• select() requires the kernel to iterate through all FDs passed to the select() call, check their state and register CBs. When an event on any FD happend, kernel has to iterate through these FDs again to deregister CB. Note that every time you get a new connection with accept() system call, you will get a new FD
  • Linux uses epoll_ctl() to manage the registration. Call epoll_wait to wait for updates about the list of files you’re interested in.
  • Level-triggered epoll() inherits from the “thundering herd” semantics form select() - get list of every FDs you are interested that is operatable. This means everyone will wake up from epoll_wait() and try accept() - all but one will get EAGAIN (Resource temporarily unavailable)!
  • In edge-triggered, only 1 thread is triggered, but the threads may wake up unneccearily only to get EAGAIN, because on awake, it does not/can not know how many connections kernel received originally - use level-triggered + EPOLLEXCLUSIVE to wake up 1 thread at a time
  • In the case of writing to FD with level triggered, your thread will get constant notification that FD is ready, even though you may not be able to write yet because your resource to FD is not available yet.
  • In the case of writing to FD with edge triggered, you get triggered by FD is ready ONCE and write at much as you want until your write(2), and then you wait for the next signal
  • Reading from FD with level-triggered is useful when you don’t want to consume all data at once and want epoll to keep triggring while data is available. However, for performance, probably want to use edge-triggered with all data buffered and will be eventually handled
  • Poll() is very similar to select() and returns more status,i.e., it also requires the full list of FDs to watch on each invocation

  • Poll() can perform better than select() if you have a sparse set of FDs you are interested, because select() accepts a RANGE of FDs and bitsets for FDs you want for reads/writes/exceptions

  • It is also called IO multiplexing because one thread can watch over the change of multiple FDs, i.e., the processing thread itself is reused.
  • Note that call to select() and poll() are still blocking