Understanding Apache MPMs: Prefork, Worker, and Event Explained

For Apache 2.4.10 (released July 21, 2014) there are three stable Multi‑Processing Modules (MPMs): prefork, worker, and event, representing the evolution of Apache's processing models.

# httpd -V
Server version: Apache/2.4.10 (Unix)
Server built:   Dec 29 2014 11:23:13
Server's Module Magic Number: 20120211:36
Server loaded:  APR 1.5.1, APR-UTIL 1.5.3
Compiled using: APR 1.5.1, APR-UTIL 1.5.3
Architecture:   64-bit
Server MPM:     event
threaded:     yes (fixed thread count)
forked:     yes (variable process count)

When compiling Apache you can select an MPM with the configure flag: --with-mpm=prefork|worker|event Or build all three as shared modules: --enable-mpms-shared=all In httpd.conf you load the desired MPM module (the .so files are generated in the modules directory):

#LoadModule mpm_prefork_module modules/mod_mpm_prefork.so
LoadModule mpm_worker_module modules/mod_mpm_worker.so
#LoadModule mpm_event_module modules/mod_mpm_event.so

1. prefork MPM

Prefork is the oldest and most stable mode. Apache forks a number of child processes at startup; each process handles one request with a single thread. Advantages: mature, stable, compatible with all modules, no thread‑safety concerns (e.g., mod_php). Disadvantages: higher memory usage per process and poorer performance under high concurrency because requests wait in a queue for an available process.

<IfModule mpm_prefork_module>
StartServers             5
MinSpareServers          5
MaxSpareServers         10
MaxRequestWorkers      250
MaxConnectionsPerChild   0
</IfModule>

2. worker MPM

Worker combines multiple processes with multiple threads per process. Each child process creates several threads, each handling a request, reducing memory consumption compared with prefork and improving performance under high load. The mixed model improves stability: if a thread crashes, only its process is affected, not the whole server.

Advantages: lower memory footprint, better high‑concurrency performance. Disadvantages: must handle thread‑safety; long‑lived keep‑alive connections can tie up threads, potentially exhausting the thread pool.

<IfModule mpm_worker_module>
StartServers             3
MinSpareThreads         75
MaxSpareThreads        250
ThreadsPerChild         25
MaxRequestWorkers      400
MaxConnectionsPerChild   0
</IfModule>

3. event MPM

Event is the newest stable mode, similar to worker but adds a dedicated thread to manage keep‑alive connections, preventing idle threads from consuming resources. This improves request handling in high‑concurrency scenarios. Event falls back to worker when incompatible modules are loaded. It requires Linux 2.6+ with epoll support. Modern Apache also supports SSL with event, although older documentation claimed otherwise.

<IfModule mpm_event_module>
StartServers             3
MinSpareThreads         75
MaxSpareThreads        250
ThreadsPerChild         25
MaxRequestWorkers      400
MaxConnectionsPerChild   0
</IfModule>

Performance testing

Using ab with 200 concurrent connections and 200,000 requests, the three MPMs showed similar throughput for static HTML:

prefork: 9556 QPS
worker : 11038 QPS
event  : 10224 QPS

For a simple PHP script ( echo "hello world";) the results were:

prefork: 6094 QPS
worker : 7411 QPS
event  : 7089 QPS

For PHP, FastCGI or php‑fpm is recommended. Modern browsers open multiple keep‑alive connections per domain, making high concurrency a common scenario. While Apache remains mature, Nginx often outperforms it in many use cases due to its lighter architecture.