cmd/dist: avoid CPU underutilization starting from GOMAXPROCS=2 runtime

This CL is doing now is:
change maxbg to increase test parallelism.
adjust test sequence.

This CL speeds up the go tool dist test,
most of the speed up is due to the fact that the
three time-consuming tests
cmd/internal/testdir and API check and runtime/race
can be done in parallel with the GOMAXPROCS=2 runtime
on a machine with enough CPU cores.

In windows with an 8-core 16-thread CPU,
this CL can complete all other tests before
GOMAXPROCS=2 runtime -cpu=1,2,4 -quick completes.

Fixes #65164

Change-Id: I56ed7031d58be3bece9f975bfc73e5c834d0a4fa
GitHub-Last-Rev: 18cffb7
GitHub-Pull-Request: #65703
Reviewed-on: https://go-review.googlesource.com/c/go/+/563916
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Commit-Queue: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: David Chase <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

src/cmd/dist/test.go

@@ -711,24 +711,6 @@ func (t *tester) registerTests() {
 			})
 	}
 
-	// Runtime CPU tests.
-	if !t.compileOnly && t.hasParallelism() {
-		for i := 1; i <= 4; i *= 2 {
-			t.registerTest(fmt.Sprintf("GOMAXPROCS=2 runtime -cpu=%d -quick", i),
-				&goTest{
-					variant:   "cpu" + strconv.Itoa(i),
-					timeout:   300 * time.Second,
-					cpu:       strconv.Itoa(i),
-					short:     true,
-					testFlags: []string{"-quick"},
-					// We set GOMAXPROCS=2 in addition to -cpu=1,2,4 in order to test runtime bootstrap code,
-					// creation of first goroutines and first garbage collections in the parallel setting.
-					env: []string{"GOMAXPROCS=2"},
-					pkg: "runtime",
-				})
-		}
-	}
-
 	// GOEXPERIMENT=rangefunc tests
 	if !t.compileOnly {
 		t.registerTest("GOEXPERIMENT=rangefunc go test iter",
@@ -864,10 +846,6 @@ func (t *tester) registerTests() {
 			})
 	}
 
-	if t.raceDetectorSupported() {
-		t.registerRaceTests()
-	}
-
 	const cgoHeading = "Testing cgo"
 	if t.cgoEnabled {
 		t.registerCgoTests(cgoHeading)
@@ -883,6 +861,40 @@ func (t *tester) registerTests() {
 			})
 	}
 
+	// Only run the API check on fast development platforms.
+	// Every platform checks the API on every GOOS/GOARCH/CGO_ENABLED combination anyway,
+	// so we really only need to run this check once anywhere to get adequate coverage.
+	// To help developers avoid trybot-only failures, we try to run on typical developer machines
+	// which is darwin,linux,windows/amd64 and darwin/arm64.
+	//
+	// The same logic applies to the release notes that correspond to each api/next file.
+	if goos == "darwin" || ((goos == "linux" || goos == "windows") && goarch == "amd64") {
+		t.registerTest("API release note check", &goTest{variant: "check", pkg: "cmd/relnote", testFlags: []string{"-check"}})
+		t.registerTest("API check", &goTest{variant: "check", pkg: "cmd/api", timeout: 5 * time.Minute, testFlags: []string{"-check"}})
+	}
+
+	// Runtime CPU tests.
+	if !t.compileOnly && t.hasParallelism() {
+		for i := 1; i <= 4; i *= 2 {
+			t.registerTest(fmt.Sprintf("GOMAXPROCS=2 runtime -cpu=%d -quick", i),
+				&goTest{
+					variant:   "cpu" + strconv.Itoa(i),
+					timeout:   300 * time.Second,
+					cpu:       strconv.Itoa(i),
+					short:     true,
+					testFlags: []string{"-quick"},
+					// We set GOMAXPROCS=2 in addition to -cpu=1,2,4 in order to test runtime bootstrap code,
+					// creation of first goroutines and first garbage collections in the parallel setting.
+					env: []string{"GOMAXPROCS=2"},
+					pkg: "runtime",
+				})
+		}
+	}
+
+	if t.raceDetectorSupported() {
+		t.registerRaceTests()
+	}
+
 	if goos != "android" && !t.iOS() {
 		// Only start multiple test dir shards on builders,
 		// where they get distributed to multiple machines.
@@ -907,17 +919,6 @@ func (t *tester) registerTests() {
 			)
 		}
 	}
-	// Only run the API check on fast development platforms.
-	// Every platform checks the API on every GOOS/GOARCH/CGO_ENABLED combination anyway,
-	// so we really only need to run this check once anywhere to get adequate coverage.
-	// To help developers avoid trybot-only failures, we try to run on typical developer machines
-	// which is darwin,linux,windows/amd64 and darwin/arm64.
-	//
-	// The same logic applies to the release notes that correspond to each api/next file.
-	if goos == "darwin" || ((goos == "linux" || goos == "windows") && goarch == "amd64") {
-		t.registerTest("API check", &goTest{variant: "check", pkg: "cmd/api", timeout: 5 * time.Minute, testFlags: []string{"-check"}})
-		t.registerTest("API release note check", &goTest{variant: "check", pkg: "cmd/relnote", testFlags: []string{"-check"}})
-	}
 }
 
 // addTest adds an arbitrary test callback to the test list.
@@ -1313,6 +1314,23 @@ func (t *tester) runPending(nextTest *distTest) {
 		}(w)
 	}
 
+	maxbg := maxbg
+	// for runtime.NumCPU() < 4 ||  runtime.GOMAXPROCS(0) == 1, do not change maxbg.
+	// Because there is not enough CPU to parallel the testing of multiple packages.
+	if runtime.NumCPU() > 4 && runtime.GOMAXPROCS(0) != 1 {
+		for _, w := range worklist {
+			// See go.dev/issue/65164
+			// 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()
+				break
+			}
+		}
+	}
+
 	started := 0
 	ended := 0
 	var last *distTest