Goroutines not Using Max CPU and Cores

Question

I'm realizing my very first Golang application and I'm having some issues on using MAX CPU & Cores when using GoRoutines and I don't really know why.

When using a tool such as htop , CPU isn't used at its max power and only 1..4 threads are active at time. Also, all cores are active but they are around 25%-40% utilization.

I used:

func MaxParallelism() int {
    maxProcs := runtime.GOMAXPROCS(0)
    numCPU := runtime.NumCPU()
    if maxProcs < numCPU {
        return maxProcs
    }
    return numCPU
}

In order to get the number of goroutines to be instantiated.

Here's How I set up the application:

//Common Channel for the goroutines
tasks := make(chan *exec.Cmd, 64)

    //Spawning Max Cores Number goroutines (8)
    var wg sync.WaitGroup
    cores := MaxParallelism()
    for i := 0; i < cores; i++ {
        wg.Add(1)
        go func(num int, w *sync.WaitGroup) {
            defer w.Done()
            var (
                out []byte
                err error
            )
            for cmd := range tasks {
                out, err = cmd.Output()
                if err != nil {
                    fmt.Printf("Can't get stdout:", err)
                }
                . . .
            }
        }(i, &wg)
    }

    //Generate Tasks
    for i := 0; i < 100000; i++ {
      tasks <- exec.Command(cmd1, args...)
      tasks <- exec.Command(cmd2, args...)
    }

close(tasks)
// wait for the workers to finish
wg.Wait()

I share two screenshots of htop while executing the application

I don't know If it May help but I'm launching it through Intellij Idea .

How do I use Max CPU properly and Cores?

Thanks in advance.

Answer 1

Goroutines and threads are not the same. Ergo you should not expect any CPU affinity. See more for details here http://tleyden.github.io/blog/2014/10/30/goroutines-vs-threads/ .

Here are some of the advantages of Goroutines over threads:

• You can run more goroutines on a typical system than you can threads.
• Goroutines have growable segmented stacks.
• Goroutines have a faster startup time than threads.
• Goroutines come with built-in primitives to communicate safely between themselves (channels).
• Goroutines allow you to avoid having to resort to mutex locking when sharing data structures.
• Goroutines are multiplexed onto a small number of OS threads, rather than a 1:1 mapping.
• You can write massively concurrent servers without having to resort to evented programming.

EDIT: Answer to your question in the comments. There is no definitive answer. As others mentioned, it depends on what your code does. You may never end up using 100% CPU if you, for example, do I/O, which is slow. So no matter how many routines you start, I/O is slow. Contrarily, if your goroutine has a very tight loop doing just some computation then it's likely that 8 goroutines will consume your 8 CPUs completely.

Answer 2

After reading around, getting some hints and trying some stuff, it came out that the code was written just fine and there were not real problems caused by it.

Essentially, Go scales pretty good and, the more tasks you generate, the more CPU resources are used.

For instance, setting an higher range on the for , eg 1.000.000 , makes Go use all the cores available at ~55% of their possibilities.

I hope it will help someone.