Concurrency, Goroutines and GOMAXPROCS

Introduction

When new people join the Go-Miami (http://www.meetup.com/Go-Miami/) group they always write that they want to learn more about Go’s concurrency model. Concurrency seems to be the big buzz word around the language. It was for me when I first started hearing about Go. It was Rob Pike’s Go Concurrency Patterns (http://www.youtube.com/watch?v=f6kdp27TYZs) video that finally convinced me I needed to learn this language.

To understand how Go makes writing concurrent programs easier and less prone to errors, we first need to understand what a concurrent program is and the problems that result from such programs. I will not be talking about CSP (Communicating Sequential Processes) in this post, which is the basis for Go’s implementation of channels. This post will focus on what a concurrent program is, the role that goroutines play and how the GOMAXPROCS environment variable and runtime function affects the behavior of the Go runtime and the programs we write.

Processes and Threads

When we run an application, like the browser I am using to write this post, a process is created by the operating system for the application. The job of the process is to act like a container for all the resources the application uses and maintains as it runs. These resources include things like a memory address space, handles to files, devices and threads.

A thread is a path of execution that is scheduled by the operating system to execute the code we write in our functions against a processor. A process starts out with one thread, the main thread, and when that thread terminates the process terminates. This is because the main thread is the origin for the application. The main thread can then in turn launch more threads and those threads can launch even more threads.

The operating system schedules a thread to run on an available processor regardless of which process the thread belongs to. Each operating system has its own algorithms that make these decisions and it is best for us to write concurrent programs that are not specific to one algorithm or the other. Plus these algorithms change with every new release of an operating system, so it is dangerous game to play.

Goroutines and Parallelism

Any function or method in Go can be created as a goroutine. We can consider that the main function is executing as a goroutine, however the Go runtime does not start that goroutine. Goroutines are considered to be lightweight because they use little memory and resources plus their initial stack size is small. Prior to version 1.2 the stack size started at 4K and now as of version 1.4 it starts at 8K. The stack has the ability to grow as needed.

The operating system schedules threads to run against available processors and the Go runtime schedules goroutines to run within a logical processor (https://www.ardanlabs.com/blog/2015/02/scheduler-tracing-in-go.html) that is bound to a single operating system thread. By default, the Go runtime allocates a single logical processor to execute all the goroutines that are created for our program. Even with this single logical processor and operating system thread, hundreds of thousands of goroutines can be scheduled to run concurrently with amazing efficiency and performance. It is not recommended to add more that one logical processor, but if you want to run goroutines in parallel, Go provides the ability to add more via the GOMAXPROCS environment variable or runtime function.

Concurrency is not Parallelism. Parallelism is when two or more threads are executing code simultaneously against different processors. If you configure the runtime to use more than one logical processor, the scheduler will distribute goroutines between these logical processors which will result in goroutines running on different operating system threads. However, to have true parallelism you need to run your program on a machine with multiple physical processors. If not, then the goroutines will be running concurrently against a single physical processor, even though the Go runtime is using multiple logical processors.

Concurrency Example

Let’s build a small program that shows Go running goroutines concurrently. In this example we are running the code with one logical processor: