title: Why is poll() better than select()?
body: <p>It is said select() is not scalable because it needs to go over an array with the size of max num of file describers (FD): complexity O(max_num_FD). And it is said that poll() is better because it only goes over an array with size of num of active FD(): complexity O(num of active FD) what does active FD mean?</p>
  
  <p>Is poll() a popular way used by large scale servers with many available data at a time? Usually which socket approach does a large scale server usually use in reality?</p>