diff --git a/core/load/adaptiveshedder_test.go b/core/load/adaptiveshedder_test.go
index 48e1343f..7be62b7e 100644
--- a/core/load/adaptiveshedder_test.go
+++ b/core/load/adaptiveshedder_test.go
@@ -135,6 +135,7 @@ func TestAdaptiveShedderShouldDrop(t *testing.T) {
 		passCounter:     passCounter,
 		rtCounter:       rtCounter,
 		windows:         buckets,
+		dropTime:        syncx.NewAtomicDuration(),
 		droppedRecently: syncx.NewAtomicBool(),
 	}
 	// cpu >=  800, inflight < maxPass
@@ -160,6 +161,40 @@ func TestAdaptiveShedderShouldDrop(t *testing.T) {
 	}
 	shedder.avgFlying = 80
 	assert.False(t, shedder.shouldDrop())
+
+	// cpu >=  800, inflight < maxPass
+	systemOverloadChecker = func(int64) bool {
+		return true
+	}
+	shedder.avgFlying = 80
+	shedder.flying = 80
+	_, err := shedder.Allow()
+	assert.NotNil(t, err)
+}
+
+func TestAdaptiveShedderStillHot(t *testing.T) {
+	logx.Disable()
+	passCounter := newRollingWindow()
+	rtCounter := newRollingWindow()
+	for i := 0; i < 10; i++ {
+		if i > 0 {
+			time.Sleep(bucketDuration)
+		}
+		passCounter.Add(float64((i + 1) * 100))
+		for j := i*10 + 1; j <= i*10+10; j++ {
+			rtCounter.Add(float64(j))
+		}
+	}
+	shedder := &adaptiveShedder{
+		passCounter:     passCounter,
+		rtCounter:       rtCounter,
+		windows:         buckets,
+		dropTime:        syncx.NewAtomicDuration(),
+		droppedRecently: syncx.ForAtomicBool(true),
+	}
+	assert.False(t, shedder.stillHot())
+	shedder.dropTime.Set(-coolOffDuration * 2)
+	assert.False(t, shedder.stillHot())
 }
 
 func BenchmarkAdaptiveShedder_Allow(b *testing.B) {
diff --git a/core/load/sheddergroup_test.go b/core/load/sheddergroup_test.go
index f58f1588..1e3b82e9 100644
--- a/core/load/sheddergroup_test.go
+++ b/core/load/sheddergroup_test.go
@@ -13,3 +13,8 @@ func TestGroup(t *testing.T) {
 		assert.NotNil(t, limiter)
 	})
 }
+
+func TestShedderClose(t *testing.T) {
+	var nop nopCloser
+	assert.Nil(t, nop.Close())
+}
diff --git a/core/syncx/limit.go b/core/syncx/limit.go
index 04fa3aaa..d36c3fc9 100644
--- a/core/syncx/limit.go
+++ b/core/syncx/limit.go
@@ -6,18 +6,22 @@ import (
 	"github.com/tal-tech/go-zero/core/lang"
 )
 
-var ErrReturn = errors.New("discarding limited token, resource pool is full, someone returned multiple times")
+// ErrLimitReturn indicates that the more than borrowed elements were returned.
+var ErrLimitReturn = errors.New("discarding limited token, resource pool is full, someone returned multiple times")
 
+// Limit controls the concurrent requests.
 type Limit struct {
 	pool chan lang.PlaceholderType
 }
 
+// NewLimit creates a Limit that can borrow n elements from it concurrently.
 func NewLimit(n int) Limit {
 	return Limit{
 		pool: make(chan lang.PlaceholderType, n),
 	}
 }
 
+// Borrow borrows an element from Limit in blocking mode.
 func (l Limit) Borrow() {
 	l.pool <- lang.Placeholder
 }
@@ -28,10 +32,12 @@ func (l Limit) Return() error {
 	case <-l.pool:
 		return nil
 	default:
-		return ErrReturn
+		return ErrLimitReturn
 	}
 }
 
+// TryBorrow tries to borrow an element from Limit, in non-blocking mode.
+// If success, true returned, false for otherwise.
 func (l Limit) TryBorrow() bool {
 	select {
 	case l.pool <- lang.Placeholder:
diff --git a/core/syncx/limit_test.go b/core/syncx/limit_test.go
index 338e23c5..1465dbb0 100644
--- a/core/syncx/limit_test.go
+++ b/core/syncx/limit_test.go
@@ -13,5 +13,5 @@ func TestLimit(t *testing.T) {
 	assert.False(t, limit.TryBorrow())
 	assert.Nil(t, limit.Return())
 	assert.Nil(t, limit.Return())
-	assert.Equal(t, ErrReturn, limit.Return())
+	assert.Equal(t, ErrLimitReturn, limit.Return())
 }
diff --git a/core/syncx/timeoutlimit_test.go b/core/syncx/timeoutlimit_test.go
index a1e54704..5f6e3fe2 100644
--- a/core/syncx/timeoutlimit_test.go
+++ b/core/syncx/timeoutlimit_test.go
@@ -29,5 +29,5 @@ func TestTimeoutLimit(t *testing.T) {
 	assert.Equal(t, ErrTimeout, limit.Borrow(time.Millisecond*100))
 	assert.Nil(t, limit.Return())
 	assert.Nil(t, limit.Return())
-	assert.Equal(t, ErrReturn, limit.Return())
+	assert.Equal(t, ErrLimitReturn, limit.Return())
 }
diff --git a/rest/handler/maxconnshandler.go b/rest/handler/maxconnshandler.go
index d4a51233..6fffc880 100644
--- a/rest/handler/maxconnshandler.go
+++ b/rest/handler/maxconnshandler.go
@@ -28,7 +28,7 @@ func MaxConns(n int) func(http.Handler) http.Handler {
 
 				next.ServeHTTP(w, r)
 			} else {
-				internal.Errorf(r, "Concurrent connections over %d, rejected with code %d",
+				internal.Errorf(r, "concurrent connections over %d, rejected with code %d",
 					n, http.StatusServiceUnavailable)
 				w.WriteHeader(http.StatusServiceUnavailable)
 			}