Thomas Hunter II - Multithreaded JavaScript: Concurrency Beyond the Event Loop

• 2021-04-19: First Release

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Computers used to be much simpler. Thats not to say they were easy to use or write code for, but conceptually there was a lot less to work with. PCs in the 1980s typically had a single 8-bit CPU core, and not a whole lot of memory. You typically could only run a single program at one time. What we think of these days as operating systems would not even be running at the same time as the program the user was interacting with.

Eventually, people wanted to run more than one program at once, and multitasking was born. This allowed operating systems to run several programs at the same time by switching execution between them. Programs could decide when it would be an appropriate time to let another program run by yielding execution to the operating system. This approach is called cooperative multitasking.

In a cooperative multitasking environment, when a program fails to yield execution for any reason, no other program can continue executing. This interruption to other programs is not desirable, so eventually operating systems moved toward preemptive multitasking. In this model, the operating system would determine which program would run on the CPU at which time, using its own notion of scheduling, rather than relying on the programs themselves to be the sole deciders of when to switch execution. To this day, almost every operating system uses this approach, even on multi-core systems, because we generally have more programs running that we have CPU cores.

Running multiple tasks at once is extremely useful for both programmers and users. Before threads, a single program (that is, a single process) could not have multiple tasks running at the same time. Instead, programmers wishing to perform tasks concurrently would either have to split up the task into smaller chunks and schedule them inside the process, or run separate tasks in separate processes and have them communicate with each other.

Even today, in some high-level languages the appropriate way to run multiple tasks at once is to run additional processes. In some languages, like Ruby and Python, theres a global interpreter lock (GIL), meaning only one thread can be executing at a given time. While this makes memory management far more practical, it makes multithreaded programming not as attractive to programmers, and instead multiple processes are employed.

Until fairly recently, JavaScript was a language where the only multitasking mechanisms available were splitting tasks up and scheduling their pieces for later execution, and in the case of Node.js, running additional processes. Today, in all major JavaScript environments, we have access to threads, and unlike Ruby and Python, we dont have a GIL making them effectively useless for performing CPU-intensive tasks. Instead, other trade-offs are made, like not sharing JavaScript objects across threads (at least not directly). Still, threads are useful to JavaScript developers for cordoning off CPU-intensive tasks. In the browser, there are also special-purpose threads that have feature sets available to them that are different from the main thread.

The purpose of this book is to explore and explain JavaScript threads as a programming concept and tool. Youll learn how to use them, and more importantly, when to use them. Not every problem needs to be solved with threads. Not even every CPU-intensive problem needs to be solved with threads. Its the job of software developers to evaluate problems and tools to determine the most appropriate solutions. The aim here is to give you another tool, and enough knowledge around it to knew when to use it and how.