unwoops.go

package main

import (
	"fmt"
	"math/rand"
	"time"
)

// This program starts numProcs goroutines which each generate numSteps random
// numbers and prints the amount of time it takes.
//
// It scales perfectly on an i9 until we reach the number of cores, 6 (yes we have
// 12 hyperthreads but each pair shares a floating point core).

const numSteps = 1e6

func run(numProcs int) {
	t := time.Now()
	ch := make(chan bool)

	for i := 0; i < numProcs; i++ {
		go func() {
			// Generate a rand source for the thread.
			// Instead of accessing the threadsafe default source.
			// Which causes a lot of lock contention!
			r := rand.New(rand.NewSource(rand.Int63()))
			for i := 0; i < numSteps; i++ {
				_ = r.Float64()
			}
			ch <- true
		}()
	}

	for i := 0; i < numProcs; i++ {
		<-ch
	}
	fmt.Println(time.Now().Sub(t))
}

func main() {
	// Better!
	run(1) // 7.475166ms
	run(2) // 6.727281ms
	run(4) // 6.186384ms
	run(6) // 6.240453ms
	run(8) // 12.316837ms
}