Never start a goroutine without knowing how it will stop

In Go, goroutines are cheap to create and efficient to schedule. The Go runtime has been written for programs with tens of thousands of goroutines as the norm, hundreds of thousands are not unexpected. But goroutines do have a finite cost in terms of memory footprint; you cannot create an infinite number of them.

Every time you use the go keyword in your program to launch a goroutine, you must know how, and when, that goroutine will exit. If you don’t know the answer, that’s a potential memory leak.

Consider this trivial code snippet:

ch := somefunction()
go func() {
        for range ch { }
}()

This code obtains a channel of int from somefunction and starts a goroutine to drain it. When will this goroutine exit? It will only exit when ch is closed. When will that occur? It’s hard to say, ch is returned by somefunction. So, depending on the state of somefunction, ch might never be closed, causing the goroutine to quietly leak.

In your design, some goroutines may run until the program exits, for example a background goroutine watching a configuration file, or the main conn.Accept loop in your server. However, these goroutines are rare enough I don’t consider them an exception to this rule.

Every time you write the statement go in a program, you should consider the question of how, and under what conditions, the goroutine you are about to start, will end.

Threads are a strange abstraction

When you think about it, threads are a strange abstraction.

From the programmer’s point of view, threads are great. It’s as if you can create virtual CPUs, on the fly, and the operating system will take care of simulating these virtual CPUs on top of real ones.

But on an implementation level, what is a simple abstraction can lead you, the programmer, into a trap. You don’t have an infinite number of CPUs and applying threaded abstractions in an unbounded manner invites you to overwhelm the real CPU if you try to actually use all these virtual CPUs simultaneously, or overwhelm the address space if they sit idle due to the overhead needed to maintain the illusion.

Careful tuning of one or both of the number of threads in use by the program, or the amount of memory each thread is allocated is needed whenever threads as a concurrency primitive are used in anger. So much for abstraction.

Go’s goroutines sit right in the middle, the same programmer interface as threads, a nice imperative coding model, but also efficient implementation model based on coroutines.