What is the difference between concurrency and parallelism in Go?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Define concurrency

   Structure the program as independent tasks (goroutines) that can be started, paused, and resumed.

2. Step 2

   Define parallelism

   Recognize it as simultaneous execution requiring multiple CPU cores at the same instant.

3. Step 3

   Express concurrency in Go

   Launch goroutines with the go keyword and coordinate them with channels or sync primitives.

4. Step 4

   Enable parallelism

   Set GOMAXPROCS (default = number of cores) so the scheduler runs goroutines on multiple OS threads.

5. Step 5

   Observe the relationship

   The same concurrent code runs interleaved on one core or in parallel on many, without code changes.

Code Example

package main

import (
	"fmt"
	"runtime"
	"sync"
)

func main() {
	runtime.GOMAXPROCS(runtime.NumCPU()) // allow parallelism
	var wg sync.WaitGroup
	for i := 0; i < 3; i++ {
		wg.Add(1)
		go func(id int) { // concurrency: independent tasks
			defer wg.Done()
			fmt.Println("task", id)
		}(i)
	}
	wg.Wait()
}

Follow-up Questions

• What does GOMAXPROCS control and what is its default?
• Can a concurrent Go program run on a single core?
• How do goroutines differ from OS threads?
• When does adding more goroutines stop improving performance?
• How do channels help coordinate concurrent goroutines?