fix: Use a pool of LabelPairs to reduce memory allocs (#2838)

Author: bryanhuhta

pkg/phlaredb/block_querier.go

@@ -1989,8 +1989,18 @@ func (b *singleBlockQuerier) Series(ctx context.Context, params *ingestv1.Series
 
 func (b *singleBlockQuerier) getUniqueLabelsSets(postings index.Postings, names []string, fingerprints *map[uint64]struct{}) ([]*typesv1.Labels, error) {
 	var labelsSets []*typesv1.Labels
+
+	// This memory will be re-used between posting iterations to avoid
+	// re-allocating many *typesv1.LabelPair objects.
+	matchedLabelsPool := make(phlaremodel.Labels, len(names))
+	for i := range matchedLabelsPool {
+		matchedLabelsPool[i] = &typesv1.LabelPair{}
+	}
+
 	for postings.Next() {
-		matchedLabels := make(phlaremodel.Labels, 0, len(names))
+		// Reset the pool.
+		matchedLabelsPool = matchedLabelsPool[:0]
+
 		for _, name := range names {
 			value, err := b.index.LabelValueFor(postings.At(), name)
 			if err != nil {
@@ -1999,22 +2009,29 @@ func (b *singleBlockQuerier) getUniqueLabelsSets(postings index.Postings, names
 				}
 				return nil, err
 			}
-			matchedLabels = append(matchedLabels, &typesv1.LabelPair{
-				Name:  name,
-				Value: value,
-			})
+
+			// Expand the pool's length and add this label to the end. The pool
+			// will always have enough capacity for all the labels.
+			matchedLabelsPool = matchedLabelsPool[:len(matchedLabelsPool)+1]
+			matchedLabelsPool[len(matchedLabelsPool)-1].Name = name
+			matchedLabelsPool[len(matchedLabelsPool)-1].Value = value
 		}
 
-		fp := matchedLabels.Hash()
+		fp := matchedLabelsPool.Hash()
 		_, ok := (*fingerprints)[fp]
 		if ok {
 			continue
 		}
 		(*fingerprints)[fp] = struct{}{}
 
-		labelsSets = append(labelsSets, &typesv1.Labels{
-			Labels: matchedLabels,
-		})
+		// Copy every element from the pool to a new slice.
+		labels := &typesv1.Labels{
+			Labels: make([]*typesv1.LabelPair, 0, len(matchedLabelsPool)),
+		}
+		for _, label := range matchedLabelsPool {
+			labels.Labels = append(labels.Labels, label.CloneVT())
+		}
+		labelsSets = append(labelsSets, labels)
 	}
 	return labelsSets, nil
 }

pkg/phlaredb/block_querier_test.go

@@ -230,6 +230,21 @@ func (f *fakeQuerier) SelectMatchingProfiles(ctx context.Context, params *ingest
 	return iter.NewSliceIterator(profiles), nil
 }
 
+func openSingleBlockQuerierIndex(t *testing.T, blockID string) *singleBlockQuerier {
+	t.Helper()
+
+	reader, err := index.NewFileReader(fmt.Sprintf("testdata/%s/index.tsdb", blockID))
+	require.NoError(t, err)
+
+	q := &singleBlockQuerier{
+		metrics: newBlocksMetrics(nil),
+		meta:    &block.Meta{ULID: ulid.MustParse(blockID)},
+		opened:  true, // Skip trying to open the block.
+		index:   reader,
+	}
+	return q
+}
+
 func TestSelectMatchingProfilesCleanUp(t *testing.T) {
 	defer goleak.VerifyNone(t, goleak.IgnoreCurrent())
 
@@ -245,15 +260,7 @@ func TestSelectMatchingProfilesCleanUp(t *testing.T) {
 
 func Test_singleBlockQuerier_Series(t *testing.T) {
 	ctx := context.Background()
-	reader, err := index.NewFileReader("testdata/01HA2V3CPSZ9E0HMQNNHH89WSS/index.tsdb")
-	assert.NoError(t, err)
-
-	q := &singleBlockQuerier{
-		metrics: newBlocksMetrics(nil),
-		meta:    &block.Meta{ULID: ulid.MustParse("01HA2V3CPSZ9E0HMQNNHH89WSS")},
-		opened:  true, // Skip trying to open the block.
-		index:   reader,
-	}
+	q := openSingleBlockQuerierIndex(t, "01HA2V3CPSZ9E0HMQNNHH89WSS")
 
 	t.Run("get all names", func(t *testing.T) {
 		want := []string{
@@ -1055,13 +1062,15 @@ func Test_singleBlockQuerier_ProfileTypes(t *testing.T) {
 }
 
 func Benchmark_singleBlockQuerier_Series(b *testing.B) {
+	const id = "01HA2V3CPSZ9E0HMQNNHH89WSS"
+
 	ctx := context.Background()
-	reader, err := index.NewFileReader("testdata/01HA2V3CPSZ9E0HMQNNHH89WSS/index.tsdb")
+	reader, err := index.NewFileReader(fmt.Sprintf("testdata/%s/index.tsdb", id))
 	assert.NoError(b, err)
 
 	q := &singleBlockQuerier{
 		metrics: newBlocksMetrics(nil),
-		meta:    &block.Meta{ULID: ulid.MustParse("01HA2V3CPSZ9E0HMQNNHH89WSS")},
+		meta:    &block.Meta{ULID: ulid.MustParse(id)},
 		opened:  true, // Skip trying to open the block.
 		index:   reader,
 	}
@@ -1083,6 +1092,15 @@ func Benchmark_singleBlockQuerier_Series(b *testing.B) {
 			})
 		}
 	})
+
+	b.Run("UI request", func(b *testing.B) {
+		for n := 0; n < b.N; n++ {
+			q.Series(ctx, &ingestv1.SeriesRequest{ //nolint:errcheck
+				Matchers:   []string{},
+				LabelNames: []string{"pyroscope_app", "service_name", "__profile_type__", "__type__", "__name__"},
+			})
+		}
+	})
 }
 
 func Benchmark_singleBlockQuerier_LabelNames(b *testing.B) {