Start goroutines once and use channels to distribute work

eratosthenes_parallel.go

@@ -3,26 +3,68 @@ package main
 import (
 	"fmt"
 	"os"
+	"runtime"
 	"strconv"
 	"time"
 )
 
+type work struct {
+	i int64
+	n0, n1 int64
+}
+
+type result bool
+
+func markMultiples(sieve []uint64, workCh chan work, resultCh chan result) {
+	for {
+		w := <-workCh
+		for j := w.n0; j < w.n1; j += 2 * w.i {
+			sieve[j>>7] |= 1 << ((j & 0x7F) >> 1)
+		}
+		resultCh <- true
+	}
+}
+
 func main() {
 	n, err := strconv.ParseInt(os.Args[1], 10, 64)
 	if err != nil {
 		panic(err)
 	}
+	nCpus := int64(runtime.NumCPU())
 	t0 := time.Now()
 	sieve := make([]uint64, (n>>7)+1)
-	var found int64
+	workCh := make(chan work)
+	resultCh := make(chan result)
 	var i int64
+	for i = 0; i < nCpus; i++ {
+		go markMultiples(sieve, workCh, resultCh)
+	}
+	var found int64
 	i = 2
 	found++
 	for i = 3; i < n; i += 2 {
 		if (sieve[i>>7] & (1 << ((i & 0x7F) >> 1))) == 0 {
 			found++
-			for j := i * i; j < n; j += 2 * i {
-				sieve[j>>7] |= 1 << ((j & 0x7F) >> 1)
+
+			p := nCpus
+			var jnext int64
+			// XXX - i * i can overflow.
+			// bail out before that happens
+			if n / i < i {
+				continue;
+			}
+			workers := 0
+			for j := i * i; j < n; j = jnext {
+				jnext = ((n-j)/(2*i)/p+1)*(2*i) + j
+				if jnext > n {
+					jnext = n
+				}
+				workCh <-work{ i, j, jnext }
+				workers++
+				p--
+			}
+			for k  := 0; k < workers; k++ {
+				_ = <-resultCh
 			}
 		}
 	}