.4",
    "Zend\/bench.php": {
        "instructions": "2238547261"
    },
    "Zend\/bench.php JIT": {
        "instructions": "602045601"
    },
    "Symfony Demo 2.2.3": {
        "instructions": "39825708"
    },
    "Symfony Demo 2.2.3 JIT": {
        "instructions": "34312908"
    },
    "Wordpress 6.2": {
        "instructions": "122750078"
    },
    "Wordpress 6.2 JIT": {
        "instructions": "93605841"
    }
}

Disabled internal function interrupts

https://github.com/dktapps/php-src/actions/runs/17739651420/job/50410223738

{
    "branch": "2\/merge",
    "Zend\/bench.php": {
        "instructions": "2235291018"
    },
    "Zend\/bench.php JIT": {
        "instructions": "602044206"
    },
    "Symfony Demo 2.2.3": {
        "instructions": "39759966"
    },
    "Symfony Demo 2.2.3 JIT": {
        "instructions": "34414535"
    },
    "Wordpress 6.2": {
        "instructions": "122607185"
    },
    "Wordpress 6.2 JIT": {
        "instructions": "93576566"
    }
}

Disabled loop interrupts:

https://github.com/dktapps/php-src/actions/runs/17739667715/job/50410276410

{
    "branch": "3\/merge",
    "Zend\/bench.php": {
        "instructions": "2238546893"
    },
    "Zend\/bench.php JIT": {
        "instructions": "602044524"
    },
    "Symfony Demo 2.2.3": {
        "instructions": "39818052"
    },
    "Symfony Demo 2.2.3 JIT": {
        "instructions": "34420746"
    },
    "Wordpress 6.2": {
        "instructions": "122749574"
    },
    "Wordpress 6.2 JIT": {
        "instructions": "93611056"
    }
}

Disabled `ZEND_VM_INTERRUPT_CHECK()`

https://github.com/dktapps/php-src/actions/runs/17739683417/job/50410330496

 {
    "branch": "4\/merge",
    "Zend\/bench.php": {
        "instructions": "2166770913"
    },
    "Zend\/bench.php JIT": {
        "instructions": "602038150"
    },
    "Symfony Demo 2.2.3": {
        "instructions": "39618324"
    },
    "Symfony Demo 2.2.3 JIT": {
        "instructions": "34253136"
    },
    "Wordpress 6.2": {
        "instructions": "121592519"
    },
    "Wordpress 6.2 JIT": {
        "instructions": "93386286"
    }
}

[arnaud-lb]

Thank you! I've computed the differences:

Disabled internal function interrupts vs baseline:

Zend/bench.php          -0.15%
Zend/bench.php JIT      -0.00%
Symfony Demo 2.2.3      -0.17%
Symfony Demo 2.2.3 JIT  +0.30%
Wordpress 6.2           -0.12%
Wordpress 6.2 JIT       -0.03%

Disabled ZEND_VM_INTERRUPT_CHECK() vs baseline:

Zend/bench.php          -3.21%
Zend/bench.php JIT      -0.00%
Symfony Demo 2.2.3      -0.52%
Symfony Demo 2.2.3 JIT  -0.17%
Wordpress 6.2           -0.94%
Wordpress 6.2 JIT       -0.23%

These are not negligible. This is worth investigating.