Can APC improve the speed of CLI scripts when used with pcntl_fork()?

Question

APC works by storing the opcodes from PHP files in shared memory. When PHP is used with a web server (eg Apache) then the shared memory has a long life. When called from the commandline, then the APC cache is created and destroyed for each process. APC is disabled on the commadnline by default, probably due to this.

I have a theory that there will be benefits from using APC if a PHP process is forked (with pcntl_fork() as presumably the same opcode cache can be used. This may only apply to files included after the fork.

What's the best way to benchmark this? Can anyone either test this or explain whether the theory is correct or not?

<?php
if (pcntl_fork()) {
    // parent
    include 'huge-file.php';
} else {
    // child
    sleep(1); // stop race condition
    include 'huge-file.php'; // will this use APC's cache?
}

Answer 1

APC has an impact in CLI mode in two cases:

• The same file is included repeatedly, either by the same script (e.g. a file containing some data cache), or by multiple processes
• You use apc_fetch() (always returns false if apc is disabled)

Note that APC disables itself in CLI by default; you have to enable it with apc.enable_cli=1.

Here is a quick benchmark:

<?php                                                                           

for ($i = 0; $i < 1000; ++$i) {                                                 
    // cache.php contains the output of 'var_export(get_defined_constants(true))'
    require 'cache.php';                                                   
}

Result without apc:

$ time php test.php

real    0m1.219s
user    0m1.208s
sys     0m0.008s

Result with apc:

$ time php -dapc.enable_cli=1 test.php

real    0m0.252s
user    0m0.244s
sys     0m0.004s

In this case, APC does have a significant impact on performance.

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With pcntl_fork(), APC should have exactly the same impact than running multiple PHP scripts under apache's mod_php or php-fpm: if multiple children scripts include the same files, the included files will be parsed only once.

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In PHP 5.5, the bundled opcache extension replacing APC also optimizes the code, so it should not only impact require performance, but also the performance of the code itself.