Avoid growing slice when mapping points to shards

coordinator/points_writer.go

@@ -98,13 +98,15 @@ func NewPointsWriter() *PointsWriter {
 
 // ShardMapping contains a mapping of shards to points.
 type ShardMapping struct {
+	n      int
 	Points map[uint64][]models.Point  // The points associated with a shard ID
 	Shards map[uint64]*meta.ShardInfo // The shards that have been mapped, keyed by shard ID
 }
 
 // NewShardMapping creates an empty ShardMapping.
-func NewShardMapping() *ShardMapping {
+func NewShardMapping(n int) *ShardMapping {
 	return &ShardMapping{
+		n:      n,
 		Points: map[uint64][]models.Point{},
 		Shards: map[uint64]*meta.ShardInfo{},
 	}
@@ -112,6 +114,9 @@ func NewShardMapping() *ShardMapping {
 
 // MapPoint adds the point to the ShardMapping, associated with the given shardInfo.
 func (s *ShardMapping) MapPoint(shardInfo *meta.ShardInfo, p models.Point) {
+	if cap(s.Points[shardInfo.ID]) < s.n {
+		s.Points[shardInfo.ID] = make([]models.Point, 0, s.n)
+	}
 	s.Points[shardInfo.ID] = append(s.Points[shardInfo.ID], p)
 	s.Shards[shardInfo.ID] = shardInfo
 }
@@ -218,7 +223,7 @@ func (w *PointsWriter) MapShards(wp *WritePointsRequest) (*ShardMapping, error)
 		list = list.Append(*sg)
 	}
 
-	mapping := NewShardMapping()
+	mapping := NewShardMapping(len(wp.Points))
 	for _, p := range wp.Points {
 		sg := list.ShardGroupAt(p.Time())
 		if sg == nil {

coordinator/points_writer_test.go

@@ -286,7 +286,7 @@ func TestPointsWriter_WritePoints(t *testing.T) {
 
 		// copy to prevent data race
 		theTest := test
-		sm := coordinator.NewShardMapping()
+		sm := coordinator.NewShardMapping(16)
 		sm.MapPoint(
 			&meta.ShardInfo{ID: uint64(1), Owners: []meta.ShardOwner{
 				{NodeID: 1},