perf(chunkenc): intern the custom values for native histograms

The custom values are the "le" bucket boundaries of native histograms
with custom buckets. They are never modified. It is ok to not copy them
when iterating a chunk, just reference them.

If we will ever have a function that modifies the custom values, like
'trim' for example. That function will have to make a copy on write.

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

model/histogram/float_histogram.go

@@ -73,10 +73,8 @@ func (h *FloatHistogram) Copy() *FloatHistogram {
 	}
 
 	if h.UsesCustomBuckets() {
-		if len(h.CustomValues) != 0 {
-			c.CustomValues = make([]float64, len(h.CustomValues))
-			copy(c.CustomValues, h.CustomValues)
-		}
+		// Custom values are interned, so no need to copy them.
+		c.CustomValues = h.CustomValues
 	} else {
 		c.ZeroThreshold = h.ZeroThreshold
 		c.ZeroCount = h.ZeroCount
@@ -117,9 +115,8 @@ func (h *FloatHistogram) CopyTo(to *FloatHistogram) {
 
 		to.NegativeSpans = clearIfNotNil(to.NegativeSpans)
 		to.NegativeBuckets = clearIfNotNil(to.NegativeBuckets)
-
-		to.CustomValues = resize(to.CustomValues, len(h.CustomValues))
-		copy(to.CustomValues, h.CustomValues)
+		// Custom values are interned, so no need to copy them.
+		to.CustomValues = h.CustomValues
 	} else {
 		to.ZeroThreshold = h.ZeroThreshold
 		to.ZeroCount = h.ZeroCount
@@ -130,7 +127,8 @@ func (h *FloatHistogram) CopyTo(to *FloatHistogram) {
 		to.NegativeBuckets = resize(to.NegativeBuckets, len(h.NegativeBuckets))
 		copy(to.NegativeBuckets, h.NegativeBuckets)
 
-		to.CustomValues = clearIfNotNil(to.CustomValues)
+		// Custom values are interned, so no need to reset them.
+		to.CustomValues = nil
 	}
 
 	to.PositiveSpans = resize(to.PositiveSpans, len(h.PositiveSpans))

model/histogram/histogram.go

@@ -102,10 +102,8 @@ func (h *Histogram) Copy() *Histogram {
 	}
 
 	if h.UsesCustomBuckets() {
-		if len(h.CustomValues) != 0 {
-			c.CustomValues = make([]float64, len(h.CustomValues))
-			copy(c.CustomValues, h.CustomValues)
-		}
+		// Custom values are interned, it's ok to copy by reference.
+		c.CustomValues = h.CustomValues
 	} else {
 		c.ZeroThreshold = h.ZeroThreshold
 		c.ZeroCount = h.ZeroCount
@@ -146,9 +144,8 @@ func (h *Histogram) CopyTo(to *Histogram) {
 
 		to.NegativeSpans = clearIfNotNil(to.NegativeSpans)
 		to.NegativeBuckets = clearIfNotNil(to.NegativeBuckets)
-
-		to.CustomValues = resize(to.CustomValues, len(h.CustomValues))
-		copy(to.CustomValues, h.CustomValues)
+		// Custom values are interned, it's ok to copy by reference.
+		to.CustomValues = h.CustomValues
 	} else {
 		to.ZeroThreshold = h.ZeroThreshold
 		to.ZeroCount = h.ZeroCount
@@ -158,8 +155,8 @@ func (h *Histogram) CopyTo(to *Histogram) {
 
 		to.NegativeBuckets = resize(to.NegativeBuckets, len(h.NegativeBuckets))
 		copy(to.NegativeBuckets, h.NegativeBuckets)
-
-		to.CustomValues = clearIfNotNil(to.CustomValues)
+		// Custom values are interned, no need to reset.
+		to.CustomValues = nil
 	}
 
 	to.PositiveSpans = resize(to.PositiveSpans, len(h.PositiveSpans))
@@ -379,9 +376,8 @@ func (h *Histogram) ToFloat(fh *FloatHistogram) *FloatHistogram {
 		fh.ZeroCount = 0
 		fh.NegativeSpans = clearIfNotNil(fh.NegativeSpans)
 		fh.NegativeBuckets = clearIfNotNil(fh.NegativeBuckets)
-
-		fh.CustomValues = resize(fh.CustomValues, len(h.CustomValues))
-		copy(fh.CustomValues, h.CustomValues)
+		// Custom values are interned, it's ok to copy by reference.
+		fh.CustomValues = h.CustomValues
 	} else {
 		fh.ZeroThreshold = h.ZeroThreshold
 		fh.ZeroCount = float64(h.ZeroCount)
@@ -395,7 +391,8 @@ func (h *Histogram) ToFloat(fh *FloatHistogram) *FloatHistogram {
 			currentNegative += float64(b)
 			fh.NegativeBuckets[i] = currentNegative
 		}
-		fh.CustomValues = clearIfNotNil(fh.CustomValues)
+		// Custom values are interned, no need to reset.
+		fh.CustomValues = nil
 	}
 
 	fh.PositiveSpans = resize(fh.PositiveSpans, len(h.PositiveSpans))

tsdb/chunkenc/float_histogram.go

@@ -946,8 +946,8 @@ func (it *floatHistogramIterator) AtFloatHistogram(fh *histogram.FloatHistogram)
 	fh.NegativeBuckets = resize(fh.NegativeBuckets, len(it.nBuckets))
 	copy(fh.NegativeBuckets, it.nBuckets)
 
-	fh.CustomValues = resize(fh.CustomValues, len(it.customValues))
-	copy(fh.CustomValues, it.customValues)
+	// Custom values are interned. The single copy is in this iterator.
+	fh.CustomValues = it.customValues
 
 	return it.t, fh
 }

tsdb/chunkenc/histogram.go

@@ -201,7 +201,8 @@ type HistogramAppender struct {
 	schema         int32
 	zThreshold     float64
 	pSpans, nSpans []histogram.Span
-	customValues   []float64
+	// customValues is read only after the first sample is appended.
+	customValues []float64
 
 	// Although we intend to start new chunks on counter resets, we still
 	// have to handle negative deltas for gauge histograms. Therefore, even
@@ -995,8 +996,8 @@ func (it *histogramIterator) AtHistogram(h *histogram.Histogram) (int64, *histog
 	h.NegativeBuckets = resize(h.NegativeBuckets, len(it.nBuckets))
 	copy(h.NegativeBuckets, it.nBuckets)
 
-	h.CustomValues = resize(h.CustomValues, len(it.customValues))
-	copy(h.CustomValues, it.customValues)
+	// Custom values are interned. The single copy is here in the iterator.
+	h.CustomValues = it.customValues
 
 	return it.t, h
 }
@@ -1049,8 +1050,8 @@ func (it *histogramIterator) AtFloatHistogram(fh *histogram.FloatHistogram) (int
 		fh.NegativeBuckets[i] = currentNegative
 	}
 
-	fh.CustomValues = resize(fh.CustomValues, len(it.customValues))
-	copy(fh.CustomValues, it.customValues)
+	// Custom values are interned. The single copy is here in the iterator.
+	fh.CustomValues = it.customValues
 
 	return it.t, fh
 }
@@ -1081,7 +1082,6 @@ func (it *histogramIterator) Reset(b []byte) {
 		it.atHistogramCalled = false
 		it.pBuckets, it.nBuckets = nil, nil
 		it.pSpans, it.nSpans = nil, nil
-		it.customValues = nil
 	} else {
 		it.pBuckets = it.pBuckets[:0]
 		it.nBuckets = it.nBuckets[:0]
@@ -1089,7 +1089,6 @@ func (it *histogramIterator) Reset(b []byte) {
 	if it.atFloatHistogramCalled {
 		it.atFloatHistogramCalled = false
 		it.pFloatBuckets, it.nFloatBuckets = nil, nil
-		it.customValues = nil
 	} else {
 		it.pFloatBuckets = it.pFloatBuckets[:0]
 		it.nFloatBuckets = it.nFloatBuckets[:0]
@@ -1102,6 +1101,7 @@ func (it *histogramIterator) Reset(b []byte) {
 	it.leading = 0
 	it.trailing = 0
 	it.err = nil
+	it.customValues = nil
 }
 
 func (it *histogramIterator) Next() ValueType {
@@ -1211,13 +1211,7 @@ func (it *histogramIterator) Next() ValueType {
 		} else {
 			it.nSpans = nil
 		}
-		if len(it.customValues) > 0 {
-			newCustomValues := make([]float64, len(it.customValues))
-			copy(newCustomValues, it.customValues)
-			it.customValues = newCustomValues
-		} else {
-			it.customValues = nil
-		}
+		// it.CustomValues are interned, so we don't need to copy them.
 	}
 
 	if it.atHistogramCalled {

tsdb/chunkenc/histogram_test.go

@@ -1631,7 +1631,7 @@ func TestHistogramUniqueSpansAfterNextWithAtFloatHistogram(t *testing.T) {
 	require.NotSame(t, &rh1.NegativeSpans[0], &rh2.NegativeSpans[0], "NegativeSpans should be unique between histograms")
 }
 
-func TestHistogramUniqueCustomValuesAfterNextWithAtHistogram(t *testing.T) {
+func TestHistogramCustomValuesInternedAfterNextWithAtHistogram(t *testing.T) {
 	// Create two histograms with the same schema and custom values.
 	h1 := &histogram.Histogram{
 		Schema: -53,
@@ -1674,10 +1674,10 @@ func TestHistogramUniqueCustomValuesAfterNextWithAtHistogram(t *testing.T) {
 
 	// Check that the spans and custom values for h1 and h2 are unique slices.
 	require.NotSame(t, &rh1.PositiveSpans[0], &rh2.PositiveSpans[0], "PositiveSpans should be unique between histograms")
-	require.NotSame(t, &rh1.CustomValues[0], &rh2.CustomValues[0], "CustomValues should be unique between histograms")
+	require.Same(t, &rh1.CustomValues[0], &rh2.CustomValues[0], "CustomValues should be unique between histograms")
 }
 
-func TestHistogramUniqueCustomValuesAfterNextWithAtFloatHistogram(t *testing.T) {
+func TestHistogramCustomValuesInternedAfterNextWithAtFloatHistogram(t *testing.T) {
 	// Create two histograms with the same schema and custom values.
 	h1 := &histogram.Histogram{
 		Schema: -53,
@@ -1720,7 +1720,7 @@ func TestHistogramUniqueCustomValuesAfterNextWithAtFloatHistogram(t *testing.T)
 
 	// Check that the spans and custom values for h1 and h2 are unique slices.
 	require.NotSame(t, &rh1.PositiveSpans[0], &rh2.PositiveSpans[0], "PositiveSpans should be unique between histograms")
-	require.NotSame(t, &rh1.CustomValues[0], &rh2.CustomValues[0], "CustomValues should be unique between histograms")
+	require.Same(t, &rh1.CustomValues[0], &rh2.CustomValues[0], "CustomValues should be unique between histograms")
 }
 
 func BenchmarkAppendable(b *testing.B) {