vmalert: reduce allocations for Prometheus resp parse (#3435)

Method `metrics()` now pre-allocates slices for labels
and results from query responses. This reduces the number 
of allocations on the hot path for instant requests.

Signed-off-by: hagen1778 <roman@victoriametrics.com>

app/vmalert/datasource/datasource.go

@@ -54,6 +54,19 @@ func (m *Metric) SetLabel(key, value string) {
 	m.AddLabel(key, value)
 }
 
+// SetLabels sets the given map as Metric labels
+func (m *Metric) SetLabels(ls map[string]string) {
+	var i int
+	m.Labels = make([]Label, len(ls))
+	for k, v := range ls {
+		m.Labels[i] = Label{
+			Name:  k,
+			Value: v,
+		}
+		i++
+	}
+}
+
 // AddLabel appends the given label to the label set
 func (m *Metric) AddLabel(key, value string) {
 	m.Labels = append(m.Labels, Label{Name: key, Value: value})

app/vmalert/datasource/vm_prom_api.go

@@ -32,19 +32,17 @@ type promInstant struct {
 }
 
 func (r promInstant) metrics() ([]Metric, error) {
-	var result []Metric
+	result := make([]Metric, len(r.Result))
 	for i, res := range r.Result {
 		f, err := strconv.ParseFloat(res.TV[1].(string), 64)
 		if err != nil {
 			return nil, fmt.Errorf("metric %v, unable to parse float64 from %s: %w", res, res.TV[1], err)
 		}
 		var m Metric
-		for k, v := range r.Result[i].Labels {
-			m.AddLabel(k, v)
-		}
+		m.SetLabels(res.Labels)
 		m.Timestamps = append(m.Timestamps, int64(res.TV[0].(float64)))
 		m.Values = append(m.Values, f)
-		result = append(result, m)
+		result[i] = m
 	}
 	return result, nil
 }

app/vmalert/datasource/vm_prom_api_test.go

@@ -0,0 +1,20 @@
+package datasource
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func BenchmarkMetrics(b *testing.B) {
+	payload := []byte(`[{"metric":{"__name__":"vm_rows"},"value":[1583786142,"13763"]},{"metric":{"__name__":"vm_requests", "foo":"bar", "baz": "qux"},"value":[1583786140,"2000"]}]`)
+
+	var pi promInstant
+	if err := json.Unmarshal(payload, &pi.Result); err != nil {
+		b.Fatalf(err.Error())
+	}
+	b.Run("Instant", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			_, _ = pi.metrics()
+		}
+	})
+}

app/vmalert/datasource/vm_test.go

@@ -7,6 +7,7 @@ import (
 	"net/http/httptest"
 	"net/url"
 	"reflect"
+	"sort"
 	"strconv"
 	"strings"
 	"testing"
@@ -74,7 +75,7 @@ func TestVMInstantQuery(t *testing.T) {
 		case 5:
 			w.Write([]byte(`{"status":"success","data":{"resultType":"matrix"}}`))
 		case 6:
-			w.Write([]byte(`{"status":"success","data":{"resultType":"vector","result":[{"metric":{"__name__":"vm_rows"},"value":[1583786142,"13763"]},{"metric":{"__name__":"vm_requests"},"value":[1583786140,"2000"]}]}}`))
+			w.Write([]byte(`{"status":"success","data":{"resultType":"vector","result":[{"metric":{"__name__":"vm_rows","foo":"bar"},"value":[1583786142,"13763"]},{"metric":{"__name__":"vm_requests","foo":"baz"},"value":[1583786140,"2000"]}]}}`))
 		case 7:
 			w.Write([]byte(`{"status":"success","data":{"resultType":"scalar","result":[1583786142, "1"]}}`))
 		}
@@ -115,19 +116,17 @@ func TestVMInstantQuery(t *testing.T) {
 	}
 	expected := []Metric{
 		{
-			Labels:     []Label{{Value: "vm_rows", Name: "__name__"}},
+			Labels:     []Label{{Value: "vm_rows", Name: "__name__"}, {Value: "bar", Name: "foo"}},
 			Timestamps: []int64{1583786142},
 			Values:     []float64{13763},
 		},
 		{
-			Labels:     []Label{{Value: "vm_requests", Name: "__name__"}},
+			Labels:     []Label{{Value: "vm_requests", Name: "__name__"}, {Value: "baz", Name: "foo"}},
 			Timestamps: []int64{1583786140},
 			Values:     []float64{2000},
 		},
 	}
-	if !reflect.DeepEqual(m, expected) {
-		t.Fatalf("unexpected metric %+v want %+v", m, expected)
-	}
+	metricsEqual(t, m, expected)
 
 	m, req, err := pq.Query(ctx, query, ts) // 7 - scalar
 	if err != nil {
@@ -158,13 +157,36 @@ func TestVMInstantQuery(t *testing.T) {
 	if len(m) != 1 {
 		t.Fatalf("expected 1 metric  got %d in %+v", len(m), m)
 	}
-	exp := Metric{
-		Labels:     []Label{{Value: "constantLine(10)", Name: "name"}},
-		Timestamps: []int64{1611758403},
-		Values:     []float64{10},
+	exp := []Metric{
+		{
+			Labels:     []Label{{Value: "constantLine(10)", Name: "name"}},
+			Timestamps: []int64{1611758403},
+			Values:     []float64{10},
+		},
 	}
-	if !reflect.DeepEqual(m[0], exp) {
-		t.Fatalf("unexpected metric %+v want %+v", m[0], expected)
+	metricsEqual(t, m, exp)
+}
+
+func metricsEqual(t *testing.T, gotM, expectedM []Metric) {
+	for i, exp := range expectedM {
+		got := gotM[i]
+		gotTS, expTS := got.Timestamps, exp.Timestamps
+		if !reflect.DeepEqual(gotTS, expTS) {
+			t.Fatalf("unexpected timestamps %+v want %+v", gotTS, expTS)
+		}
+		gotV, expV := got.Values, exp.Values
+		if !reflect.DeepEqual(gotV, expV) {
+			t.Fatalf("unexpected values %+v want %+v", gotV, expV)
+		}
+		sort.Slice(got.Labels, func(i, j int) bool {
+			return got.Labels[i].Name < got.Labels[j].Name
+		})
+		sort.Slice(exp.Labels, func(i, j int) bool {
+			return exp.Labels[i].Name < exp.Labels[j].Name
+		})
+		if !reflect.DeepEqual(exp.Labels, got.Labels) {
+			t.Fatalf("unexpected labels %+v want %+v", got.Labels, exp.Labels)
+		}
 	}
 }