Histogram performance: optimize floatBucketIterator

Signed-off-by: Linas Medziunas <linas.medziunas@gmail.com>

model/histogram/float_histogram.go

@@ -434,25 +434,25 @@ func (h *FloatHistogram) DetectReset(previous *FloatHistogram) bool {
 	}
 	currIt := h.floatBucketIterator(true, h.ZeroThreshold, h.Schema)
 	prevIt := previous.floatBucketIterator(true, h.ZeroThreshold, h.Schema)
-	if detectReset(currIt, prevIt) {
+	if detectReset(&currIt, &prevIt) {
 		return true
 	}
 	currIt = h.floatBucketIterator(false, h.ZeroThreshold, h.Schema)
 	prevIt = previous.floatBucketIterator(false, h.ZeroThreshold, h.Schema)
-	return detectReset(currIt, prevIt)
+	return detectReset(&currIt, &prevIt)
 }
 
-func detectReset(currIt, prevIt BucketIterator[float64]) bool {
+func detectReset(currIt, prevIt *floatBucketIterator) bool {
 	if !prevIt.Next() {
 		return false // If no buckets in previous histogram, nothing can be reset.
 	}
-	prevBucket := prevIt.At()
+	prevBucket := prevIt.strippedAt()
 	if !currIt.Next() {
 		// No bucket in current, but at least one in previous
 		// histogram. Check if any of those are non-zero, in which case
 		// this is a reset.
 		for {
-			if prevBucket.Count != 0 {
+			if prevBucket.count != 0 {
 				return true
 			}
 			if !prevIt.Next() {
@@ -460,10 +460,10 @@ func detectReset(currIt, prevIt BucketIterator[float64]) bool {
 			}
 		}
 	}
-	currBucket := currIt.At()
+	currBucket := currIt.strippedAt()
 	for {
 		// Forward currIt until we find the bucket corresponding to prevBucket.
-		for currBucket.Index < prevBucket.Index {
+		for currBucket.index < prevBucket.index {
 			if !currIt.Next() {
 				// Reached end of currIt early, therefore
 				// previous histogram has a bucket that the
@@ -471,7 +471,7 @@ func detectReset(currIt, prevIt BucketIterator[float64]) bool {
 				// remaining buckets in the previous histogram
 				// are unpopulated, this is a reset.
 				for {
-					if prevBucket.Count != 0 {
+					if prevBucket.count != 0 {
 						return true
 					}
 					if !prevIt.Next() {
@@ -479,18 +479,18 @@ func detectReset(currIt, prevIt BucketIterator[float64]) bool {
 					}
 				}
 			}
-			currBucket = currIt.At()
+			currBucket = currIt.strippedAt()
 		}
-		if currBucket.Index > prevBucket.Index {
+		if currBucket.index > prevBucket.index {
 			// Previous histogram has a bucket the current one does
 			// not have. If it's populated, it's a reset.
-			if prevBucket.Count != 0 {
+			if prevBucket.count != 0 {
 				return true
 			}
 		} else {
 			// We have reached corresponding buckets in both iterators.
 			// We can finally compare the counts.
-			if currBucket.Count < prevBucket.Count {
+			if currBucket.count < prevBucket.count {
 				return true
 			}
 		}
@@ -498,35 +498,39 @@ func detectReset(currIt, prevIt BucketIterator[float64]) bool {
 			// Reached end of prevIt without finding offending buckets.
 			return false
 		}
-		prevBucket = prevIt.At()
+		prevBucket = prevIt.strippedAt()
 	}
 }
 
 // PositiveBucketIterator returns a BucketIterator to iterate over all positive
 // buckets in ascending order (starting next to the zero bucket and going up).
 func (h *FloatHistogram) PositiveBucketIterator() BucketIterator[float64] {
-	return h.floatBucketIterator(true, 0, h.Schema)
+	it := h.floatBucketIterator(true, 0, h.Schema)
+	return &it
 }
 
 // NegativeBucketIterator returns a BucketIterator to iterate over all negative
 // buckets in descending order (starting next to the zero bucket and going
 // down).
 func (h *FloatHistogram) NegativeBucketIterator() BucketIterator[float64] {
-	return h.floatBucketIterator(false, 0, h.Schema)
+	it := h.floatBucketIterator(false, 0, h.Schema)
+	return &it
 }
 
 // PositiveReverseBucketIterator returns a BucketIterator to iterate over all
 // positive buckets in descending order (starting at the highest bucket and
 // going down towards the zero bucket).
 func (h *FloatHistogram) PositiveReverseBucketIterator() BucketIterator[float64] {
-	return newReverseFloatBucketIterator(h.PositiveSpans, h.PositiveBuckets, h.Schema, true)
+	it := newReverseFloatBucketIterator(h.PositiveSpans, h.PositiveBuckets, h.Schema, true)
+	return &it
 }
 
 // NegativeReverseBucketIterator returns a BucketIterator to iterate over all
 // negative buckets in ascending order (starting at the lowest bucket and going
 // up towards the zero bucket).
 func (h *FloatHistogram) NegativeReverseBucketIterator() BucketIterator[float64] {
-	return newReverseFloatBucketIterator(h.NegativeSpans, h.NegativeBuckets, h.Schema, false)
+	it := newReverseFloatBucketIterator(h.NegativeSpans, h.NegativeBuckets, h.Schema, false)
+	return &it
 }
 
 // AllBucketIterator returns a BucketIterator to iterate over all negative,
@@ -537,8 +541,8 @@ func (h *FloatHistogram) NegativeReverseBucketIterator() BucketIterator[float64]
 func (h *FloatHistogram) AllBucketIterator() BucketIterator[float64] {
 	return &allFloatBucketIterator{
 		h:         h,
-		leftIter:  h.NegativeReverseBucketIterator(),
-		rightIter: h.PositiveBucketIterator(),
+		leftIter:  newReverseFloatBucketIterator(h.NegativeSpans, h.NegativeBuckets, h.Schema, false),
+		rightIter: h.floatBucketIterator(true, 0, h.Schema),
 		state:     -1,
 	}
 }
@@ -551,8 +555,8 @@ func (h *FloatHistogram) AllBucketIterator() BucketIterator[float64] {
 func (h *FloatHistogram) AllReverseBucketIterator() BucketIterator[float64] {
 	return &allFloatBucketIterator{
 		h:         h,
-		leftIter:  h.PositiveReverseBucketIterator(),
-		rightIter: h.NegativeBucketIterator(),
+		leftIter:  newReverseFloatBucketIterator(h.PositiveSpans, h.PositiveBuckets, h.Schema, true),
+		rightIter: h.floatBucketIterator(false, 0, h.Schema),
 		state:     -1,
 	}
 }
@@ -683,11 +687,11 @@ func (h *FloatHistogram) reconcileZeroBuckets(other *FloatHistogram) float64 {
 // targetSchema prior to iterating (without mutating FloatHistogram).
 func (h *FloatHistogram) floatBucketIterator(
 	positive bool, absoluteStartValue float64, targetSchema int32,
-) *floatBucketIterator {
+) floatBucketIterator {
 	if targetSchema > h.Schema {
 		panic(fmt.Errorf("cannot merge from schema %d to %d", h.Schema, targetSchema))
 	}
-	i := &floatBucketIterator{
+	i := floatBucketIterator{
 		baseBucketIterator: baseBucketIterator[float64, float64]{
 			schema:   h.Schema,
 			positive: positive,
@@ -705,11 +709,11 @@ func (h *FloatHistogram) floatBucketIterator(
 	return i
 }
 
-// reverseFloatbucketiterator is a low-level constructor for reverse bucket iterators.
+// reverseFloatBucketIterator is a low-level constructor for reverse bucket iterators.
 func newReverseFloatBucketIterator(
 	spans []Span, buckets []float64, schema int32, positive bool,
-) *reverseFloatBucketIterator {
-	r := &reverseFloatBucketIterator{
+) reverseFloatBucketIterator {
+	r := reverseFloatBucketIterator{
 		baseBucketIterator: baseBucketIterator[float64, float64]{
 			schema:   schema,
 			spans:    spans,
@@ -737,6 +741,8 @@ type floatBucketIterator struct {
 	targetSchema       int32   // targetSchema is the schema to merge to and must be ≤ schema.
 	origIdx            int32   // The bucket index within the original schema.
 	absoluteStartValue float64 // Never return buckets with an upper bound ≤ this value.
+
+	boundReachedStartValue bool // Has getBound reached absoluteStartValue already?
 }
 
 func (i *floatBucketIterator) At() Bucket[float64] {
@@ -800,9 +806,10 @@ mergeLoop: // Merge together all buckets from the original schema that fall into
 	}
 	// Skip buckets before absoluteStartValue.
 	// TODO(beorn7): Maybe do something more efficient than this recursive call.
-	if getBound(i.currIdx, i.targetSchema) <= i.absoluteStartValue {
+	if !i.boundReachedStartValue && getBound(i.currIdx, i.targetSchema) <= i.absoluteStartValue {
 		return i.Next()
 	}
+	i.boundReachedStartValue = true
 	return true
 }
 
@@ -843,8 +850,9 @@ func (i *reverseFloatBucketIterator) Next() bool {
 }
 
 type allFloatBucketIterator struct {
-	h                   *FloatHistogram
-	leftIter, rightIter BucketIterator[float64]
+	h         *FloatHistogram
+	leftIter  reverseFloatBucketIterator
+	rightIter floatBucketIterator
 	// -1 means we are iterating negative buckets.
 	// 0 means it is time for the zero bucket.
 	// 1 means we are iterating positive buckets.

model/histogram/float_histogram_test.go

@@ -16,6 +16,7 @@ package histogram
 import (
 	"fmt"
 	"math"
+	"math/rand"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -2291,3 +2292,51 @@ func TestFloatBucketIteratorTargetSchema(t *testing.T) {
 	}
 	require.False(t, it.Next(), "negative iterator not exhausted")
 }
+
+func BenchmarkFloatHistogramAllBucketIterator(b *testing.B) {
+	rng := rand.New(rand.NewSource(0))
+
+	fh := createRandomFloatHistogram(rng, 50)
+
+	b.ReportAllocs() // the current implementation reports 1 alloc
+	b.ResetTimer()
+
+	for n := 0; n < b.N; n++ {
+		for it := fh.AllBucketIterator(); it.Next(); {
+		}
+	}
+}
+
+func BenchmarkFloatHistogramDetectReset(b *testing.B) {
+	rng := rand.New(rand.NewSource(0))
+
+	fh := createRandomFloatHistogram(rng, 50)
+
+	b.ReportAllocs() // the current implementation reports 0 allocs
+	b.ResetTimer()
+
+	for n := 0; n < b.N; n++ {
+		// Detect against the itself (no resets is the worst case input).
+		fh.DetectReset(fh)
+	}
+}
+
+func createRandomFloatHistogram(rng *rand.Rand, spanNum int32) *FloatHistogram {
+	f := &FloatHistogram{}
+	f.PositiveSpans, f.PositiveBuckets = createRandomSpans(rng, spanNum)
+	f.NegativeSpans, f.NegativeBuckets = createRandomSpans(rng, spanNum)
+	return f
+}
+
+func createRandomSpans(rng *rand.Rand, spanNum int32) ([]Span, []float64) {
+	Spans := make([]Span, spanNum)
+	Buckets := make([]float64, 0)
+	for i := 0; i < int(spanNum); i++ {
+		Spans[i].Offset = rng.Int31n(spanNum) + 1
+		Spans[i].Length = uint32(rng.Int31n(spanNum) + 1)
+		for j := 0; j < int(Spans[i].Length); j++ {
+			Buckets = append(Buckets, float64(rng.Int31n(spanNum)+1))
+		}
+	}
+	return Spans, Buckets
+}

model/histogram/generic.go

@@ -53,6 +53,13 @@ type Bucket[BC BucketCount] struct {
 	Index int32
 }
 
+// strippedBucket is Bucket without bound values (which are expensive to calculate
+// and not used in certain use cases).
+type strippedBucket[BC BucketCount] struct {
+	count BC
+	index int32
+}
+
 // String returns a string representation of a Bucket, using the usual
 // mathematical notation of '['/']' for inclusive bounds and '('/')' for
 // non-inclusive bounds.
@@ -101,13 +108,12 @@ type baseBucketIterator[BC BucketCount, IBC InternalBucketCount] struct {
 	currIdx   int32 // The actual bucket index.
 }
 
-func (b baseBucketIterator[BC, IBC]) At() Bucket[BC] {
+func (b *baseBucketIterator[BC, IBC]) At() Bucket[BC] {
 	return b.at(b.schema)
 }
 
-// at is an internal version of the exported At to enable using a different
-// schema.
-func (b baseBucketIterator[BC, IBC]) at(schema int32) Bucket[BC] {
+// at is an internal version of the exported At to enable using a different schema.
+func (b *baseBucketIterator[BC, IBC]) at(schema int32) Bucket[BC] {
 	bucket := Bucket[BC]{
 		Count: BC(b.currCount),
 		Index: b.currIdx,
@@ -124,6 +130,14 @@ func (b baseBucketIterator[BC, IBC]) at(schema int32) Bucket[BC] {
 	return bucket
 }
 
+// strippedAt returns current strippedBucket (which lacks bucket bounds but is cheaper to compute).
+func (b *baseBucketIterator[BC, IBC]) strippedAt() strippedBucket[BC] {
+	return strippedBucket[BC]{
+		count: BC(b.currCount),
+		index: b.currIdx,
+	}
+}
+
 // compactBuckets is a generic function used by both Histogram.Compact and
 // FloatHistogram.Compact. Set deltaBuckets to true if the provided buckets are
 // deltas. Set it to false if the buckets contain absolute counts.

model/histogram/histogram.go

@@ -148,13 +148,15 @@ func (h *Histogram) ZeroBucket() Bucket[uint64] {
 // PositiveBucketIterator returns a BucketIterator to iterate over all positive
 // buckets in ascending order (starting next to the zero bucket and going up).
 func (h *Histogram) PositiveBucketIterator() BucketIterator[uint64] {
-	return newRegularBucketIterator(h.PositiveSpans, h.PositiveBuckets, h.Schema, true)
+	it := newRegularBucketIterator(h.PositiveSpans, h.PositiveBuckets, h.Schema, true)
+	return &it
 }
 
 // NegativeBucketIterator returns a BucketIterator to iterate over all negative
 // buckets in descending order (starting next to the zero bucket and going down).
 func (h *Histogram) NegativeBucketIterator() BucketIterator[uint64] {
-	return newRegularBucketIterator(h.NegativeSpans, h.NegativeBuckets, h.Schema, false)
+	it := newRegularBucketIterator(h.NegativeSpans, h.NegativeBuckets, h.Schema, false)
+	return &it
 }
 
 // CumulativeBucketIterator returns a BucketIterator to iterate over a
@@ -325,14 +327,14 @@ type regularBucketIterator struct {
 	baseBucketIterator[uint64, int64]
 }
 
-func newRegularBucketIterator(spans []Span, buckets []int64, schema int32, positive bool) *regularBucketIterator {
+func newRegularBucketIterator(spans []Span, buckets []int64, schema int32, positive bool) regularBucketIterator {
 	i := baseBucketIterator[uint64, int64]{
 		schema:   schema,
 		spans:    spans,
 		buckets:  buckets,
 		positive: positive,
 	}
-	return &regularBucketIterator{i}
+	return regularBucketIterator{i}
 }
 
 func (r *regularBucketIterator) Next() bool {