all: add lower limit to the number of threads

mkill.go

@@ -19,10 +19,11 @@ import (
 )
 
 var (
-	pid       = os.Getpid()
-	maxThread = int32(runtime.NumCPU()) + 2 // 2 meaning runtime sysmon thread + template thread
-	interval  = time.Second
-	debug     = false
+	pid        = os.Getpid()
+	minThreads = int32(runtime.NumCPU()) + 2 // minimum number of threads required by the runtime
+	maxThreads = int32(runtime.NumCPU()) + 2 // 2 meaning runtime sysmon thread + template thread
+	interval   = time.Second
+	debug      = false
 )
 
 // NumM returns the number of running threads.
@@ -41,14 +42,15 @@ func NumM() int {
 }
 
 // GOMAXTHREADS sets the maximum number of system threads that allowed in a Go program
-// and returns the previous setting. If n < 1, it does not change the current setting.
-// The default allowed number of threads of a program is runtime.NumCPU() + 2.
+// and returns the previous setting. If n is lower than minimum required number of threads,
+// it does not change the current setting.
+// The minimum allowed number of threads of a program is runtime.NumCPU() + 2.
 func GOMAXTHREADS(n int) int {
-	if n < 1 {
-		return int(atomic.LoadInt32(&maxThread))
+	if n < int(minThreads) {
+		return int(atomic.LoadInt32(&maxThreads))
 	}
 
-	return int(atomic.SwapInt32(&maxThread, int32(n)))
+	return int(atomic.SwapInt32(&maxThreads, int32(n)))
 }
 
 // Wait waits until the number of threads meet the GOMAXTHREADS settings.
@@ -82,7 +84,7 @@ func checkwork() {
 func init() {
 	checkwork()
 	if debug {
-		fmt.Printf("mkill: pid %v, maxThread %v, interval %v\n", pid, maxThread, interval)
+		fmt.Printf("mkill: pid %v, maxThread %v, interval %v\n", pid, maxThreads, interval)
 	}
 
 	wg := sync.WaitGroup{}
@@ -92,10 +94,10 @@ func init() {
 			select {
 			case <-t.C:
 				n := NumM()
-				nkill := int32(n) - atomic.LoadInt32(&maxThread)
+				nkill := int32(n) - atomic.LoadInt32(&maxThreads)
 				if nkill <= 0 {
 					if debug {
-						fmt.Printf("mkill: checked #threads total %v / max %v\n", n, maxThread)
+						fmt.Printf("mkill: checked #threads total %v / max %v\n", n, maxThreads)
 					}
 					continue
 				}

mkill_test.go

@@ -7,6 +7,8 @@ package mkill_test
 import (
 	"context"
 	"fmt"
+	"runtime"
+	"sync"
 	"testing"
 	"time"
 
@@ -17,17 +19,29 @@ func TestMKill(t *testing.T) {
 	mkill.GOMAXTHREADS(10)
 
 	// create a lot of threads by sleep gs
+	wg := sync.WaitGroup{}
+	wg.Add(100000)
 	for i := 0; i < 100000; i++ {
 		go func() {
-			time.Sleep(time.Second * 10)
+			time.Sleep(time.Second * 1)
+			wg.Done()
 		}()
 	}
 
-	ctx, cancel := context.WithTimeout(context.Background(), time.Second*100)
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second*10)
 	defer cancel()
 	ok := mkill.Wait(ctx)
 	if !ok {
-		t.Fatal("mkill failed in 100s")
+		t.Fatal("mkill failed in 10s")
+	}
+	wg.Wait()
+}
+
+func TestMinThreads(t *testing.T) {
+	old := mkill.GOMAXTHREADS(0)
+	n := runtime.NumCPU()
+	if mkill.GOMAXTHREADS(n-1) != old {
+		t.Fatalf("number of threads is less than required in the runtime")
 	}
 }