Make Counter faster for simple Inc or Add with an integer

prometheus/counter.go

@@ -15,6 +15,10 @@ package prometheus
 
 import (
 	"errors"
+	"math"
+	"sync/atomic"
+
+	dto "github.com/prometheus/client_model/go"
 )
 
 // Counter is a Metric that represents a single numerical value that only ever
@@ -42,27 +46,73 @@ type Counter interface {
 type CounterOpts Opts
 
 // NewCounter creates a new Counter based on the provided CounterOpts.
+//
+// The returned implementation tracks the counter value in two separate
+// variables, a float64 and a uint64. The latter is used to track calls of the
+// Inc method and calls of the Add method with a value that can be represented
+// as a uint64. This allows atomic increments of the counter with optimal
+// performance. (It is common to have an Inc call in very hot execution paths.)
+// Both internal tracking values are added up in the Write method. This has to
+// be taken into account when it comes to precision and overflow behavior.
 func NewCounter(opts CounterOpts) Counter {
 	desc := NewDesc(
 		BuildFQName(opts.Namespace, opts.Subsystem, opts.Name),
 		opts.Help,
 		nil,
 		opts.ConstLabels,
 	)
-	result := &counter{value: value{desc: desc, valType: CounterValue, labelPairs: desc.constLabelPairs}}
+	result := &counter{desc: desc, labelPairs: desc.constLabelPairs}
 	result.init(result) // Init self-collection.
 	return result
 }
 
 type counter struct {
-	value
+	// valBits contains the bits of the represented float64 value, while
+	// valInt stores values that are exact integers. Both have to go first
+	// in the struct to guarantee alignment for atomic operations.
+	// http://golang.org/pkg/sync/atomic/#pkg-note-BUG
+	valBits uint64
+	valInt  uint64
+
+	selfCollector
+	desc *Desc
+
+	labelPairs []*dto.LabelPair
+}
+
+func (c *counter) Desc() *Desc {
+	return c.desc
 }
 
 func (c *counter) Add(v float64) {
 	if v < 0 {
 		panic(errors.New("counter cannot decrease in value"))
 	}
-	c.value.Add(v)
+	ival := uint64(v)
+	if float64(ival) == v {
+		atomic.AddUint64(&c.valInt, ival)
+		return
+	}
+
+	for {
+		oldBits := atomic.LoadUint64(&c.valBits)
+		newBits := math.Float64bits(math.Float64frombits(oldBits) + v)
+		if atomic.CompareAndSwapUint64(&c.valBits, oldBits, newBits) {
+			return
+		}
+	}
+}
+
+func (c *counter) Inc() {
+	atomic.AddUint64(&c.valInt, 1)
+}
+
+func (c *counter) Write(out *dto.Metric) error {
+	fval := math.Float64frombits(atomic.LoadUint64(&c.valBits))
+	ival := atomic.LoadUint64(&c.valInt)
+	val := fval + float64(ival)
+
+	return populateMetric(CounterValue, val, c.labelPairs, out)
 }
 
 // CounterVec is a Collector that bundles a set of Counters that all share the
@@ -85,11 +135,10 @@ func NewCounterVec(opts CounterOpts, labelNames []string) *CounterVec {
 	)
 	return &CounterVec{
 		metricVec: newMetricVec(desc, func(lvs ...string) Metric {
-			result := &counter{value: value{
-				desc:       desc,
-				valType:    CounterValue,
-				labelPairs: makeLabelPairs(desc, lvs),
-			}}
+			if len(lvs) != len(desc.variableLabels) {
+				panic(errInconsistentCardinality)
+			}
+			result := &counter{desc: desc, labelPairs: makeLabelPairs(desc, lvs)}
 			result.init(result) // Init self-collection.
 			return result
 		}),

prometheus/counter_test.go

@@ -28,13 +28,27 @@ func TestCounterAdd(t *testing.T) {
 		ConstLabels: Labels{"a": "1", "b": "2"},
 	}).(*counter)
 	counter.Inc()
-	if expected, got := 1., math.Float64frombits(counter.valBits); expected != got {
+	if expected, got := 0.0, math.Float64frombits(counter.valBits); expected != got {
 		t.Errorf("Expected %f, got %f.", expected, got)
 	}
+	if expected, got := uint64(1), counter.valInt; expected != got {
+		t.Errorf("Expected %d, got %d.", expected, got)
+	}
 	counter.Add(42)
-	if expected, got := 43., math.Float64frombits(counter.valBits); expected != got {
+	if expected, got := 0.0, math.Float64frombits(counter.valBits); expected != got {
 		t.Errorf("Expected %f, got %f.", expected, got)
 	}
+	if expected, got := uint64(43), counter.valInt; expected != got {
+		t.Errorf("Expected %d, got %d.", expected, got)
+	}
+
+	counter.Add(24.42)
+	if expected, got := 24.42, math.Float64frombits(counter.valBits); expected != got {
+		t.Errorf("Expected %f, got %f.", expected, got)
+	}
+	if expected, got := uint64(43), counter.valInt; expected != got {
+		t.Errorf("Expected %d, got %d.", expected, got)
+	}
 
 	if expected, got := "counter cannot decrease in value", decreaseCounter(counter).Error(); expected != got {
 		t.Errorf("Expected error %q, got %q.", expected, got)
@@ -43,7 +57,7 @@ func TestCounterAdd(t *testing.T) {
 	m := &dto.Metric{}
 	counter.Write(m)
 
-	if expected, got := `label:<name:"a" value:"1" > label:<name:"b" value:"2" > counter:<value:43 > `, m.String(); expected != got {
+	if expected, got := `label:<name:"a" value:"1" > label:<name:"b" value:"2" > counter:<value:67.42 > `, m.String(); expected != got {
 		t.Errorf("expected %q, got %q", expected, got)
 	}
 }
@@ -112,3 +126,87 @@ func expectPanic(t *testing.T, op func(), errorMsg string) {
 
 	op()
 }
+
+func TestCounterAddInf(t *testing.T) {
+	counter := NewCounter(CounterOpts{
+		Name: "test",
+		Help: "test help",
+	}).(*counter)
+
+	counter.Inc()
+	if expected, got := 0.0, math.Float64frombits(counter.valBits); expected != got {
+		t.Errorf("Expected %f, got %f.", expected, got)
+	}
+	if expected, got := uint64(1), counter.valInt; expected != got {
+		t.Errorf("Expected %d, got %d.", expected, got)
+	}
+
+	counter.Add(math.Inf(1))
+	if expected, got := math.Inf(1), math.Float64frombits(counter.valBits); expected != got {
+		t.Errorf("valBits expected %f, got %f.", expected, got)
+	}
+	if expected, got := uint64(1), counter.valInt; expected != got {
+		t.Errorf("valInts expected %d, got %d.", expected, got)
+	}
+
+	counter.Inc()
+	if expected, got := math.Inf(1), math.Float64frombits(counter.valBits); expected != got {
+		t.Errorf("Expected %f, got %f.", expected, got)
+	}
+	if expected, got := uint64(2), counter.valInt; expected != got {
+		t.Errorf("Expected %d, got %d.", expected, got)
+	}
+
+	m := &dto.Metric{}
+	counter.Write(m)
+
+	if expected, got := `counter:<value:inf > `, m.String(); expected != got {
+		t.Errorf("expected %q, got %q", expected, got)
+	}
+}
+
+func TestCounterAddLarge(t *testing.T) {
+	counter := NewCounter(CounterOpts{
+		Name: "test",
+		Help: "test help",
+	}).(*counter)
+
+	// large overflows the underlying type and should therefore be stored in valBits.
+	large := float64(math.MaxUint64 + 1)
+	counter.Add(large)
+	if expected, got := large, math.Float64frombits(counter.valBits); expected != got {
+		t.Errorf("valBits expected %f, got %f.", expected, got)
+	}
+	if expected, got := uint64(0), counter.valInt; expected != got {
+		t.Errorf("valInts expected %d, got %d.", expected, got)
+	}
+
+	m := &dto.Metric{}
+	counter.Write(m)
+
+	if expected, got := fmt.Sprintf("counter:<value:%0.16e > ", large), m.String(); expected != got {
+		t.Errorf("expected %q, got %q", expected, got)
+	}
+}
+
+func TestCounterAddSmall(t *testing.T) {
+	counter := NewCounter(CounterOpts{
+		Name: "test",
+		Help: "test help",
+	}).(*counter)
+	small := 0.000000000001
+	counter.Add(small)
+	if expected, got := small, math.Float64frombits(counter.valBits); expected != got {
+		t.Errorf("valBits expected %f, got %f.", expected, got)
+	}
+	if expected, got := uint64(0), counter.valInt; expected != got {
+		t.Errorf("valInts expected %d, got %d.", expected, got)
+	}
+
+	m := &dto.Metric{}
+	counter.Write(m)
+
+	if expected, got := fmt.Sprintf("counter:<value:%0.0e > ", small), m.String(); expected != got {
+		t.Errorf("expected %q, got %q", expected, got)
+	}
+}

prometheus/gauge.go

@@ -13,6 +13,14 @@
 
 package prometheus
 
+import (
+	"math"
+	"sync/atomic"
+	"time"
+
+	dto "github.com/prometheus/client_model/go"
+)
+
 // Gauge is a Metric that represents a single numerical value that can
 // arbitrarily go up and down.
 //
@@ -48,13 +56,74 @@ type Gauge interface {
 type GaugeOpts Opts
 
 // NewGauge creates a new Gauge based on the provided GaugeOpts.
+//
+// The returned implementation is optimized for a fast Set method. If you have a
+// choice for managing the value of a Gauge via Set vs. Inc/Dec/Add/Sub, pick
+// the former. For example, the Inc method of the returned Gauge is slower than
+// the Inc method of a Counter returned by NewCounter. This matches the typical
+// scenarios for Gauges and Counters, where the former tends to be Set-heavy and
+// the latter Inc-heavy.
 func NewGauge(opts GaugeOpts) Gauge {
-	return newValue(NewDesc(
+	desc := NewDesc(
 		BuildFQName(opts.Namespace, opts.Subsystem, opts.Name),
 		opts.Help,
 		nil,
 		opts.ConstLabels,
-	), GaugeValue, 0)
+	)
+	result := &gauge{desc: desc, labelPairs: desc.constLabelPairs}
+	result.init(result) // Init self-collection.
+	return result
+}
+
+type gauge struct {
+	// valBits contains the bits of the represented float64 value. It has
+	// to go first in the struct to guarantee alignment for atomic
+	// operations.  http://golang.org/pkg/sync/atomic/#pkg-note-BUG
+	valBits uint64
+
+	selfCollector
+
+	desc       *Desc
+	labelPairs []*dto.LabelPair
+}
+
+func (g *gauge) Desc() *Desc {
+	return g.desc
+}
+
+func (g *gauge) Set(val float64) {
+	atomic.StoreUint64(&g.valBits, math.Float64bits(val))
+}
+
+func (g *gauge) SetToCurrentTime() {
+	g.Set(float64(time.Now().UnixNano()) / 1e9)
+}
+
+func (g *gauge) Inc() {
+	g.Add(1)
+}
+
+func (g *gauge) Dec() {
+	g.Add(-1)
+}
+
+func (g *gauge) Add(val float64) {
+	for {
+		oldBits := atomic.LoadUint64(&g.valBits)
+		newBits := math.Float64bits(math.Float64frombits(oldBits) + val)
+		if atomic.CompareAndSwapUint64(&g.valBits, oldBits, newBits) {
+			return
+		}
+	}
+}
+
+func (g *gauge) Sub(val float64) {
+	g.Add(val * -1)
+}
+
+func (g *gauge) Write(out *dto.Metric) error {
+	val := math.Float64frombits(atomic.LoadUint64(&g.valBits))
+	return populateMetric(GaugeValue, val, g.labelPairs, out)
 }
 
 // GaugeVec is a Collector that bundles a set of Gauges that all share the same
@@ -77,7 +146,12 @@ func NewGaugeVec(opts GaugeOpts, labelNames []string) *GaugeVec {
 	)
 	return &GaugeVec{
 		metricVec: newMetricVec(desc, func(lvs ...string) Metric {
-			return newValue(desc, GaugeValue, 0, lvs...)
+			if len(lvs) != len(desc.variableLabels) {
+				panic(errInconsistentCardinality)
+			}
+			result := &gauge{desc: desc, labelPairs: makeLabelPairs(desc, lvs)}
+			result.init(result) // Init self-collection.
+			return result
 		}),
 	}
 }

prometheus/gauge_test.go

@@ -83,7 +83,7 @@ func TestGaugeConcurrency(t *testing.T) {
 		}
 		start.Done()
 
-		if expected, got := <-result, math.Float64frombits(gge.(*value).valBits); math.Abs(expected-got) > 0.000001 {
+		if expected, got := <-result, math.Float64frombits(gge.(*gauge).valBits); math.Abs(expected-got) > 0.000001 {
 			t.Fatalf("expected approx. %f, got %f", expected, got)
 			return false
 		}
@@ -147,7 +147,7 @@ func TestGaugeVecConcurrency(t *testing.T) {
 		start.Done()
 
 		for i := range sStreams {
-			if expected, got := <-results[i], math.Float64frombits(gge.WithLabelValues(string('A'+i)).(*value).valBits); math.Abs(expected-got) > 0.000001 {
+			if expected, got := <-results[i], math.Float64frombits(gge.WithLabelValues(string('A'+i)).(*gauge).valBits); math.Abs(expected-got) > 0.000001 {
 				t.Fatalf("expected approx. %f, got %f", expected, got)
 				return false
 			}