Use StateLocker in MinMaxSumCount (#546)

* Add MinMaxSumCount stress test

* Reimplement MinMaxSumCount using StateLocker

* Address PR comments

* Round #2 of PR comments

Co-authored-by: Rahul Patel <rahulpa@google.com>

api/core/number.go

@@ -34,6 +34,20 @@ const (
 	Uint64NumberKind
 )
 
+// Zero returns a zero value for a given NumberKind
+func (k NumberKind) Zero() Number {
+	switch k {
+	case Int64NumberKind:
+		return NewInt64Number(0)
+	case Float64NumberKind:
+		return NewFloat64Number(0.)
+	case Uint64NumberKind:
+		return NewUint64Number(0)
+	default:
+		return Number(0)
+	}
+}
+
 // Minimum returns the minimum representable value
 // for a given NumberKind
 func (k NumberKind) Minimum() Number {

sdk/metric/aggregator/minmaxsumcount/mmsc.go

@@ -20,17 +20,17 @@ import (
 	"go.opentelemetry.io/otel/api/core"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
 	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/internal"
 )
 
 type (
 	// Aggregator aggregates measure events, keeping only the max,
 	// sum, and count.
 	Aggregator struct {
-		// current has to be aligned for 64-bit atomic operations.
-		current state
-		// checkpoint has to be aligned for 64-bit atomic operations.
-		checkpoint state
-		kind       core.NumberKind
+		// states has to be aligned for 64-bit atomic operations.
+		states [2]state
+		lock   internal.StateLocker
+		kind   core.NumberKind
 	}
 
 	state struct {
@@ -48,104 +48,116 @@ var _ aggregator.MinMaxSumCount = &Aggregator{}
 // New returns a new measure aggregator for computing min, max, sum, and
 // count.  It does not compute quantile information other than Max.
 //
-// Note that this aggregator maintains each value using independent
-// atomic operations, which introduces the possibility that
-// checkpoints are inconsistent.  For greater consistency and lower
-// performance, consider using Array or DDSketch aggregators.
+// This aggregator uses the StateLocker pattern to guarantee
+// the count, sum, min and max are consistent within a checkpoint
 func New(desc *export.Descriptor) *Aggregator {
+	kind := desc.NumberKind()
 	return &Aggregator{
-		kind:    desc.NumberKind(),
-		current: unsetMinMaxSumCount(desc.NumberKind()),
+		kind: kind,
+		states: [2]state{
+			{
+				count: core.NewUint64Number(0),
+				sum:   kind.Zero(),
+				min:   kind.Maximum(),
+				max:   kind.Minimum(),
+			},
+			{
+				count: core.NewUint64Number(0),
+				sum:   kind.Zero(),
+				min:   kind.Maximum(),
+				max:   kind.Minimum(),
+			},
+		},
 	}
 }
 
-func unsetMinMaxSumCount(kind core.NumberKind) state {
-	return state{min: kind.Maximum(), max: kind.Minimum()}
-}
-
 // Sum returns the sum of values in the checkpoint.
 func (c *Aggregator) Sum() (core.Number, error) {
-	return c.checkpoint.sum, nil
+	c.lock.Lock()
+	defer c.lock.Unlock()
+	return c.checkpoint().sum, nil
 }
 
 // Count returns the number of values in the checkpoint.
 func (c *Aggregator) Count() (int64, error) {
-	return int64(c.checkpoint.count.AsUint64()), nil
+	c.lock.Lock()
+	defer c.lock.Unlock()
+	return c.checkpoint().count.CoerceToInt64(core.Uint64NumberKind), nil
 }
 
 // Min returns the minimum value in the checkpoint.
-// The error value aggregator.ErrEmptyDataSet will be returned if
-// (due to a race condition) the checkpoint was set prior to
-// current.min being computed in Update().
-//
-// Note: If a measure's recorded values for a given checkpoint are
-// all equal to NumberKind.Maximum(), Min() will return ErrEmptyDataSet
+// The error value aggregator.ErrEmptyDataSet will be returned
+// if there were no measurements recorded during the checkpoint.
 func (c *Aggregator) Min() (core.Number, error) {
-	if c.checkpoint.min == c.kind.Maximum() {
-		return core.Number(0), aggregator.ErrEmptyDataSet
+	c.lock.Lock()
+	defer c.lock.Unlock()
+	if c.checkpoint().count.IsZero(core.Uint64NumberKind) {
+		return c.kind.Zero(), aggregator.ErrEmptyDataSet
 	}
-	return c.checkpoint.min, nil
+	return c.checkpoint().min, nil
 }
 
 // Max returns the maximum value in the checkpoint.
-// The error value aggregator.ErrEmptyDataSet will be returned if
-// (due to a race condition) the checkpoint was set prior to
-// current.max being computed in Update().
-//
-// Note: If a measure's recorded values for a given checkpoint are
-// all equal to NumberKind.Minimum(), Max() will return ErrEmptyDataSet
+// The error value aggregator.ErrEmptyDataSet will be returned
+// if there were no measurements recorded during the checkpoint.
 func (c *Aggregator) Max() (core.Number, error) {
-	if c.checkpoint.max == c.kind.Minimum() {
-		return core.Number(0), aggregator.ErrEmptyDataSet
+	c.lock.Lock()
+	defer c.lock.Unlock()
+	if c.checkpoint().count.IsZero(core.Uint64NumberKind) {
+		return c.kind.Zero(), aggregator.ErrEmptyDataSet
 	}
-	return c.checkpoint.max, nil
+	return c.checkpoint().max, nil
 }
 
 // Checkpoint saves the current state and resets the current state to
-// the empty set.  Since no locks are taken, there is a chance that
-// the independent Min, Max, Sum, and Count are not consistent with each
-// other.
+// the empty set.
 func (c *Aggregator) Checkpoint(ctx context.Context, desc *export.Descriptor) {
-	// N.B. There is no atomic operation that can update all three
-	// values at once without a memory allocation.
-	//
-	// This aggregator is intended to trade this correctness for
-	// speed.
-	//
-	// Therefore, atomically swap fields independently, knowing
-	// that individually the three parts of this aggregation could
-	// be spread across multiple collections in rare cases.
+	c.lock.SwapActiveState(c.resetCheckpoint)
+}
 
-	c.checkpoint.count.SetUint64(c.current.count.SwapUint64Atomic(0))
-	c.checkpoint.sum = c.current.sum.SwapNumberAtomic(core.Number(0))
-	c.checkpoint.max = c.current.max.SwapNumberAtomic(c.kind.Minimum())
-	c.checkpoint.min = c.current.min.SwapNumberAtomic(c.kind.Maximum())
+// checkpoint returns the "cold" state, i.e. state collected prior to the
+// most recent Checkpoint() call
+func (c *Aggregator) checkpoint() *state {
+	return &c.states[c.lock.ColdIdx()]
+}
+
+func (c *Aggregator) resetCheckpoint() {
+	checkpoint := c.checkpoint()
+
+	checkpoint.count.SetUint64(0)
+	checkpoint.sum.SetNumber(c.kind.Zero())
+	checkpoint.min.SetNumber(c.kind.Maximum())
+	checkpoint.max.SetNumber(c.kind.Minimum())
 }
 
 // Update adds the recorded measurement to the current data set.
 func (c *Aggregator) Update(_ context.Context, number core.Number, desc *export.Descriptor) error {
 	kind := desc.NumberKind()
 
-	c.current.count.AddUint64Atomic(1)
-	c.current.sum.AddNumberAtomic(kind, number)
+	cIdx := c.lock.Start()
+	defer c.lock.End(cIdx)
+
+	current := &c.states[cIdx]
+	current.count.AddUint64Atomic(1)
+	current.sum.AddNumberAtomic(kind, number)
 
 	for {
-		current := c.current.min.AsNumberAtomic()
+		cmin := current.min.AsNumberAtomic()
 
-		if number.CompareNumber(kind, current) >= 0 {
+		if number.CompareNumber(kind, cmin) >= 0 {
 			break
 		}
-		if c.current.min.CompareAndSwapNumber(current, number) {
+		if current.min.CompareAndSwapNumber(cmin, number) {
 			break
 		}
 	}
 	for {
-		current := c.current.max.AsNumberAtomic()
+		cmax := current.max.AsNumberAtomic()
 
-		if number.CompareNumber(kind, current) <= 0 {
+		if number.CompareNumber(kind, cmax) <= 0 {
 			break
 		}
-		if c.current.max.CompareAndSwapNumber(current, number) {
+		if current.max.CompareAndSwapNumber(cmax, number) {
 			break
 		}
 	}
@@ -159,14 +171,22 @@ func (c *Aggregator) Merge(oa export.Aggregator, desc *export.Descriptor) error
 		return aggregator.NewInconsistentMergeError(c, oa)
 	}
 
-	c.checkpoint.sum.AddNumber(desc.NumberKind(), o.checkpoint.sum)
-	c.checkpoint.count.AddNumber(core.Uint64NumberKind, o.checkpoint.count)
+	// Lock() synchronizes Merge() and Checkpoint() to ensure all operations of
+	// Merge() are performed on the same state.
+	c.lock.Lock()
+	defer c.lock.Unlock()
 
-	if c.checkpoint.min.CompareNumber(desc.NumberKind(), o.checkpoint.min) > 0 {
-		c.checkpoint.min.SetNumber(o.checkpoint.min)
+	current := c.checkpoint()
+	ocheckpoint := o.checkpoint()
+
+	current.count.AddNumber(core.Uint64NumberKind, ocheckpoint.count)
+	current.sum.AddNumber(desc.NumberKind(), ocheckpoint.sum)
+
+	if current.min.CompareNumber(desc.NumberKind(), ocheckpoint.min) > 0 {
+		current.min.SetNumber(ocheckpoint.min)
 	}
-	if c.checkpoint.max.CompareNumber(desc.NumberKind(), o.checkpoint.max) < 0 {
-		c.checkpoint.max.SetNumber(o.checkpoint.max)
+	if current.max.CompareNumber(desc.NumberKind(), ocheckpoint.max) < 0 {
+		current.max.SetNumber(ocheckpoint.max)
 	}
 	return nil
 }

sdk/metric/aggregator/minmaxsumcount/mmsc_test.go

@@ -66,12 +66,8 @@ var (
 func TestMain(m *testing.M) {
 	fields := []ottest.FieldOffset{
 		{
-			Name:   "Aggregator.current",
-			Offset: unsafe.Offsetof(Aggregator{}.current),
-		},
-		{
-			Name:   "Aggregator.checkpoint",
-			Offset: unsafe.Offsetof(Aggregator{}.checkpoint),
+			Name:   "Aggregator.states",
+			Offset: unsafe.Offsetof(Aggregator{}.states),
 		},
 		{
 			Name:   "state.count",

sdk/metric/minmaxsumcount_stress_test.go

@@ -0,0 +1,83 @@
+// Copyright 2020, OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// This test is too large for the race detector.  This SDK uses no locks
+// that the race detector would help with, anyway.
+// +build !race
+
+package metric_test
+
+import (
+	"context"
+	"math/rand"
+	"testing"
+	"time"
+
+	"go.opentelemetry.io/otel/api/core"
+	"go.opentelemetry.io/otel/sdk/export/metric"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator/minmaxsumcount"
+)
+
+func TestStressInt64MinMaxSumCount(t *testing.T) {
+	desc := metric.NewDescriptor("some_metric", metric.MeasureKind, nil, "", "", core.Int64NumberKind)
+	mmsc := minmaxsumcount.New(desc)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	go func() {
+		rnd := rand.New(rand.NewSource(time.Now().Unix()))
+		v := rnd.Int63() % 103
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			default:
+				_ = mmsc.Update(ctx, core.NewInt64Number(v), desc)
+			}
+			v++
+		}
+	}()
+
+	startTime := time.Now()
+	for time.Since(startTime) < time.Second {
+		mmsc.Checkpoint(context.Background(), desc)
+
+		s, _ := mmsc.Sum()
+		c, _ := mmsc.Count()
+		min, e1 := mmsc.Min()
+		max, e2 := mmsc.Max()
+		if c == 0 && (e1 == nil || e2 == nil || s.AsInt64() != 0) {
+			t.Fail()
+		}
+		if c != 0 {
+			if e1 != nil || e2 != nil {
+				t.Fail()
+			}
+			lo, hi, sum := min.AsInt64(), max.AsInt64(), s.AsInt64()
+
+			if hi-lo+1 != c {
+				t.Fail()
+			}
+			if c == 1 {
+				if lo != hi || lo != sum {
+					t.Fail()
+				}
+			} else {
+				if hi*(hi+1)/2-(lo-1)*lo/2 != sum {
+					t.Fail()
+				}
+			}
+		}
+	}
+}

sdk/metric/stress_test.go

@@ -1,4 +1,4 @@
-// Copyright 2019, OpenTelemetry Authors
+// Copyright 2020, OpenTelemetry Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.