Create goroutines adaptively during metrics gathering

prometheus/registry.go

@@ -18,6 +18,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"runtime"
 	"sort"
 	"sync"
 	"unicode/utf8"
@@ -202,6 +203,13 @@ func (errs MultiError) Error() string {
 	return buf.String()
 }
 
+// Append appends the provided error if it is not nil.
+func (errs *MultiError) Append(err error) {
+	if err != nil {
+		*errs = append(*errs, err)
+	}
+}
+
 // MaybeUnwrap returns nil if len(errs) is 0. It returns the first and only
 // contained error as error if len(errs is 1). In all other cases, it returns
 // the MultiError directly. This is helpful for returning a MultiError in a way
@@ -368,22 +376,12 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 	)
 
 	r.mtx.RLock()
-	metricFamiliesByName := make(map[string]*dto.MetricFamily, len(r.dimHashesByName))
-
-	// Scatter.
-	// (Collectors could be complex and slow, so we call them all at once.)
 	wg.Add(len(r.collectorsByID))
-	go func() {
-		wg.Wait()
-		close(metricChan)
-	}()
+	metricFamiliesByName := make(map[string]*dto.MetricFamily, len(r.dimHashesByName))
+	collectors := make(chan Collector, len(r.collectorsByID))
 	for _, collector := range r.collectorsByID {
-		go func(collector Collector) {
-			defer wg.Done()
-			collector.Collect(metricChan)
-		}(collector)
+		collectors <- collector
 	}
-
 	// In case pedantic checks are enabled, we have to copy the map before
 	// giving up the RLock.
 	if r.pedanticChecksEnabled {
@@ -392,78 +390,126 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 			registeredDescIDs[id] = struct{}{}
 		}
 	}
-
 	r.mtx.RUnlock()
 
+	collectWorker := func() {
+		for {
+			select {
+			case collector := <-collectors:
+				collector.Collect(metricChan)
+				wg.Done()
+			default:
+				return
+			}
+		}
+	}
+
+	// Start the first worker now to make sure at least is running.
+	go collectWorker()
+
+	// Close the metricChan once all collectors are collected.
+	go func() {
+		wg.Wait()
+		close(metricChan)
+	}()
+
 	// Drain metricChan in case of premature return.
 	defer func() {
 		for range metricChan {
 		}
 	}()
 
-	// Gather.
+collectLoop:
+	for {
+		select {
+		case metric, ok := <-metricChan:
+			if !ok {
+				// metricChan is closed, we are done.
+				break collectLoop
+			}
+			errs.Append(processMetric(
+				metric, metricFamiliesByName,
+				metricHashes, dimHashes,
+				registeredDescIDs,
+			))
+		default:
+			if len(collectors) == 0 {
+				// All collectors are being worked on. Just
+				// process metrics from now on.
 				for metric := range metricChan {
-		// This could be done concurrently, too, but it required locking
-		// of metricFamiliesByName (and of metricHashes if checks are
-		// enabled). Most likely not worth it.
+					errs.Append(processMetric(
+						metric, metricFamiliesByName,
+						metricHashes, dimHashes,
+						registeredDescIDs,
+					))
+				}
+				break collectLoop
+			}
+			// Start more workers.
+			go collectWorker()
+			runtime.Gosched()
+		}
+	}
+	return normalizeMetricFamilies(metricFamiliesByName), errs.MaybeUnwrap()
+}
+
+// processMetric is an internal helper method only used by the Gather method.
+func processMetric(
+	metric Metric,
+	metricFamiliesByName map[string]*dto.MetricFamily,
+	metricHashes map[uint64]struct{},
+	dimHashes map[string]uint64,
+	registeredDescIDs map[uint64]struct{},
+) error {
 	desc := metric.Desc()
 	dtoMetric := &dto.Metric{}
 	if err := metric.Write(dtoMetric); err != nil {
-			errs = append(errs, fmt.Errorf(
-				"error collecting metric %v: %s", desc, err,
-			))
-			continue
+		return fmt.Errorf("error collecting metric %v: %s", desc, err)
 	}
 	metricFamily, ok := metricFamiliesByName[desc.fqName]
 	if ok {
 		if metricFamily.GetHelp() != desc.help {
-				errs = append(errs, fmt.Errorf(
+			return fmt.Errorf(
 				"collected metric %s %s has help %q but should have %q",
 				desc.fqName, dtoMetric, desc.help, metricFamily.GetHelp(),
-				))
-				continue
+			)
 		}
 		// TODO(beorn7): Simplify switch once Desc has type.
 		switch metricFamily.GetType() {
 		case dto.MetricType_COUNTER:
 			if dtoMetric.Counter == nil {
-					errs = append(errs, fmt.Errorf(
+				return fmt.Errorf(
 					"collected metric %s %s should be a Counter",
 					desc.fqName, dtoMetric,
-					))
-					continue
+				)
 			}
 		case dto.MetricType_GAUGE:
 			if dtoMetric.Gauge == nil {
-					errs = append(errs, fmt.Errorf(
+				return fmt.Errorf(
 					"collected metric %s %s should be a Gauge",
 					desc.fqName, dtoMetric,
-					))
-					continue
+				)
 			}
 		case dto.MetricType_SUMMARY:
 			if dtoMetric.Summary == nil {
-					errs = append(errs, fmt.Errorf(
+				return fmt.Errorf(
 					"collected metric %s %s should be a Summary",
 					desc.fqName, dtoMetric,
-					))
-					continue
+				)
 			}
 		case dto.MetricType_UNTYPED:
 			if dtoMetric.Untyped == nil {
-					errs = append(errs, fmt.Errorf(
+				return fmt.Errorf(
 					"collected metric %s %s should be Untyped",
 					desc.fqName, dtoMetric,
-					))
-					continue
+				)
 			}
 		case dto.MetricType_HISTOGRAM:
 			if dtoMetric.Histogram == nil {
-					errs = append(errs, fmt.Errorf(
+				return fmt.Errorf(
 					"collected metric %s %s should be a Histogram",
 					desc.fqName, dtoMetric,
-					))
-					continue
+				)
 			}
 		default:
 			panic("encountered MetricFamily with invalid type")
@@ -485,34 +531,27 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 		case dtoMetric.Histogram != nil:
 			metricFamily.Type = dto.MetricType_HISTOGRAM.Enum()
 		default:
-				errs = append(errs, fmt.Errorf(
-					"empty metric collected: %s", dtoMetric,
-				))
-				continue
+			return fmt.Errorf("empty metric collected: %s", dtoMetric)
 		}
 		metricFamiliesByName[desc.fqName] = metricFamily
 	}
 	if err := checkMetricConsistency(metricFamily, dtoMetric, metricHashes, dimHashes); err != nil {
-			errs = append(errs, err)
-			continue
+		return err
 	}
-		if r.pedanticChecksEnabled {
+	if registeredDescIDs != nil {
 		// Is the desc registered at all?
 		if _, exist := registeredDescIDs[desc.id]; !exist {
-				errs = append(errs, fmt.Errorf(
+			return fmt.Errorf(
 				"collected metric %s %s with unregistered descriptor %s",
 				metricFamily.GetName(), dtoMetric, desc,
-				))
-				continue
+			)
 		}
 		if err := checkDescConsistency(metricFamily, dtoMetric, desc); err != nil {
-				errs = append(errs, err)
-				continue
+			return err
 		}
 	}
 	metricFamily.Metric = append(metricFamily.Metric, dtoMetric)
-	}
-	return normalizeMetricFamilies(metricFamiliesByName), errs.MaybeUnwrap()
+	return nil
 }
 
 // Gatherers is a slice of Gatherer instances that implements the Gatherer