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copied!<p>Speaking with my experience with running large IRC servers, we used to use select() and poll() (because epoll()/kqueue() weren't available). At around about 700 simultaneous clients, the server would be using 100% of a CPU (the irc server wasn't multithreaded). However, interestingly the server would still perform well. At around 4,000 clients, the server would start to lag.</p> <p>The reason for this was that at around 700ish clients, when we'd get back to select() there would be one client available for processing. The for() loops scanning to find out which client it was would be eating up most of the CPU. As we got more clients, we'd start getting more and more clients needing processing in each call to select(), so we'd become more efficient.</p> <p>Moving to epoll()/kqueue(), similar spec'd machines would trivially deal with 10,000 clients, with some (admitidly more powerful machines, but still machines that would be considered tiny by todays standards), have held 30,000 clients without breaking a sweat.</p> <p>Experiments I've seen with SIGIO seem to suggest it works well for applications where latency is extremely important, where there are only a few active clients doing very little individual work.</p> <p>I'd recommend using epoll()/kqueue() over select()/poll() in almost any situation. I've not experimented with splitting clients between threads. To be honest, I've never found a service that needed more optimsation work done on the front end client processing to justify the experimentation with threads.</p>

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