To enable the PHP JIT compiler, SSH into your server as root and edit the following file:
/etc/php8.0-sp/conf.d/opcache.ini
Add the following line to the end of the file:
opcache.jit_buffer_size=100M
Finally, restart PHP by running this command:
sudo service php8.0-fpm-sp restart
To verify the JIT was enabled, create a page that calls the phpinfo() function, request the page from your browser, and verify opcache.jit_buffer_size shows the value of 100M rather than 0.
Learn more about PHP's JIT compiler in our PHP 8.0 announcement.