fix for comments

Signed-off-by: Ziqi Zhao <zhaoziqi9146@gmail.com>

prometheus/histogram.go

@@ -761,8 +761,9 @@ func (h *histogram) Observe(v float64) {
 	h.observe(v, h.findBucket(v))
 }
 
-// ObserveWithExemplar should not be called in high-frequency settings,
+// ObserveWithExemplar should not be called in a high-frequency setting
-// since it isn't lock-free for native histograms with configured exemplars.
+// for a native histogram with configured exemplars. For this case,
+// the implementation isn't lock-free and might suffer from lock contention.
 func (h *histogram) ObserveWithExemplar(v float64, e Labels) {
 	i := h.findBucket(v)
 	h.observe(v, i)
@@ -843,6 +844,8 @@ func (h *histogram) Write(out *dto.Metric) error {
 			}}
 		}
 
+		// If exemplars are not configured, the cap will be 0.
+		// So append is not needed in this case.
 		if cap(h.nativeExemplars.exemplars) > 0 {
 			h.nativeExemplars.Lock()
 			his.Exemplars = append(his.Exemplars, h.nativeExemplars.exemplars...)
@@ -1709,7 +1712,7 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
 		nIdx = len(n.exemplars)
 	}
 
-	if otIdx != -1 && time.Since(ot) > n.ttl {
+	if otIdx != -1 && e.Timestamp.AsTime().Sub(ot) > n.ttl {
 		rIdx = otIdx
 	} else {
 		// In the previous for loop, when calculating the closest pair of exemplars,

prometheus/histogram_test.go

@@ -1314,7 +1314,7 @@ func TestNativeHistogramExemplar(t *testing.T) {
 		{
 			name: "remove exemplar with oldest timestamp, the removed index is smaller than inserted index",
 			addFunc: func(h *histogram) {
-				time.Sleep(10 * time.Second)
+				h.now = func() time.Time { return time.Now().Add(time.Second * 11) }
 				h.ObserveWithExemplar(6, Labels{"id": "1"})
 			},
 			expectedValues: []float64{0, 4, 6},
@@ -1324,14 +1324,7 @@ func TestNativeHistogramExemplar(t *testing.T) {
 	for _, tc := range tcs {
 		t.Run(tc.name, func(t *testing.T) {
 			tc.addFunc(h)
-			if len(h.nativeExemplars.exemplars) != len(tc.expectedValues) {
+			compareNativeExemplarValues(t, h.nativeExemplars.exemplars, tc.expectedValues)
-				t.Errorf("the count of exemplars is not %d", len(tc.expectedValues))
-			}
-			for i, e := range h.nativeExemplars.exemplars {
-				if e.GetValue() != tc.expectedValues[i] {
-					t.Errorf("the %dth exemplar value %v is not as expected: %v", i, e.GetValue(), tc.expectedValues[i])
-				}
-			}
 		})
 	}
 
@@ -1385,14 +1378,7 @@ func TestNativeHistogramExemplar(t *testing.T) {
 	for _, tc := range tcs {
 		t.Run(tc.name, func(t *testing.T) {
 			tc.addFunc(h)
-			if len(h.nativeExemplars.exemplars) != len(tc.expectedValues) {
+			compareNativeExemplarValues(t, h.nativeExemplars.exemplars, tc.expectedValues)
-				t.Errorf("the count of exemplars is not %d", len(tc.expectedValues))
-			}
-			for i, e := range h.nativeExemplars.exemplars {
-				if e.GetValue() != tc.expectedValues[i] {
-					t.Errorf("the %dth exemplar value %v is not as expected: %v", i, e.GetValue(), tc.expectedValues[i])
-				}
-			}
 		})
 	}
 
@@ -1425,14 +1411,18 @@ func TestNativeHistogramExemplar(t *testing.T) {
 	for _, tc := range tcs {
 		t.Run(tc.name, func(t *testing.T) {
 			tc.addFunc(h)
-			if len(h.nativeExemplars.exemplars) != len(tc.expectedValues) {
+			compareNativeExemplarValues(t, h.nativeExemplars.exemplars, tc.expectedValues)
-				t.Errorf("the count of exemplars is not %d", len(tc.expectedValues))
-			}
-			for i, e := range h.nativeExemplars.exemplars {
-				if e.GetValue() != tc.expectedValues[i] {
-					t.Errorf("the %dth exemplar value %v is not as expected: %v", i, e.GetValue(), tc.expectedValues[i])
-				}
-			}
 		})
 	}
 }
+
+func compareNativeExemplarValues(t *testing.T, exps []*dto.Exemplar, values []float64) {
+	if len(exps) != len(values) {
+		t.Errorf("the count of exemplars is not %d", len(values))
+	}
+	for i, e := range exps {
+		if e.GetValue() != values[i] {
+			t.Errorf("the %dth exemplar value %v is not as expected: %v", i, e.GetValue(), values[i])
+		}
+	}
+}