scrape: re-use memory in TargetsFromGroup

Common service discovery mechanisms such as Kubernetes can generate a
lot of target groups, so this function was allocating a lot of memory
which then immediately became garbage. Re-using the structures across
an entire Sync saves effort.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

cmd/promtool/main.go

@@ -1250,8 +1250,11 @@ func checkTargetGroupsForAlertmanager(targetGroups []*targetgroup.Group, amcfg *
 }
 
 func checkTargetGroupsForScrapeConfig(targetGroups []*targetgroup.Group, scfg *config.ScrapeConfig) error {
+	var targets []*scrape.Target
+	lb := labels.NewBuilder(labels.EmptyLabels())
 	for _, tg := range targetGroups {
-		_, failures := scrape.TargetsFromGroup(tg, scfg, false)
+		var failures []error
+		targets, failures = scrape.TargetsFromGroup(tg, scfg, false, targets, lb)
 		if len(failures) > 0 {
 			first := failures[0]
 			return first

scrape/scrape.go

@@ -490,9 +490,11 @@ func (sp *scrapePool) Sync(tgs []*targetgroup.Group) {
 
 	sp.targetMtx.Lock()
 	var all []*Target
+	var targets []*Target
+	lb := labels.NewBuilder(labels.EmptyLabels())
 	sp.droppedTargets = []*Target{}
 	for _, tg := range tgs {
-		targets, failures := TargetsFromGroup(tg, sp.config, sp.noDefaultPort)
+		targets, failures := TargetsFromGroup(tg, sp.config, sp.noDefaultPort, targets, lb)
 		for _, err := range failures {
 			level.Error(sp.logger).Log("msg", "Creating target failed", "err", err)
 		}

scrape/target.go

@@ -482,11 +482,10 @@ func PopulateLabels(lb *labels.Builder, cfg *config.ScrapeConfig, noDefaultPort
 }
 
 // TargetsFromGroup builds targets based on the given TargetGroup and config.
-func TargetsFromGroup(tg *targetgroup.Group, cfg *config.ScrapeConfig, noDefaultPort bool) ([]*Target, []error) {
-	targets := make([]*Target, 0, len(tg.Targets))
+func TargetsFromGroup(tg *targetgroup.Group, cfg *config.ScrapeConfig, noDefaultPort bool, targets []*Target, lb *labels.Builder) ([]*Target, []error) {
+	targets = targets[:0]
 	failures := []error{}
 
-	lb := labels.NewBuilder(labels.EmptyLabels())
 	for i, tlset := range tg.Targets {
 		lb.Reset(labels.EmptyLabels())
 

scrape/target_test.go

@@ -375,7 +375,8 @@ func TestTargetsFromGroup(t *testing.T) {
 		ScrapeTimeout:  model.Duration(10 * time.Second),
 		ScrapeInterval: model.Duration(1 * time.Minute),
 	}
-	targets, failures := TargetsFromGroup(&targetgroup.Group{Targets: []model.LabelSet{{}, {model.AddressLabel: "localhost:9090"}}}, &cfg, false)
+	lb := labels.NewBuilder(labels.EmptyLabels())
+	targets, failures := TargetsFromGroup(&targetgroup.Group{Targets: []model.LabelSet{{}, {model.AddressLabel: "localhost:9090"}}}, &cfg, false, nil, lb)
 	if len(targets) != 1 {
 		t.Fatalf("Expected 1 target, got %v", len(targets))
 	}
@@ -464,9 +465,11 @@ scrape_configs:
 				}
 				targets = append(targets, labels)
 			}
+			var tgets []*Target
+			lb := labels.NewBuilder(labels.EmptyLabels())
 			group := &targetgroup.Group{Targets: targets}
 			for i := 0; i < b.N; i++ {
-				targets, _ := TargetsFromGroup(group, config.ScrapeConfigs[0], false)
+				tgets, _ = TargetsFromGroup(group, config.ScrapeConfigs[0], false, tgets, lb)
 				if len(targets) != nTargets {
 					b.Fatalf("Expected %d targets, got %d", nTargets, len(targets))
 				}