Understanding the Basics of Concurrency

Overview

A concurrent program can execute multiple tasks at the same time. Concurrent programming is, in general, a lot harder than sequential programming, but there are several reasons why a program may benefit from being concurrent:

• Efficiency: The smartphones and desktop computers of today have multiple CPU cores that can execute multiple tasks in parallel. If we manage to split a big task into subtasks that can be run in parallel, it is theoretically possible to divide the running time of the big task by the number of CPU cores. For programs that run on machines with one single core, there can still be a gain in performance if a task is I/O bound. While one subtask is waiting for I/O, other subtasks can still perform useful work on the CPU.

• Responsiveness and low latency contexts: For applications with a graphical user interface, it is important to never block the UI so the application becomes unresponsive. To prevent unresponsiveness, it is common to let long-running tasks (like loading a file from disk or fetching some data from the network) execute in separate background threads so that the thread responsible for the UI is never blocked by long-running tasks. Another example where low latency matters is real-time audio. The function responsible for producing buffers of audio data is executed in a separate high-priority thread, while the rest of the program can run in lower-priority threads to handle the UI and so on.

• Simulation: Concurrency can make it easier to simulate systems that are concurrent in the real world. After all, most things around us happen concurrently, and sometimes it is very hard to model concurrent flows with a sequential programming model. We will not focus on simulation in this course, but will instead focus on performance-related aspects of concurrency.

Concurrency solves many problems for us but introduces new ones, as we will discuss next.

What makes concurrent programming hard?

There are several reasons why concurrent programming is hard, and if you have written concurrent programs before, you have most likely already encountered the ones listed here: