uranusjr commented on a change in pull request #17945:
URL: https://github.com/apache/airflow/pull/17945#discussion_r700869979



##########
File path: airflow/jobs/scheduler_job.py
##########
@@ -970,48 +971,61 @@ def _start_queued_dagruns(
         session: Session,
     ) -> int:
         """Find DagRuns in queued state and decide moving them to running 
state"""
-        dag_runs = self._get_next_dagruns_to_examine(State.QUEUED, session)
-
-        active_runs_of_dags = defaultdict(
-            lambda: 0,
-            session.query(DagRun.dag_id, func.count('*'))
-            .filter(  # We use `list` here because SQLA doesn't accept a set
-                # We use set to avoid duplicate dag_ids
-                DagRun.dag_id.in_(list({dr.dag_id for dr in dag_runs})),
-                DagRun.state == State.RUNNING,
-            )
-            .group_by(DagRun.dag_id)
-            .all(),
+        active_dags = (
+            session.query(DM)
+            .filter(DM.is_active == expression.true(), DM.is_paused == 
expression.false())
+            .all()
         )
+        max_number = conf.getint('scheduler', 
'max_dagruns_per_loop_to_schedule', fallback=20)
+        # There's a possibility of exceeding this max_number or going below it 
as a result of the logic below
+        if len(active_dags) > max_number:
+            max_number = 1
+        elif len(active_dags) > 0:
+            max_number = round(max_number / len(active_dags))
 
-        def _update_state(dag_run):
-            dag_run.state = State.RUNNING
-            dag_run.start_date = timezone.utcnow()
-            expected_start_date = 
dag.following_schedule(dag_run.execution_date)
-            if expected_start_date:
-                schedule_delay = dag_run.start_date - expected_start_date
-                Stats.timing(
-                    f'dagrun.schedule_delay.{dag.dag_id}',
-                    schedule_delay,
-                )
+        for dag_model in active_dags:
 
-        for dag_run in dag_runs:
-            try:
-                dag = dag_run.dag = self.dagbag.get_dag(dag_run.dag_id, 
session=session)
-            except SerializedDagNotFound:
-                self.log.exception("DAG '%s' not found in serialized_dag 
table", dag_run.dag_id)
-                continue
-            active_runs = active_runs_of_dags[dag_run.dag_id]
-            if dag.max_active_runs and active_runs >= dag.max_active_runs:
-                self.log.debug(
-                    "DAG %s already has %d active runs, not moving any more 
runs to RUNNING state %s",
-                    dag.dag_id,
-                    active_runs,
-                    dag_run.execution_date,
+            queued_dagruns = 
DR.get_next_queued_dagruns_for_dag(dag_model.dag_id, session, max_number)
+
+            active_runs_of_dags = defaultdict(
+                lambda: 0,
+                session.query(DagRun.dag_id, func.count('*'))
+                .filter(
+                    DagRun.dag_id == dag_model.dag_id,
+                    DagRun.state == State.RUNNING,
                 )
-            else:
-                active_runs_of_dags[dag_run.dag_id] += 1
-                _update_state(dag_run)
+                .group_by(DagRun.dag_id)
+                .all(),
+            )
+
+            def _update_state(dag_run):
+                dag_run.state = State.RUNNING
+                dag_run.start_date = timezone.utcnow()
+                expected_start_date = 
dag.following_schedule(dag_run.execution_date)

Review comment:
       `following_schedule` is deprecated so we should avoid using it.




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