Unlocking Performance: A Deep Dive into PHP OPCache

As web applications grow in complexity, ensuring optimal performance becomes paramount. One key tool in a PHP developer's arsenal is OPCache, a powerful extension designed to improve the performance of PHP scripts. In this article, we will explore what OPCache is, how it works, and how to configure it to boost your PHP application's performance.

1. What is PHP OPCache?

PHP OPCache is an opcode caching mechanism. It works by storing precompiled script bytecode in shared memory, eliminating the need for PHP to load and parse scripts on each request. This results in significant performance gains, especially for applications with a high volume of requests.

2. How Does OPCache Work?

When a PHP script is executed, the PHP engine compiles it into opcode, a low-level representation of the code. Without OPCache, this process happens every time a script runs. With OPCache, once a script is compiled, its opcode is stored in memory. Subsequent requests for the same script fetch the precompiled opcode directly from memory, bypassing the parsing and compilation stages.

OPCache includes three layers of caching: the original shared memory cache, the file cache introduced in PHP 7, and the preloading functionality added in PHP 7.4.

2.1 Shared Memory Cache (Original Cache):

The shared memory cache is the primary layer of OPCache, introduced in the earliest versions of PHP. It stores precompiled PHP scripts (bytecode) in shared memory, allowing for quick retrieval and execution without needing to recompile the script on every request. This drastically improves performance by reducing CPU load and script execution time.

2.2 File Cache (Introduced in PHP 7):

The file cache was added in PHP 7 as a secondary layer of caching. It serves as a fallback mechanism when the shared memory cache is full or unavailable. Instead of recompiling scripts, PHP can read the precompiled bytecode directly from the disk. While not as fast as shared memory, the file cache still provides a significant performance boost compared to recompiling scripts on every request.

2.3 Preloading (Introduced in PHP 7.4):

Preloading is the latest addition, introduced in PHP 7.4. This feature allows developers to load and compile a set of scripts into OPCache at the start of the PHP process, making them available for all subsequent requests. Preloaded files remain in memory and are not subject to OPCache’s usual eviction policies, ensuring consistent performance for critical parts of the application.

Each of these caching layers contributes to enhancing PHP application performance, with shared memory offering the fastest access, file caching providing a reliable fallback, and preloading ensuring that essential scripts are always ready to execute.

3. Benefits of Using OPCache

1. Improved Performance: Reduces script execution time by caching precompiled bytecode.
2. Reduced Server Load: Lowers CPU usage by minimizing repetitive compilation processes.
3. Enhanced Scalability: Supports more concurrent users by freeing up server resources.
4. Lower Latency: Faster script execution leads to reduced response times.

4. Key Configuration Options

1. opcache.memory_consumption: Amount of memory allocated to store precompiled code.
2. opcache.interned_strings_buffer: Memory allocated for storing interned strings.
3. opcache.max_accelerated_files: Maximum number of files that can be cached.
4. opcache.revalidate_freq: Frequency (in seconds) to check for script updates. Setting to 0 checks every request, which can negate performance benefits.

5. Fine-Tuning OPCache

Increase Memory Consumption: Depending on your application's size, you may need to increase the opcache.memory_consumption setting.

Optimize File Limit: Adjust the opcache.max_accelerated_files setting based on the number of scripts in your application.

Fine-tuning PHP OPCache configuration should be based on your application's specific needs. The best approach is to monitor performance and hit/miss ratios. I'm using the OPCache GUI tool (link below) to monitor and adjust settings as needed:

OPCache GUI: https://github.com/amnuts/opcache-gui

Magento Application - Fine Tuning Opcache configurations using Opcache GUI

Based on the OPCache GUI status for my application, there's no need to increase the "memory consumption" or "max accelerated files" settings.

However, I need to slightly increase the interned strings buffer because it is currently full.

Important: If the assigned memory is full, slightly increase the memory size. If the memory already has plenty of space, increasing the size further won't make any difference, except that it may prevent the server from using that memory for other purposes.

That's it! I hope it helps.