cmd/dist: increase test parallelism starting with GOMAXPROCS=2 runtime

Fixes golang#65164

Change-Id: Ia10952f50a3c2a7868e30dcdba333927947b3ac3

src/cmd/dist/test.go

@@ -1272,8 +1272,8 @@ func (t *tester) runPending(nextTest *distTest) {
 	worklist := t.worklist
 	t.worklist = nil
 	for _, w := range worklist {
-		w.start = make(chan bool)
-		w.end = make(chan struct{})
+		w.start = make(chan bool, runtime.NumCPU()*2)
+		w.end = make(chan struct{}, runtime.NumCPU()*2)
 		// w.cmd must be set up to write to w.out. We can't check that, but we
 		// can check for easy mistakes.
 		if w.cmd.Stdout == nil || w.cmd.Stdout == os.Stdout || w.cmd.Stderr == nil || w.cmd.Stderr == os.Stderr {
@@ -1303,6 +1303,21 @@ func (t *tester) runPending(nextTest *distTest) {
 		}(w)
 	}
 
+	// for runtime.NumCPU() > 4 , do not change maxbg.
+	// Because there is not enough CPU to parallel the testing of multiple packages.
+	if runtime.NumCPU() > 4 {
+		for _, w := range worklist {
+			// because GOMAXPROCS=2 runtime CPU usage is low,
+			// so increase maxbg to avoid slowing down execution with low CPU usage.
+			// This makes testing a single package slower,
+			// but testing multiple packages together faster.
+			if strings.Contains(w.dt.heading, "GOMAXPROCS=2 runtime") {
+				maxbg = runtime.NumCPU() * 2
+				break
+			}
+		}
+	}
+
 	started := 0
 	ended := 0
 	var last *distTest