LoadScope scheduler: Sort scopes by number of tests to assign biggest scopes first (#778)

Follow up to # 632.

---------

Co-authored-by: Jörg Kohlsdorf <joerg@cornershopapp.com>

changelog/632.feature.rst

@@ -0,0 +1 @@
+``--dist=loadscope`` now sorts scopes by number of tests to assign largest scopes early -- in many cases this should improve overall test session running time, as there is less chance of a large scope being left to be processed near the end of the session, leaving other workers idle.

src/xdist/scheduler/loadscope.py

@@ -350,11 +350,18 @@ class LoadScopeScheduling:
             return
 
         # Determine chunks of work (scopes)
+        unsorted_workqueue = OrderedDict()
         for nodeid in self.collection:
             scope = self._split_scope(nodeid)
-            work_unit = self.workqueue.setdefault(scope, default=OrderedDict())
+            work_unit = unsorted_workqueue.setdefault(scope, default=OrderedDict())
             work_unit[nodeid] = False
 
+        # Insert tests scopes into work queue ordered by number of tests.
+        for scope, nodeids in sorted(
+            unsorted_workqueue.items(), key=lambda item: -len(item[1])
+        ):
+            self.workqueue[scope] = nodeids
+
         # Avoid having more workers than work
         extra_nodes = len(self.nodes) - len(self.workqueue)
 

testing/acceptance_test.py

@@ -1232,6 +1232,22 @@ class TestLoadScope:
             "test_a.py::TestB", result.outlines
         ) in ({"gw0": 10}, {"gw1": 10})
 
+    def test_workqueue_ordered_by_size(self, pytester: pytest.Pytester) -> None:
+        test_file = """
+            import pytest
+            @pytest.mark.parametrize('i', range({}))
+            def test(i):
+                pass
+        """
+        pytester.makepyfile(test_a=test_file.format(10), test_b=test_file.format(20))
+        result = pytester.runpytest("-n2", "--dist=loadscope", "-v")
+        assert get_workers_and_test_count_by_prefix(
+            "test_a.py::test", result.outlines
+        ) == {"gw1": 10}
+        assert get_workers_and_test_count_by_prefix(
+            "test_b.py::test", result.outlines
+        ) == {"gw0": 20}
+
     def test_module_single_start(self, pytester: pytest.Pytester) -> None:
         """Fix test suite never finishing in case all workers start with a single test (#277)."""
         test_file1 = """