Don't recompute runahead unnecessarily in main loop

cylc/flow/scheduler.py

@@ -343,6 +343,10 @@ def __init__(self, id_: str, options: 'Values') -> None:
         # {install_target: platform}
         self.incomplete_ri_map: Dict[str, Dict] = {}
 
+        # This flag must be initialised to True, to compute the runahead
+        # limit at the start of the first main loop iteration.
+        self.has_updated: bool = True
+
     async def install(self):
         """Get the filesystem in the right state to run the flow.
         * Validate flowfiles
@@ -1618,8 +1622,13 @@ async def _main_loop(self) -> None:
 
         tinit = time()
 
-        self.pool.compute_runahead()
-        self.pool.release_runahead_tasks()
+        if self.has_updated or self.pool.tasks_removed:
+            # The runahead limit might need recomputing.
+            self.pool.compute_runahead()
+            self.pool.release_runahead_tasks()
+            # Reset tasks_removed (this is the only use of this flag).
+            self.pool.tasks_removed = False
+
         # If applicable, set stop mode or shutdown on task failure:
         await self.workflow_shutdown()
 
@@ -1704,19 +1713,19 @@ async def _main_loop(self) -> None:
         # List of task whose states have changed.
         updated_task_list = [
             t for t in self.pool.get_tasks() if t.state.is_updated]
-        has_updated = updated_task_list or self.is_updated
+        self.has_updated = bool(updated_task_list) or self.is_updated
 
         if updated_task_list and self.is_restart_timeout_wait:
             # Stop restart timeout if action has been triggered.
             with suppress(KeyError):
                 self.timers[self.EVENT_RESTART_TIMEOUT].stop()
                 self.is_restart_timeout_wait = False
 
-        if has_updated or self.data_store_mgr.updates_pending:
+        if self.has_updated or self.data_store_mgr.updates_pending:
             # Update the datastore.
             await self.update_data_structure()
 
-        if has_updated:
+        if self.has_updated:
             if not self.is_reloaded and self.is_stalled:
                 # (A reload cannot un-stall workflow by itself)
                 self.is_stalled = False
@@ -1754,7 +1763,7 @@ async def _main_loop(self) -> None:
             )
         )
 
-        if not has_updated and not self.stop_mode:
+        if not self.has_updated and not self.stop_mode:
             # Has the workflow stalled?
             self.check_workflow_stalled()
 

cylc/flow/task_pool.py

@@ -431,25 +431,25 @@ def compute_runahead(self, force=False) -> bool:
             )
         else:
             # Find the earliest point with incomplete tasks.
-            for point, itasks in sorted(self.get_tasks_by_point().items()):
-                # All n=0 tasks are incomplete by definition, but Cylc 7
-                # ignores failed ones (it does not ignore submit-failed!).
-                if (
-                    cylc.flow.flags.cylc7_back_compat and
-                    all(
-                        itask.state(TASK_STATUS_FAILED)
-                        for itask in itasks
-                    )
-                ):
-                    continue
-                base_point = point
-                break
+            if cylc.flow.flags.cylc7_back_compat:
+                for point, itasks in sorted(self.get_tasks_by_point().items()):
+                    # All n=0 tasks are incomplete by definition, but Cylc 7
+                    # ignores failed ones (it does not ignore submit-failed!).
+                    if (
+                        all(
+                            itask.state(TASK_STATUS_FAILED)
+                            for itask in itasks
+                        )
+                    ):
+                        continue
+                    base_point = point
+                    break
+            else:
+                base_point = min(self.active_tasks)
 
         if base_point is None:
             return False
 
-        LOG.debug(f"Runahead: base point {base_point}")
-
         if self._prev_runahead_base_point is None:
             self._prev_runahead_base_point = base_point