nginx : Its Multithreaded but uses multiple processes?

Question

I'm trying to understand what makes Nginx so fast, and I have a few questions.

As I understand it, Apache either spawns a new process to serve each request OR spawns a new thread to serve each request. Since each new thread shares virtual address space the memory usage keeps climbs if there are a number of concurrent requests coming in.

Nginx solves this by having just one listening process(Master), with a single execution thread AND 2 or 3(number is configurable) worker processes. This Master process/thread is running an event loop. Effectively waiting for any incoming request. When a request comes in it gives that request to one of the worker processes.

Please correct me if my above understanding is not correct

If the above is correct, then I have a few questions:

1. Isn't the worker process going to spawn multiple threads and going to run into the same problem as apache ?

2. Or is nginx fast because its event based architecture uses nonblocking-IO underneath it all. Maybe the worker process spawns threads which do only non-blocking-IO, is that it ?

3. What "exactly" is "event based architecture", can someone really simplify it, for soemone like me to understand. Does it just pertain to non-blocking-io or something else as well ?

I got a reference of c10k, I am trying to go through it, but I don't think its about event based arch. it seems more for nonblocking IO.

Answer 1

Apache uses multiple threads to provide each request with it's own thread of execution. This is necessary to avoid blocking when using synchronous I/O.

Nginx uses only asynchronous I/O, which makes blocking a non-issue. The only reason nginx uses multiple processes, is to make full use of multi-core, multi-CPU and hyper-threading systems. Even with SMP support, the kernel cannot schedule a single thread of execution over multiple CPUs. It requires at least one process or thread per logical CPU.

So the difference is, nginx requires only enough worker processes to get the full benefit of SMP, whereas Apache's architecture necessitates creating a new thread (each with it's own stack of around ~8MB) per request. Obviously, at high concurrency, Apache will use much more memory and suffer greater overhead from maintaining large numbers of threads.