Switching to php-fpm and Apache 2 mpm_event Worker Instead of mpm_prefork in Debian 10, 11, 12

Switching to PHP-FPM

Starting with a fairly stock Debian 10 PHP/Apache2 install which defaults to the mpm_prefork module and mod_php. It seems to be safe to install php-fpm in place, as it won't clobber the existing PHP and Apache configuration:

sudo apt install php-fpm

See the following comments after the install:

Creating config file /etc/php/7.3/fpm/php.ini with new version
NOTICE: Not enabling PHP 7.3 FPM by default.
NOTICE: To enable PHP 7.3 FPM in Apache2 do:
NOTICE: a2enmod proxy_fcgi setenvif
NOTICE: a2enconf php7.3-fpm
NOTICE: You are seeing this message because you have apache2 package installed.

So now we just need to enable the additional apache modules to support php-fpm:

a2enmod proxy_fcgi setenvif
a2enconf php7.3-fpm

The old mod_php config/module is safe to disable; it's config is located in /etc/apache2/mods-enabled/php7.3.conf; disable the module with:

sudo a2dismod php7.3                                                                                                                        

At this point once the apache process is restarted, the php-fpm configuration will take over.

Switching to the mpm_event Apache Module

Simple as disabling the prefork module, and enabling the event module as follows:

sudo a2dismod mpm_prefork
sudo a2enmod mpm_event

Restart apache and voila.

The configuration file for the mpm_event module is located at /etc/apache2/mods-enabled/mpm_event.conf. Good luck with that configuration; here are some (hopefully up-to-date and helpful) tips.

Updates and Example Config

This all works the same in Debian 11 and 12; here are example configs for mpm_event and the php-fpm pool (conservate example from a 4 core, 16GB RAM server, that leaves room for other services):

/etc/apache2/mods-enabled/mpm_event.conf

# event MPM
# StartServers: initial number of server processes to start - Default: 3
# MinSpareThreads: minimum number of worker threads which are kept spare - Default: 75
# MaxSpareThreads: maximum number of worker threads which are kept spare - Default: 250
# ThreadsPerChild: constant number of worker threads in each server process - Default: 25
# ServerLimit: Upper limit on configurable number of processes - Default: 16
# ThreadLimit: Sets the upper limit on the configurable number of threads per child process - Default: 64
# MaxRequestWorkers: maximum number of worker threads: Default: ServerLimit*ThreadsPerChild = 400
# MaxConnectionsPerChild: maximum number of requests a server process serves - Default: 0
<IfModule mpm_event_module>
        ThreadLimit     64
        ThreadsPerChild 64
        ServerLimit     3
</IfModule>

/etc/php/7.3/fpm/pool.d/www.conf

pm = static
pm.max_children = 100
pm.max_requests = 1000

PHP-FPM's pm = dynamic is often suggested but is much more complicated to tune (interplay between additional config params) and generally I want the webserver available and ready to go at full capacity at all times so I use static.

May as Well Tweak Opcache Too

opcache.memory_consumption=256
opcache.max_accelerated_files=100000
opcache.interned_strings_buffer=64
opcache.max_wasted_percentage=10
opcache.validate_timestamps=0
opcache.jit=tracing
opcache.jit_buffer_size=128M

Ensure the webserver is reloaded after deployment when setting opcach.validate_timestamps=0, and if you are feeling bold turn on the new JIT compiler available starting in PHP 8.0.