accept() with sockets shared between multiple processes (based on Apache preforking)

Question

I\'m working on some Python code modeled on Apache\'s MPM prefork server. I am more an applications programmer than a network programmer and it\'s been 10 years since I read

Answer 1

In respect of (1), the listener is merely a reference to the existence of socket on which to accept connections. Since Apache can accept connections on multiple sockets at the same time, eg., 80/443, then there are multiple listener sockets. Each child process would need to listen on all of these sockets when it comes its time. Since accept() can only be done on one socket at a time, it is preceded by the poll/select so it is known on which listener socket the accept should be performed.

In respect of (2), it is a global or cross process mutex. That is, one process locking it will block out other processes trying to acquire the same lock. Although accept() will technically serialise the processes, the presence of multiple listener sockets means you cannot rely on that as you don't know before hand which socket to perform accept on. Even where a single listener socket, the reason for the accept mutex is that if there are large numbers of processes handling requests, then it could be quite expensive if operating system wakes up all processes to see which then has accept() return for it. Since Apache in prefork mode may have 100+ processes, that could cause a problem.

So, if you have only a single listener socket and know you only have a few processes wanting to do the accept() call then you can possibly do away with the cross process accept mutex.