Go: Asynchronous Preemption

Illustration created for “A Journey With Go”, made from the original Go Gopher, created by Renee French.

ℹ️ This article is based on Go 1.14.

The preemption is an important part of the scheduler that lets it distribute the running time among the goroutines. Indeed, without preemption, a long-running goroutine that hogs the CPU would block the other goroutines from being scheduled. The version 1.14 introduces a new technique of asynchronous preemption, giving more power and control to the scheduler.

For more details about the previous behavior and its drawback, I suggest you read my article “Go: Goroutine and Preemption.”

Workflow

Let’s start with an example where preemption is needed. Here is a code where many goroutines loop for a while without any function call, meaning no opportunity for the scheduler to preempt them:

However, when visualizing the traces from this program, we clearly see the goroutines are preempted and switch among them:

We can also see that all the blocks representing the goroutines have the same length. The…