Improve Gauge and Counter performance.

This is accomplished by using the functions from the atomic packages
instead of a mutex.

benchmark                                      old ns/op     new ns/op     delta
BenchmarkGaugeNoLabels-2                       118           9.40          -92.03%
BenchmarkGaugeNoLabels                         117           9.38          -91.98%
BenchmarkGaugeNoLabels-4                       117           9.40          -91.97%
BenchmarkCounterNoLabels-2                     137           16.8          -87.74%
BenchmarkCounterNoLabels                       136           16.8          -87.65%
BenchmarkCounterNoLabels-4                     136           16.8          -87.65%
BenchmarkGaugeWithLabelValues-4                400           279           -30.25%
BenchmarkGaugeWithLabelValues-2                398           279           -29.90%
BenchmarkGaugeWithLabelValues                  400           283           -29.25%
BenchmarkCounterWithLabelValues-4              397           286           -27.96%
BenchmarkCounterWithLabelValues-2              396           286           -27.78%
BenchmarkCounterWithLabelValues                394           285           -27.66%
BenchmarkCounterWithPreparedMappedLabels       587           454           -22.66%
BenchmarkCounterWithPreparedMappedLabels-2     581           456           -21.51%
BenchmarkCounterWithPreparedMappedLabels-4     654           539           -17.58%
BenchmarkCounterWithMappedLabels-2             1441          1218          -15.48%
BenchmarkCounterWithMappedLabels               1099          963           -12.37%
BenchmarkCounterWithMappedLabels-4             1636          1501          -8.25%

prometheus/counter_test.go

@@ -14,6 +14,7 @@
 package prometheus
 
 import (
+	"math"
 	"testing"
 
 	dto "github.com/prometheus/client_model/go"
@@ -26,11 +27,11 @@ func TestCounterAdd(t *testing.T) {
 		ConstLabels: Labels{"a": "1", "b": "2"},
 	}).(*counter)
 	counter.Inc()
-	if expected, got := 1., counter.val; expected != got {
+	if expected, got := 1., math.Float64frombits(counter.valBits); expected != got {
 		t.Errorf("Expected %f, got %f.", expected, got)
 	}
 	counter.Add(42)
-	if expected, got := 43., counter.val; expected != got {
+	if expected, got := 43., math.Float64frombits(counter.valBits); expected != got {
 		t.Errorf("Expected %f, got %f.", expected, got)
 	}
 

prometheus/gauge_test.go

@@ -82,7 +82,7 @@ func TestGaugeConcurrency(t *testing.T) {
 		}
 		start.Done()
 
-		if expected, got := <-result, gge.(*value).val; math.Abs(expected-got) > 0.000001 {
+		if expected, got := <-result, math.Float64frombits(gge.(*value).valBits); math.Abs(expected-got) > 0.000001 {
 			t.Fatalf("expected approx. %f, got %f", expected, got)
 			return false
 		}
@@ -146,7 +146,7 @@ func TestGaugeVecConcurrency(t *testing.T) {
 		start.Done()
 
 		for i := range sStreams {
-			if expected, got := <-results[i], gge.WithLabelValues(string('A'+i)).(*value).val; math.Abs(expected-got) > 0.000001 {
+			if expected, got := <-results[i], math.Float64frombits(gge.WithLabelValues(string('A'+i)).(*value).valBits); math.Abs(expected-got) > 0.000001 {
 				t.Fatalf("expected approx. %f, got %f", expected, got)
 				return false
 			}

prometheus/value.go

@@ -16,8 +16,9 @@ package prometheus
 import (
 	"errors"
 	"fmt"
+	"math"
 	"sort"
-	"sync"
+	"sync/atomic"
 
 	dto "github.com/prometheus/client_model/go"
 
@@ -44,10 +45,9 @@ var errInconsistentCardinality = errors.New("inconsistent label cardinality")
 type value struct {
 	SelfCollector
 
-	mtx        sync.RWMutex
 	desc       *Desc
 	valType    ValueType
-	val        float64
+	valBits    uint64 // These are the bits of the represented float64 value.
 	labelPairs []*dto.LabelPair
 }
 
@@ -61,7 +61,7 @@ func newValue(desc *Desc, valueType ValueType, val float64, labelValues ...strin
 	result := &value{
 		desc:       desc,
 		valType:    valueType,
-		val:        val,
+		valBits:    math.Float64bits(val),
 		labelPairs: makeLabelPairs(desc, labelValues),
 	}
 	result.Init(result)
@@ -73,10 +73,7 @@ func (v *value) Desc() *Desc {
 }
 
 func (v *value) Set(val float64) {
-	v.mtx.Lock()
-	defer v.mtx.Unlock()
-
-	v.val = val
+	atomic.StoreUint64(&v.valBits, math.Float64bits(val))
 }
 
 func (v *value) Inc() {
@@ -88,10 +85,13 @@ func (v *value) Dec() {
 }
 
 func (v *value) Add(val float64) {
-	v.mtx.Lock()
-	defer v.mtx.Unlock()
-
-	v.val += val
+	for {
+		oldBits := atomic.LoadUint64(&v.valBits)
+		newBits := math.Float64bits(math.Float64frombits(oldBits) + val)
+		if atomic.CompareAndSwapUint64(&v.valBits, oldBits, newBits) {
+			return
+		}
+	}
 }
 
 func (v *value) Sub(val float64) {
@@ -99,10 +99,7 @@ func (v *value) Sub(val float64) {
 }
 
 func (v *value) Write(out *dto.Metric) error {
-	v.mtx.RLock()
-	val := v.val
-	v.mtx.RUnlock()
-
+	val := math.Float64frombits(atomic.LoadUint64(&v.valBits))
 	return populateMetric(v.valType, val, v.labelPairs, out)
 }