Is there a better way to use asynchronous TCP sockets in C++ rather than poll or select?

Question

I recently started writing some C++ code that uses sockets, which I'd like to be asynchronous. I've read many posts about how poll and select can be used to make my sockets asynchronous (using poll or select to wait for a send or recv buffer), but on my server side I have an array of struct pollfd, where every time the listening socket accepts a connection, it adds it to the array of struct pollfd so that it can monitor that socket's recv (POLLIN).

My problem is that if I have 5000 sockets connected to my listening socket on my server, then the array of struct pollfd would be of size 5000, since it would be monitoring all the connected sockets BUT the only way I know how to check if a recv for a socket is ready, is by looping through all the items in the array of struct pollfd to find the ones whose revents equals POLLIN. This just seems kind of inefficient, when the number of connected sockets because very large. Is there a better way to do this?

How does the boost::asio library handle async_accept, async_send, etc...? How should I handle it?

Answer 1

What the heck, I will go ahead and write up an answer.

I am going to ignore the "asynchronous" vs "non-blocking" terminology because I believe it is irrelevant to your question.

You are worried about performance when handling thousands of network clients, and you are right to be worried. You have rediscovered the C10K problem. Back when the Web was young, people saw a need for a small number of fast servers to handle a large number of (relatively) slow clients. The existing select/poll type interfaces require linear scans -- in both kernel and user space -- across all sockets to determine which are ready. If many sockets are often idle, your server can wind up spending more time figuring out what work to do than doing actual work.

Fast-forward to today, where we have basically two approaches for dealing with this problem:

1) Use one thread per socket and just issue blocking reads and writes. This is usually the simplest to code, in my opinion, and modern operating systems are quite good at letting idle threads sleep peacefully out of the way without imposing any significant performance overhead. In my experience, this approach works very well for hundreds of clients; I cannot personally say how it will work for thousands.

2) Use one of the platform-specific interfaces that were introduced to tackle the C10K problem. That means epoll (Linux), kqueue (BSD/Mac), or completion ports (Windows). (If you think epoll is the same as poll, look again.) All of these will only notify your application about sockets that are actually ready, avoiding the wasteful linear scan across idle connections. There are several libraries that make these platform-specific interfaces easier to use, including libevent, libev, and Boost.Asio. You will find that all of them ultimately invoke epoll on Linux, kqueue on BSD, and so on, whenever such interfaces are available.