Inconsistent results using goroutines with sync.WaitGroup

Question

I am trying to count the number of prime numbers that are smaller than an arbitrary integer i using goroutines (in Go lang). For example, if i is 100, the result should be 25.

The following is my current implementation:

package "main"

import (
    "fmt"
    "math"
    "sync"
    "time"
)

var wg sync.WaitGroup

func isprime(x int) bool {
    if x == 2 {
        return true
    }
    if x == 1 || x%2 == 0 {
        return false
    }
    var xi = float64(x)
    for i := 3; float64(i) < (math.Pow(xi, 0.5) + 1.0); i += 2.0 {
        if x%i == 0 {
            return false
        }
    }
    return true
}

func main() {
    fmt.Print("Till what number should I count primes? ")
    var i int
    fmt.Scan(&i)

    r := 0
    pr := &r
    fmt.Println("Counting primes till ", i)
    start := time.Now()
    for x := 0; x <= i; x++ {
        wg.Add(1)
        go func(n int) {
            defer wg.Done()
            if isprime(n) {
                *pr += 1
            }
        }(x)
    }
    wg.Wait()
    elapsed := time.Since(start).Seconds()
    fmt.Println("Counted", r, "primes")
    fmt.Println("took", elapsed, "seconds")
}

When I run this program, I get the correct results for smaller values of i (till about a 1000)

But for larger values of i, the results are inconsistent and incorrect.

❯ ./main
Till what number should I count primes? 10000
Counting primes till  10000
Counted 1228 primes
took 0.006776541 seconds
❯ ./main
Till what number should I count primes? 10000
Counting primes till  10000
Counted 1227 primes
took 0.004183875 seconds
❯ ./main
Till what number should I count primes? 1000000
Counting primes till  1000000
Counted 78254 primes
took 0.441985921 seconds
❯ ./main
Till what number should I count primes? 1000000
Counting primes till  1000000
Counted 78327 primes
took 0.430042047 seconds

The fluctuation in results increases as the value of i gets larger. What is causing this? Is there any way I can make it consistent and correct?

Answer 1

You have a shared variable without proper synchronization. There is a race condition (*pr += 1). Adding mutex before and after shared variable fix it (mu.Lock(), mu.Unlock()).

Code:

var wg sync.WaitGroup
var mu sync.Mutex

func main() {
    fmt.Print("Till what number should I count primes? ")
    var i int
    fmt.Scan(&i)

    r := 0
    pr := &r
    fmt.Println("Counting primes till ", i)
    start := time.Now()
    for x := 0; x <= i; x++ {
        wg.Add(1)
        go func(n int) {
            defer wg.Done()
            if isprime(n) {
                mu.Lock()   // <= lock
                *pr += 1
                mu.Unlock()  // <= unlock
            }
        }(x)
    }
    wg.Wait()
    elapsed := time.Since(start).Seconds()
    fmt.Println("Counted", r, "primes")
    fmt.Println("took", elapsed, "seconds")
}

Output:

Till what number should I count primes? 1000000
Counting primes till  1000000
Counted 78498 primes
took 0.6783484 seconds
Till what number should I count primes? 1000000
Counting primes till  1000000
Counted 78498 primes
took 0.5428273 seconds
Till what number should I count primes? 1000000
Counting primes till  1000000
Counted 78498 primes
took 0.5521617 seconds