woops.go

// This program starts N goroutines which each generate numSteps random
// numbers and prints the amount of time it takes.
//
// See if you can you figure out why it doesn't parallelise properly.
package main

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

const numSteps = 1e6

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

	for i := 0; i < numProcs; i++ {
		go func() {
			for i := 0; i < numSteps; i++ {
				_ = rand.Float64()
			}
			ch <- true
		}()
	}

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

func main() {
	// These "should" all be equally fast if we have a multicore machine.
	// But they scale super-linearly with numProcs.
	run(1) // 20.318803ms
	run(2) // 46.067529ms
	run(4) // 154.362519ms
	run(8) // 411.398554ms
}