From 12cf4f79ff7fc56d4b5ba3b7ce2debd1cb46d27c Mon Sep 17 00:00:00 2001
From: "Peter J. Holzer" <hjp@hjp.at>
Date: Tue, 13 Jul 2021 00:13:44 +0200
Subject: [PATCH] Mark non-primes in parallel goroutines

---
 eratosthenes_parallel.go | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/eratosthenes_parallel.go b/eratosthenes_parallel.go
index b327807..517e34a 100644
--- a/eratosthenes_parallel.go
+++ b/eratosthenes_parallel.go
@@ -3,15 +3,27 @@ package main
 import (
 	"fmt"
 	"os"
+	"runtime"
 	"strconv"
+	"sync"
 	"time"
 )
 
+var markGroup sync.WaitGroup
+
+func markMultiples(sieve []uint64, i int64, n0, n1 int64) {
+	for j := n0; j < n1; j += 2 * i {
+		sieve[j>>7] |= 1 << ((j & 0x7F) >> 1)
+	}
+	markGroup.Done()
+}
+
 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
@@ -21,9 +33,16 @@ func main() {
 	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
+			for j := i * i; j < n; j = jnext {
+				jnext = ((n-j)/(2*i)/p+1)*(2*i) + j
+				markGroup.Add(1)
+				go markMultiples(sieve, i, j, jnext)
+				p--
 			}
+			markGroup.Wait()
 		}
 	}
 	d := time.Since(t0)