apachearchitecturenginx

nginx : Its Multithreaded but uses multiple processes?

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.

Solution

  • It's not very complicated from a conceptual point of view. I'll try to be clear but I have to do some simplification.

    The event based servers (like nginx and lighttpd) use a wrapper around an event monitoring system. For example. lighttpd uses libevent to abstract the more advanced high-speed event monitoring system (see libev also).

    The server keeps track of all the non blocking connections it has (both writing and reading) using a simple state machine for each connection. The event monitoring system notifies the server process when there is new data available or when it can write more data. It's like a select() on steroids, if you know socket programming. The server process then simply sends the requested file using some advanced function like sendfile() where possible or turns the request to a CGI process using a socket for communication (this socket will be monitored with the event monitoring system like the other network connections.)

    This link as a lot of great information about the internals of nginx, just in case. I hope it helps.