Reduce allocation in discovery by ~30% (#2350)

* Reduce allocation in discovery by 30%

* changelog

CHANGELOG.md

@@ -65,6 +65,8 @@ Main (unreleased)
 
 - Update `async-profiler` binaries for `pyroscope.java` to 3.0-fa937db (@aleks-p)
 
+- Reduced memory allocation in discovery components by up to 30% (@thampiotr)
+
 ### Bugfixes
 
 - Fixed issue with automemlimit logging bad messages and trying to access cgroup on non-linux builds (@dehaansa)

internal/component/discovery/discovery.go

@@ -221,21 +221,7 @@ func (c *Component) runDiscovery(ctx context.Context, d DiscovererWithMetrics) {
 
 	// function to convert and send targets in format scraper expects
 	send := func() {
-		allTargets := []Target{}
-		for _, group := range cache {
-			for _, target := range group.Targets {
-				labels := map[string]string{}
-				// first add the group labels, and then the
-				// target labels, so that target labels take precedence.
-				for k, v := range group.Labels {
-					labels[string(k)] = string(v)
-				}
-				for k, v := range target {
-					labels[string(k)] = string(v)
-				}
-				allTargets = append(allTargets, labels)
-			}
-		}
+		allTargets := toAlloyTargets(cache)
 		componentID := livedebugging.ComponentID(c.opts.ID)
 		if c.debugDataPublisher.IsActive(componentID) {
 			c.debugDataPublisher.Publish(componentID, fmt.Sprintf("%s", allTargets))
@@ -274,4 +260,29 @@ func (c *Component) runDiscovery(ctx context.Context, d DiscovererWithMetrics) {
 	}
 }
 
+func toAlloyTargets(cache map[string]*targetgroup.Group) []Target {
+	targetsCount := 0
+	for _, group := range cache {
+		targetsCount += len(group.Targets)
+	}
+	allTargets := make([]Target, 0, targetsCount)
+
+	for _, group := range cache {
+		for _, target := range group.Targets {
+			tLabels := make(map[string]string, len(group.Labels)+len(target))
+
+			// first add the group labels, and then the
+			// target labels, so that target labels take precedence.
+			for k, v := range group.Labels {
+				tLabels[string(k)] = string(v)
+			}
+			for k, v := range target {
+				tLabels[string(k)] = string(v)
+			}
+			allTargets = append(allTargets, tLabels)
+		}
+	}
+	return allTargets
+}
+
 func (c *Component) LiveDebugging(_ int) {}

internal/component/discovery/discovery_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/Masterminds/goutils"
 	"github.com/go-kit/log"
 	"github.com/prometheus/common/model"
 	"github.com/prometheus/prometheus/discovery/targetgroup"
@@ -202,6 +203,49 @@ func TestDiscoveryUpdates(t *testing.T) {
 	}
 }
 
+/*
+on darwin/arm64/Apple M2:
+Benchmark_ToAlloyTargets-8   	     150	   7549967 ns/op	12768249 B/op	   40433 allocs/op
+Benchmark_ToAlloyTargets-8   	     169	   7257841 ns/op	12767441 B/op	   40430 allocs/op
+Benchmark_ToAlloyTargets-8   	     171	   7026276 ns/op	12767394 B/op	   40430 allocs/op
+Benchmark_ToAlloyTargets-8   	     170	   7060700 ns/op	12767377 B/op	   40430 allocs/op
+Benchmark_ToAlloyTargets-8   	     170	   7034392 ns/op	12767427 B/op	   40430 allocs/op
+*/
+func Benchmark_ToAlloyTargets(b *testing.B) {
+	sharedLabels := 5
+	labelsPerTarget := 5
+	labelsLength := 10
+	targetsCount := 20_000
+
+	genLabelSet := func(size int) model.LabelSet {
+		ls := model.LabelSet{}
+		for i := 0; i < size; i++ {
+			name, _ := goutils.RandomAlphaNumeric(labelsLength)
+			value, _ := goutils.RandomAlphaNumeric(labelsLength)
+			ls[model.LabelName(name)] = model.LabelValue(value)
+		}
+		return ls
+	}
+
+	var targets = []model.LabelSet{}
+	for i := 0; i < targetsCount; i++ {
+		targets = append(targets, genLabelSet(labelsPerTarget))
+	}
+
+	cache := map[string]*targetgroup.Group{}
+	cache["test"] = &targetgroup.Group{
+		Targets: targets,
+		Labels:  genLabelSet(sharedLabels),
+		Source:  "test",
+	}
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		toAlloyTargets(cache)
+	}
+}
+
 func updateDiscoverer(comp *Component, discoverer *fakeDiscoverer) {
 	comp.discMut.Lock()
 	defer comp.discMut.Unlock()