app/vmselect/promql: eliminate memory allocation when sorting values inside float64s

app/vmselect/promql/aggr.go

@@ -819,7 +819,7 @@ func lastValue(values []float64) float64 {
 // quantiles calculates the given phis from originValues without modifying originValues, appends them to qs and returns the result.
 func quantiles(qs, phis []float64, originValues []float64) []float64 {
 	a := getFloat64s()
-	a.A = prepareForQuantileFloat64(a.A[:0], originValues)
+	a.prepareForQuantileFloat64(originValues)
 	qs = quantilesSorted(qs, phis, a.A)
 	putFloat64s(a)
 	return qs
@@ -828,22 +828,38 @@ func quantiles(qs, phis []float64, originValues []float64) []float64 {
 // quantile calculates the given phi from originValues without modifying originValues
 func quantile(phi float64, originValues []float64) float64 {
 	a := getFloat64s()
-	a.A = prepareForQuantileFloat64(a.A[:0], originValues)
+	a.prepareForQuantileFloat64(originValues)
 	q := quantileSorted(phi, a.A)
 	putFloat64s(a)
 	return q
 }
 
-// prepareForQuantileFloat64 copies items from src to dst but removes NaNs and sorts the dst
-func prepareForQuantileFloat64(dst, src []float64) []float64 {
+// prepareForQuantileFloat64 copies items from src to a but removes NaNs and sorts items in a.
+func (a *float64s) prepareForQuantileFloat64(src []float64) {
+	dst := a.A[:0]
 	for _, v := range src {
 		if math.IsNaN(v) {
 			continue
 		}
 		dst = append(dst, v)
 	}
-	sort.Float64s(dst)
-	return dst
+	a.A = dst
+	// Use sort.Sort instead of sort.Float64s in order to avoid a memory allocation
+	sort.Sort(a)
+}
+
+func (a *float64s) Len() int {
+	return len(a.A)
+}
+
+func (a *float64s) Swap(i, j int) {
+	x := a.A
+	x[i], x[j] = x[j], x[i]
+}
+
+func (a *float64s) Less(i, j int) bool {
+	x := a.A
+	return x[i] < x[j]
 }
 
 // quantilesSorted calculates the given phis over a sorted list of values, appends them to qs and returns the result.