Concurrency Models: Event-Based System

Node-Js / Netty / Vert.x are popular frameworks which uses concept of Event Loop.

Main thread submits tasks to a task queue.
The event loop constantly monitors the task queue/ring buffer and runs the task until it counters I/O tasks.
In I/O task case, the event loop pauses the task and hands it over to the OS.
Event-Loop even check for the completed IO tasks. If the task is completed, the OS will notify the program. The event loop then runs the unpaused tasks.
In OS, the syscalls select() and poll() helps to keep track of IO bound tasks.
There is no need to acquire or release locks in evented system, since events are expected to non-blocking of each other.