Don't record measurements with no subscription (#600)

Suggested by Ramon, immediately dropping the measurements from
the measures with no subscription rather than doing it at a later time
significantly improves the performance on the critical path.

Before:
BenchmarkRecord0-8            	500000000	         3.09 ns/op
BenchmarkRecord1-8            	 5000000	       366 ns/op
BenchmarkRecord8-8            	 3000000	       412 ns/op
BenchmarkRecord8_Parallel-8   	 2000000	       804 ns/op
BenchmarkRecord8_8Tags-8      	 3000000	       415 ns/op

After:
BenchmarkRecord0-8            	1000000000	         2.58 ns/op
BenchmarkRecord1-8            	30000000	        36.9 ns/op
BenchmarkRecord8-8            	20000000	        89.4 ns/op
BenchmarkRecord8_Parallel-8   	30000000	        44.8 ns/op
BenchmarkRecord8_8Tags-8      	20000000	        90.1 ns/op

stats/internal/record.go

@@ -18,7 +18,8 @@ import (
 	"go.opencensus.io/tag"
 )
 
-type Recorder func(*tag.Map, interface{})
-
 // DefaultRecorder will be called for each Record call.
-var DefaultRecorder Recorder = nil
+var DefaultRecorder func(*tag.Map, interface{})
+
+// SubscriptionReporter reports when a view subscribed with a measure.
+var SubscriptionReporter func(measure string)

stats/measure.go

@@ -19,6 +19,7 @@ import (
 	"errors"
 	"fmt"
 	"sync"
+	"sync/atomic"
 
 	"go.opencensus.io/stats/internal"
 )
@@ -37,14 +38,27 @@ type Measure interface {
 	Name() string
 	Description() string
 	Unit() string
+
+	subscribe()
+	subscribed() bool
 }
 
 type measure struct {
+	subs int32 // access atomically
+
 	name        string
 	description string
 	unit        string
 }
 
+func (m *measure) subscribe() {
+	atomic.StoreInt32(&m.subs, 1)
+}
+
+func (m *measure) subscribed() bool {
+	return atomic.LoadInt32(&m.subs) == 1
+}
+
 // Name returns the name of the measure.
 func (m *measure) Name() string {
 	return m.name
@@ -61,10 +75,12 @@ func (m *measure) Unit() string {
 }
 
 var (
-	mu           sync.RWMutex
-	measures     = make(map[string]Measure)
-	errDuplicate = errors.New("duplicate measure name")
+	mu       sync.RWMutex
+	measures = make(map[string]Measure)
+)
 
+var (
+	errDuplicate          = errors.New("duplicate measure name")
 	errMeasureNameTooLong = fmt.Errorf("measure name cannot be longer than %v", internal.MaxNameLength)
 )
 

stats/measure_float64.go

@@ -20,10 +20,21 @@ type Float64Measure struct {
 	measure
 }
 
+func (f *Float64Measure) subscribe() {
+	f.measure.subscribe()
+}
+
+func (f *Float64Measure) subscribed() bool {
+	return f.measure.subscribed()
+}
+
 // M creates a new float64 measurement.
 // Use Record to record measurements.
-func (m *Float64Measure) M(v float64) Measurement {
-	return Measurement{m: m, v: v}
+func (f *Float64Measure) M(v float64) Measurement {
+	if !f.subscribed() {
+		return Measurement{}
+	}
+	return Measurement{m: f, v: v}
 }
 
 // Float64 creates a new measure of type Float64Measure. It returns

stats/measure_int64.go

@@ -20,10 +20,21 @@ type Int64Measure struct {
 	measure
 }
 
+func (i *Int64Measure) subscribe() {
+	i.measure.subscribe()
+}
+
+func (i *Int64Measure) subscribed() bool {
+	return i.measure.subscribed()
+}
+
 // M creates a new int64 measurement.
 // Use Record to record measurements.
-func (m *Int64Measure) M(v int64) Measurement {
-	return Measurement{m: m, v: float64(v)}
+func (i *Int64Measure) M(v int64) Measurement {
+	if !i.subscribed() {
+		return Measurement{}
+	}
+	return Measurement{m: i, v: float64(v)}
 }
 
 // Int64 creates a new measure of type Int64Measure. It returns an

stats/record.go

@@ -22,12 +22,30 @@ import (
 	"go.opencensus.io/tag"
 )
 
+func init() {
+	internal.SubscriptionReporter = func(measure string) {
+		mu.Lock()
+		measures[measure].subscribe()
+		mu.Unlock()
+	}
+}
+
 // Record records one or multiple measurements with the same tags at once.
 // If there are any tags in the context, measurements will be tagged with them.
 func Record(ctx context.Context, ms ...Measurement) {
 	if len(ms) == 0 {
 		return
 	}
+	var record bool
+	for _, m := range ms {
+		if (m != Measurement{}) {
+			record = true
+			break
+		}
+	}
+	if !record {
+		return
+	}
 	if internal.DefaultRecorder != nil {
 		internal.DefaultRecorder(tag.FromContext(ctx), ms)
 	}

stats/view/worker_commands.go

@@ -22,6 +22,7 @@ import (
 	"time"
 
 	"go.opencensus.io/stats"
+	"go.opencensus.io/stats/internal"
 	"go.opencensus.io/tag"
 )
 
@@ -57,6 +58,7 @@ func (cmd *subscribeToViewReq) handleCommand(w *worker) {
 			errstr = append(errstr, fmt.Sprintf("%s: %v", view.Name, err))
 			continue
 		}
+		internal.SubscriptionReporter(view.Measure.Name())
 		vi.subscribe()
 	}
 	if len(errstr) > 0 {
@@ -135,6 +137,9 @@ type recordReq struct {
 
 func (cmd *recordReq) handleCommand(w *worker) {
 	for _, m := range cmd.ms {
+		if (m == stats.Measurement{}) { // not subscribed
+			continue
+		}
 		ref := w.getMeasureRef(m.Measure().Name())
 		for v := range ref.views {
 			v.addSample(cmd.tm, m.Value())