Proposal to improve FPointSlice and HPointSlice allocation. (#13448)

* Reusing points slice from previous series when the slice is under utilized
* Adding comments on the bench test

Signed-off-by: Alan Protasio <alanprot@gmail.com>

promql/bench_test.go

@@ -33,6 +33,10 @@ func setupRangeQueryTestData(stor *teststorage.TestStorage, _ *Engine, interval,
 	ctx := context.Background()
 
 	metrics := []labels.Labels{}
+	// Generating test series: a_X, b_X, and h_X, where X can take values of one, ten, or hundred,
+	// representing the number of series each metric name contains.
+	// Metric a_X and b_X are simple metrics where h_X is a histogram.
+	// These metrics will have data for all test time range
 	metrics = append(metrics, labels.FromStrings("__name__", "a_one"))
 	metrics = append(metrics, labels.FromStrings("__name__", "b_one"))
 	for j := 0; j < 10; j++ {
@@ -59,6 +63,9 @@ func setupRangeQueryTestData(stor *teststorage.TestStorage, _ *Engine, interval,
 	}
 	refs := make([]storage.SeriesRef, len(metrics))
 
+	// Number points for each different label value of "l" for the sparse series
+	pointsPerSparseSeries := numIntervals / 50
+
 	for s := 0; s < numIntervals; s++ {
 		a := stor.Appender(context.Background())
 		ts := int64(s * interval)
@@ -66,10 +73,18 @@ func setupRangeQueryTestData(stor *teststorage.TestStorage, _ *Engine, interval,
 			ref, _ := a.Append(refs[i], metric, ts, float64(s)+float64(i)/float64(len(metrics)))
 			refs[i] = ref
 		}
+		// Generating a sparse time series: each label value of "l" will contain data only for
+		// pointsPerSparseSeries points
+		metric := labels.FromStrings("__name__", "sparse", "l", strconv.Itoa(s/pointsPerSparseSeries))
+		_, err := a.Append(0, metric, ts, float64(s)/float64(len(metrics)))
+		if err != nil {
+			return err
+		}
 		if err := a.Commit(); err != nil {
 			return err
 		}
 	}
+
 	stor.DB.ForceHeadMMap() // Ensure we have at most one head chunk for every series.
 	stor.DB.Compact(ctx)
 	return nil
@@ -94,6 +109,10 @@ func rangeQueryCases() []benchCase {
 			expr:  "rate(a_X[1m])",
 			steps: 10000,
 		},
+		{
+			expr:  "rate(sparse[1m])",
+			steps: 10000,
+		},
 		// Holt-Winters and long ranges.
 		{
 			expr: "holt_winters(a_X[1d], 0.3, 0.3)",

promql/engine.go

@@ -1452,6 +1452,7 @@ func (ev *evaluator) eval(expr parser.Expr) (parser.Value, annotations.Annotatio
 		// Reuse objects across steps to save memory allocations.
 		var floats []FPoint
 		var histograms []HPoint
+		var prevSS *Series
 		inMatrix := make(Matrix, 1)
 		inArgs[matrixArgIndex] = inMatrix
 		enh := &EvalNodeHelper{Out: make(Vector, 0, 1)}
@@ -1512,12 +1513,13 @@ func (ev *evaluator) eval(expr parser.Expr) (parser.Value, annotations.Annotatio
 				if len(outVec) > 0 {
 					if outVec[0].H == nil {
 						if ss.Floats == nil {
-							ss.Floats = getFPointSlice(numSteps)
+							ss.Floats = reuseOrGetFPointSlices(prevSS, numSteps)
 						}
+
 						ss.Floats = append(ss.Floats, FPoint{F: outVec[0].F, T: ts})
 					} else {
 						if ss.Histograms == nil {
-							ss.Histograms = getHPointSlice(numSteps)
+							ss.Histograms = reuseOrGetHPointSlices(prevSS, numSteps)
 						}
 						ss.Histograms = append(ss.Histograms, HPoint{H: outVec[0].H, T: ts})
 					}
@@ -1526,9 +1528,11 @@ func (ev *evaluator) eval(expr parser.Expr) (parser.Value, annotations.Annotatio
 				it.ReduceDelta(stepRange)
 			}
 			histSamples := totalHPointSize(ss.Histograms)
+
 			if len(ss.Floats)+histSamples > 0 {
 				if ev.currentSamples+len(ss.Floats)+histSamples <= ev.maxSamples {
 					mat = append(mat, ss)
+					prevSS = &mat[len(mat)-1]
 					ev.currentSamples += len(ss.Floats) + histSamples
 				} else {
 					ev.error(ErrTooManySamples(env))
@@ -1678,6 +1682,7 @@ func (ev *evaluator) eval(expr parser.Expr) (parser.Value, annotations.Annotatio
 			ev.error(errWithWarnings{fmt.Errorf("expanding series: %w", err), ws})
 		}
 		mat := make(Matrix, 0, len(e.Series))
+		var prevSS *Series
 		it := storage.NewMemoizedEmptyIterator(durationMilliseconds(ev.lookbackDelta))
 		var chkIter chunkenc.Iterator
 		for i, s := range e.Series {
@@ -1697,14 +1702,14 @@ func (ev *evaluator) eval(expr parser.Expr) (parser.Value, annotations.Annotatio
 					if ev.currentSamples < ev.maxSamples {
 						if h == nil {
 							if ss.Floats == nil {
-								ss.Floats = getFPointSlice(numSteps)
+								ss.Floats = reuseOrGetFPointSlices(prevSS, numSteps)
 							}
 							ss.Floats = append(ss.Floats, FPoint{F: f, T: ts})
 							ev.currentSamples++
 							ev.samplesStats.IncrementSamplesAtStep(step, 1)
 						} else {
 							if ss.Histograms == nil {
-								ss.Histograms = getHPointSlice(numSteps)
+								ss.Histograms = reuseOrGetHPointSlices(prevSS, numSteps)
 							}
 							point := HPoint{H: h, T: ts}
 							ss.Histograms = append(ss.Histograms, point)
@@ -1720,6 +1725,7 @@ func (ev *evaluator) eval(expr parser.Expr) (parser.Value, annotations.Annotatio
 
 			if len(ss.Floats)+len(ss.Histograms) > 0 {
 				mat = append(mat, ss)
+				prevSS = &mat[len(mat)-1]
 			}
 		}
 		ev.samplesStats.UpdatePeak(ev.currentSamples)
@@ -1840,6 +1846,30 @@ func (ev *evaluator) eval(expr parser.Expr) (parser.Value, annotations.Annotatio
 	panic(fmt.Errorf("unhandled expression of type: %T", expr))
 }
 
+// reuseOrGetFPointSlices reuses the space from previous slice to create new slice if the former has lots of room.
+// The previous slices capacity is adjusted so when it is re-used from the pool it doesn't overflow into the new one.
+func reuseOrGetHPointSlices(prevSS *Series, numSteps int) (r []HPoint) {
+	if prevSS != nil && cap(prevSS.Histograms)-2*len(prevSS.Histograms) > 0 {
+		r = prevSS.Histograms[len(prevSS.Histograms):]
+		prevSS.Histograms = prevSS.Histograms[0:len(prevSS.Histograms):len(prevSS.Histograms)]
+		return
+	}
+
+	return getHPointSlice(numSteps)
+}
+
+// reuseOrGetFPointSlices reuses the space from previous slice to create new slice if the former has lots of room.
+// The previous slices capacity is adjusted so when it is re-used from the pool it doesn't overflow into the new one.
+func reuseOrGetFPointSlices(prevSS *Series, numSteps int) (r []FPoint) {
+	if prevSS != nil && cap(prevSS.Floats)-2*len(prevSS.Floats) > 0 {
+		r = prevSS.Floats[len(prevSS.Floats):]
+		prevSS.Floats = prevSS.Floats[0:len(prevSS.Floats):len(prevSS.Floats)]
+		return
+	}
+
+	return getFPointSlice(numSteps)
+}
+
 func (ev *evaluator) rangeEvalTimestampFunctionOverVectorSelector(vs *parser.VectorSelector, call FunctionCall, e *parser.Call) (parser.Value, annotations.Annotations) {
 	ws, err := checkAndExpandSeriesSet(ev.ctx, vs)
 	if err != nil {