epoll

epoll means edge polling. It means when edge states are changed, the relevant events are triggered.

Legacy Polling Mechanism

For all IO tasks, the kernel updates the file descriptor flags to signal if there is data ready to be acted upon.

This would mean, the user space applications must pass all the file descriptors that they need to check and kernel will have to update back if any flag has changed. This polling has complexity of 0(n) and the polling also involves system call which needs data to be copied between userspace and kernel space.

Thread is put to sleep

If the caller of the poll system call wants to wait for the events, the call is done with timeout as minus 1. In this case, the kernel will put the thread to sleep until there is a state change in any of the file descriptors.

Protection Layer

epoll avoids userspace from polling the file descriptors. Instead we ask kernel to create an epoll instance and ask it to monitor which ever file descriptors we're interested in.

After this, epoll monitors the file descriptors independently. Whenever there is state change, the thread is simply scheduled again by the kernel. It was earlier paused exactly at the step where it called the epoll_wait system call and now it will be scheduled again to check the events.

State change notification

In most cases, the thread state is changed to TASK_INTERRUPTIBLE, this means the thread isn't scheduled anymore by the kernel.

When there is a state change, the thread state is changed back to TASK_RUNNING and the thread is scheduled again by the kernel.