# UNDF: UNDF-2026-000000188
--- a/nova/scheduler/manager.py
+++ b/nova/scheduler/manager.py
@@ -587,7 +587,8 @@ class SchedulerManager(manager.Manager):
         Additionally, we may be working with older conductors that don't pass
         in instance_uuids.
         """
-        # The list of hosts selected for each instance
-        selected_hosts = []
+        # The list of hosts selected for each instance (list preserves order)
+        selected_hosts = []
+        selected_hosts_set = set()  # O(1) membership; avoids O(N²) in _schedule_alt

@@ -605,7 +607,8 @@ class SchedulerManager(manager.Manager):

             selected_host = hosts[0]
             selected_hosts.append(selected_host)
+            selected_hosts_set.add(selected_host)
             self._consume_selected_host(

@@ -699,7 +703,7 @@ class SchedulerManager(manager.Manager):
                 if len(selected_plus_alts) >= num_alts + 1:
                     break

-                if host.cell_uuid == cell_uuid and host not in selected_hosts:
+                if host.cell_uuid == cell_uuid and host not in selected_hosts_set:
                     if alloc_reqs_by_rp_uuid is not None: