PHP Preloading (opcache.preload)

Also Known As

TL;DR

Explanation

PHP 7.4 introduced preloading via opcache.preload — a php.ini directive pointing to a bootstrap script that calls opcache_compile_file() on framework and application files. At PHP-FPM startup these files are compiled to bytecode and mapped into shared memory, available to all worker processes with no per-request file stat or compilation cost. Frameworks like Symfony and Laravel ship preload scripts. Gains are most noticeable for large frameworks (5–10% throughput). Caveat: preloaded files cannot be updated without restarting PHP-FPM; always disable preloading in development environments.

Diagram

flowchart TD
    subgraph Without_Preload
        REQ_A[Request 1] --> OPCACHE2[OPcache loads class files<br/>on first use per worker]
        REQ_B[Request 2] --> OPCACHE3[Same class loaded again<br/>in each worker process]
    end
    subgraph With_Preload
        BOOT[PHP-FPM boots] --> PRELOAD2[opcache.preload runs<br/>loads framework classes once]
        PRELOAD2 --> SHARED[Shared memory<br/>available to ALL workers instantly]
        SHARED --> FAST2[Zero class loading overhead<br/>on any request]
    end
style PRELOAD2 fill:#1f6feb,color:#fff
style SHARED fill:#6e40c9,color:#fff
style FAST2 fill:#238636,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Preloading files that change frequently — FPM must restart for changes to take effect.
• Not setting opcache.preload_user — FPM won't preload as root by default.
• Preloading the entire vendor directory — classes that are rarely used waste shared memory.
• Not measuring the impact — preloading adds startup time and memory; benchmark before committing.

Code Examples

; php.ini — preloading not configured:
; opcache.preload =              ; Not set — no preloading
; opcache.preload_user =         ; Required — must be non-root

; Correct:
opcache.preload = /var/www/app/preload.php
opcache.preload_user = www-data

; php.ini — preload at FPM startup (PHP 7.4+)
opcache.preload      = /var/www/app/preload.php
opcache.preload_user = www-data

// preload.php — compile key files on startup:
<?php
foreach (glob('/var/www/app/src/**/*.php') as \$file) {
    opcache_compile_file(\$file);
}

; Laravel: php artisan optimize
; Symfony: php bin/console cache:warmup

; Verify:
$ php -r "print_r(opcache_get_status()['scripts']);"

; Biggest win: eliminates per-request parsing of framework files

References

• ↗ https://www.php.net/manual/en/opcache.preloading.php

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