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What is the difference between the Prefork, Worker, and Event MPMs?
What is the difference between the Prefork, Worker, and Event MPMs?
The MPM selection of Apache HTTP Server depends on performance requirements and module compatibility. 1. Prefork runs in a multi-process mode, with high stability but high memory consumption, and is suitable for scenarios where non-thread-safe modules such as mod_php are used; 2. Worker adopts a multi-threaded hybrid model, with higher memory efficiency, and is suitable for environments where modules are thread-safe and require concurrent processing; 3. Event optimizes connection management based on Worker, especially suitable for modern architectures with high traffic and support asynchronous operations. Selecting the most suitable MPM according to actual application can balance resource occupation and service stability.
Apache HTTP Server uses Multi-Processing Modules (MPMs) to handle requests. The main difference between Prefork, Worker, and Event MPMs lies in how they manage processes and threads — which directly affects performance, resource usage, and compatibility with certain modules like mod_php.
Prefork MPM: Process-Based Handling
The Prefork MPM is the oldest and most stable of the three. It works by creating multiple child processes, each handling one connection at a time. There's no threading involved — just separate processes.
- Each process is independent and doesn't share memory with others.
- It's safe for unstable modules since a crash in one process won't affect others.
- However, it uses more memory because each process has its own copy of everything.
This makes Prefork best suited for environments where stability is key and module compatibility matters — especially when using non-thread-safe modules like mod_php.
Use case:
If you're running older PHP scripts or other non-thread-safe modules, Prefork is your safest bet.
Worker MPM: Threaded and Efficient
Worker MPM uses a hybrid approach — a smaller number of processes, each managing many threads. Each thread can handle a request, so it's much more memory-efficient than Prefork.
- Threads within the same process share memory, reducing overhead.
- More scalable under high traffic.
- But it's less stable if any module isn't thread-safe — a crash in one thread might affect the whole process.
Worker is a good middle ground when you want efficiency but still need some flexibility with modules — assuming they're all thread-safe.
Key points:
- Lower memory usage
- Better concurrency
- Requires careful module selection
Event MPM: Designed for Scalability
Event MPM is an improvement over Worker. It keeps the process/thread model but adds better handling of idle connections — particularly useful with keep-alive requests.
- It separates listening, request processing, and content delivery into different roles.
- Keeps fewer threads busy by offloading waiting tasks (like slow clients).
- Great for high-traffic sites that want to minimize resource use.
However, Event MPM requires modules and backends to be fully async-friendly. If you're using something like PHP via mod_php, this might not work well unless you're on a recent PHP version that supports threading.
Ideal for:
- High-traffic websites
- Sites using reverse proxies or async backends
- Modern setups with thread-safe modules
Choosing the Right One Isn't Hard, Just Context-Dependent
It really comes down to what you're running and how much traffic you expect.
- Go with Prefork if you're using older, non-thread-safe modules.
- Choose Worker if you want better performance and know your modules are thread-safe.
- Use Event if you're optimizing for high concurrency and have a modern stack.
Most Linux distributions default to Event these days, but it's always worth checking based on your actual workload.
Basically that's it.
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