Performance: OPcache, JIT and Profiling

Where PHP time goes

PHP compiles each script to opcodes before running it. OPcache caches those opcodes in shared memory so scripts are not recompiled on every request; it is the single most important performance setting and should always be enabled in production, with opcache.validate_timestamps=0 in immutable deployments (then reset the cache on each deploy). Preloading (opcache.preload) can load framework classes into memory at server start. The JIT compiler (PHP 8.0+) helps CPU-heavy code such as maths and image processing, but typical web applications are dominated by database and network I/O, where it makes little difference. Real performance work starts with measurement: profilers such as Xdebug (development), Blackfire, Tideways or SPX show which functions and queries consume time. Common wins: fixing N+1 database queries, adding indexes, caching expensive results in Redis or APCu, using composer dump-autoload --optimize (classmap) in production, streaming large files, and moving slow work (emails, PDFs, image resizing) to background queues. Long-running application servers such as FrankenPHP, RoadRunner and Swoole keep the application booted between requests for further gains, at the cost of handling state carefully.

Opcode caching

Without OPcache every request recompiles scripts; with it, compiled opcodes are reused from memory.

Production OPcache settings

Values are typical starting points; adjust to your codebase size.

; php.ini (production)
opcache.enable=1
opcache.memory_consumption=256
opcache.interned_strings_buffer=16
opcache.max_accelerated_files=20000
opcache.validate_timestamps=0      ; code never changes in place: reset on deploy
opcache.jit_buffer_size=64M        ; JIT available; most benefit for CPU-heavy code
opcache.jit=tracing

; composer install --no-dev --classmap-authoritative --no-interaction
; after each deploy: reload php-fpm so the opcode cache starts fresh

Measure before optimising

Most slow PHP pages are slow because of database queries or external calls, not PHP itself. A profiler run takes minutes and usually points straight at the real bottleneck.

Quick check: What does OPcache do?

• Caches database query results
• Compresses HTTP responses
• Caches compiled opcodes in memory so scripts are not recompiled on each request
• Stores sessions