scheduler,spanz(ticdc): specify BtreeMap degree

Signed-off-by: Neil Shen <overvenus@gmail.com>

cdc/scheduler/internal/v3/replication/replication_manager.go

@@ -114,8 +114,11 @@ type Manager struct { //nolint:revive
 func NewReplicationManager(
 	maxTaskConcurrency int, changefeedID model.ChangeFeedID,
 ) *Manager {
+	// degreeReadHeavy is a degree optimized for read heavy map, many Ascend.
+	// There may be a large number of tables.
+	const degreeReadHeavy = 256
 	return &Manager{
-		spans:              spanz.NewBtreeMap[*ReplicationSet](),
+		spans:              spanz.NewBtreeMapWithDegree[*ReplicationSet](degreeReadHeavy),
 		runningTasks:       spanz.NewBtreeMap[*ScheduleTask](),
 		maxTaskConcurrency: maxTaskConcurrency,
 		changefeedID:       changefeedID,

pkg/spanz/btree_map.go

@@ -46,10 +46,14 @@ type BtreeMap[T any] struct {
 
 // NewBtreeMap returns a new BtreeMap.
 func NewBtreeMap[T any]() *BtreeMap[T] {
-	// Map is read heavy map, many Ascend.
-	const defaultDegree = 256
+	const defaultDegree = 16
+	return NewBtreeMapWithDegree[T](defaultDegree)
+}
+
+// NewBtreeMapWithDegree returns a new BtreeMap with the given degree.
+func NewBtreeMapWithDegree[T any](degree int) *BtreeMap[T] {
 	return &BtreeMap[T]{
-		tree: btree.NewG(defaultDegree, lessSpanItem[T]),
+		tree: btree.NewG(degree, lessSpanItem[T]),
 	}
 }