Enabling the PHP 8 JIT Compiler in Plesk

In this article, we explain how to enable and configure the PHP 8 JIT compiler in Plesk Obsidian.

Prerequisites

The following prerequisites must be met in order to enable the JIT compiler:

• Plesk Obsidian 18.0.33 or higher
• OPcache enabled in the Plesk PHP settings (opcache.enable)
• PHP 8.0, PHP 8.1, PHP 8.2, PHP 8.3, PHP 8.4, or PHP 8.5 installed (e.g., via the Plesk graphical interface)
• SSH root access

If PHP 8.0, PHP 8.1, PHP 8.2, or PHP 8.3 has not yet been installed on your creoline server, see the article PHP 8 Installation provides step-by-step instructions on how to install PHP 8 via the Plesk graphical interface.

Enabling the JIT Compiler

To enable this feature, an additional PHP configuration file for the PHP 8 FPM handler must be placed on the server.

Execute the following shell command via SSH on your creoline server:

# PHP 8.0
echo -e 'opcache.jit_buffer_size=100M\nopcache.jit=1255' > /opt/plesk/php/8.0/etc/php.d/opcache_jit.ini

# PHP 8.1
echo -e 'opcache.jit_buffer_size=100M\nopcache.jit=1255' > /opt/plesk/php/8.1/etc/php.d/opcache_jit.ini

# PHP 8.2
echo -e 'opcache.jit_buffer_size=100M\nopcache.jit=1255' > /opt/plesk/php/8.2/etc/php.d/opcache_jit.ini

# PHP 8.3
echo -e 'opcache.jit_buffer_size=100M\nopcache.jit=1255' > /opt/plesk/php/8.3/etc/php.d/opcache_jit.ini

# PHP 8.4
echo -e 'opcache.jit_buffer_size=100M\nopcache.jit=1255' > /opt/plesk/php/8.4/etc/php.d/opcache_jit.ini

# PHP 8.5
echo -e 'opcache.jit_buffer_size=100M\nopcache.jit=1255' > /opt/plesk/php/8.5/etc/php.d/opcache_jit.ini

After the configuration file has been saved, PHP 8 must be reloaded for the configuration to take effect:

# PHP 8.0
systemctl reload plesk-php80-fpm

# PHP 8.1
systemctl reload plesk-php81-fpm

# PHP 8.2
systemctl reload plesk-php82-fpm

# PHP 8.3
systemctl reload plesk-php83-fpm

# PHP 8.4
systemctl reload plesk-php84-fpm

# PHP 8.5
systemctl reload plesk-php85-fpm

To verify that the JIT compiler has been enabled correctly, you can run the following PHP script:

<?php

echo '<pre>';
print_r(opcache_get_status()['jit']);

Sample output:

Array
(
    [enabled] => 1
    [on] => 1
    [kind] => 5
    [opt_level] => 5
    [opt_flags] => 6
    [buffer_size] => 104857584
    [buffer_free] => 104855104
)

Configuring the JIT Compiler

The JIT compiler is configured using the opcache.jit variable. For example: opcache.jit=1255

Each digit represents a setting for a configuration parameter. A detailed explanation can be found in the official RFC (wiki.php.net).

CPU-specific optimization flags (C)

Parameter 1

0 - none
1 - enable AVX instruction generation

Register Allocation (R)

Parameter 2

0 - do not perform register allocation
1 - use local linear-scan register allocator
2 - use global linear-scan register allocator

JIT trigger (T)

Parameter 3

0 - JIT all functions on first script load
1 - JIT functions on first execution
2 - Profile on the first request and compile hot functions on the second request
3 - Profile on the fly and compile hot functions
4 - Compile functions with the @jit tag in doc-comments
5 - Tracing JIT

Optimization level (O)

Parameter 4

0 - Do not use JIT
1 - Minimal JIT (call standard VM handlers)
2 - Selective VM handler inlining
3 - Optimized JIT based on static type inference of individual functions
4 - Optimized JIT based on static type inference and call tree
5 - Optimized JIT based on static type inference and inner procedure analysis

Example Configuration

In our example configuration, we use the following settings:

opcache.jit=1255