dxf: add comment to explain current schedule strategy (#59033)

ref #57497

pkg/disttask/framework/scheduler/scheduler.go

@@ -505,7 +505,17 @@ func (s *BaseScheduler) scheduleSubTask(
 	var size uint64
 	subTasks := make([]*proto.Subtask, 0, len(metas))
 	for i, meta := range metas {
-		// we assign the subtask to the instance in a round-robin way.
+		// our schedule target is to maximize the resource usage of all nodes while
+		// fulfilling the target of schedule tasks in the task order, we will try
+		// to pack the subtasks of different tasks onto as minimal number of nodes
+		// as possible, to allow later tasks of higher concurrency can be scheduled
+		// and run, so we order nodes, see TaskManager.GetAllNodes and assign the
+		// subtask to the instance in a round-robin way.
+		// for example:
+		//   - we have 2 node N1 and N2 of 8 cores.
+		//   - we have 2 tasks T1 and T2, each is of thread 4 and have 1 subtask.
+		//   - subtasks of T1 and T2 are all scheduled to N1
+		//   - later we have a task T3 of thread 8, we can schedule it to N2.
 		pos := i % len(adjustedEligibleNodes)
 		instanceID := adjustedEligibleNodes[pos]
 		s.logger.Debug("create subtasks", zap.String("instanceID", instanceID))