Apache MPMs explained: prefork vs worker vs event

An Apache MPM (Multi-Processing Module) decides how Apache accepts connections and hands them to processes or threads. Exactly one MPM is active at a time, and the choice affects memory use, concurrency and which PHP handlers work. This guide explains prefork, worker and event and shows how to switch and tune them.

Before you start

• A Linux VPS with Apache 2.4 and root access.
• Know how PHP is run: mod_php or PHP-FPM. See application servers explained.

Check which Apache MPM is active

apachectl -V | grep -i mpm

On Debian/Ubuntu you can also run a2query -M.

Prefork: one process per connection

Prefork starts a pool of single-threaded child processes. Each child handles one connection at a time. It is the only MPM that works with mod_php, because older PHP extensions are not thread-safe.

• Pros: simple, isolates crashes, works with non-thread-safe code.
• Cons: each child carries a full copy of PHP, often 30–80 MB, so concurrency is limited by RAM. Idle keep-alive connections tie up whole processes.

Worker: threads inside processes

Worker runs several processes, each with many threads. One thread serves one connection. Memory per connection is much lower than prefork. The weakness: a thread waiting on an idle keep-alive connection still counts as busy.

Event: worker plus a keep-alive listener

Event is worker with a dedicated listener thread that parks idle keep-alive and lingering-close connections. Threads are only used while a request is actually being processed, so thousands of open connections need far fewer threads. Event is the default on modern distributions and the right choice when PHP runs through PHP-FPM.

Which Apache MPM should you use?

Switch from prefork to event

1. Install PHP-FPM and the proxy module: sudo apt install php-fpm and sudo a2enmod proxy_fcgi setenvif.
2. Disable mod_php and prefork, enable event:
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   sudo a2dismod php8.3 mpm_prefork && sudo a2enmod mpm_event && sudo a2enconf php8.3-fpm

3. On AlmaLinux/Rocky, edit /etc/httpd/conf.modules.d/00-mpm.conf, comment the prefork LoadModule line and uncomment mpm_event_module. PHP-FPM is used there by default.
4. Test and restart:
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   sudo apachectl configtest && sudo systemctl restart apache2

Tune the event MPM

Edit /etc/apache2/mods-available/mpm_event.conf (or a file in /etc/httpd/conf.d/):

<IfModule mpm_event_module>
    StartServers             2
    ServerLimit              8
    ThreadsPerChild          25
    MaxRequestWorkers        200
    MinSpareThreads          25
    MaxSpareThreads          75
    MaxConnectionsPerChild   10000
</IfModule>

• MaxRequestWorkers = total threads that can process requests at once. It must be ≤ ServerLimit × ThreadsPerChild.
• With event and PHP-FPM, Apache threads are light; the real limit is PHP-FPM's pm.max_children. Size that one against RAM (see PHP-FPM pool tuning).
• For prefork, estimate: MaxRequestWorkers ≈ (RAM for Apache) ÷ (average child RSS). Measure RSS with ps -o rss= -C apache2.

Troubleshooting

• "Apache is running a threaded MPM, but your PHP Module is not compiled to be threadsafe": mod_php is still enabled. Disable it and use PHP-FPM.
• Log says "server reached MaxRequestWorkers setting": requests are queuing. Raise the value if RAM allows, or find slow PHP requests holding workers.
• PHP source code shown instead of running: no PHP handler after the switch. Enable the php-fpm conf and proxy_fcgi.

FAQ: Apache MPM

Is event MPM safe for WordPress?

Yes, with PHP-FPM. WordPress never talks to the MPM directly.

Does LiteSpeed use an MPM?

No. LiteSpeed is event-driven by design and reads Apache configs without using MPMs.

Can I mix MPMs per site?

No. One MPM serves the whole Apache instance.

Next steps

• Apache virtual hosts setup
• Apache vs Nginx vs LiteSpeed
• Tune PHP-FPM pools