Restore old initial batch distribution logic in LoadScheduling

pytest orders tests for optimal sequential execution - i. e. avoiding
unnecessary setup and teardown of fixtures. So executing tests in consecutive
chunks is important for optimal performance.

Commit 09d79ac optimized test distribution for
the corner case, when the number of tests is less than 2 * number of nodes.
At the same time, it made initial test distribution worse for all other cases.
If some tests use some fixture, and these tests fit into the initial batch,
the fixture will be created min(n_tests, n_workers) times, no matter how many
other tests there are. With the old algorithm (before
09d79ac), if there are enough tests not using
the fixture, the fixture was created only once.

So restore the old behavior for typical cases where the number of tests is
much greater than the number of workers (or, strictly speaking, when there
are at least 2 tests for every node).

In my test suite, where fixtures create Docker containers, this change reduces
total run time by 10-15%.

This is a partial revert of commit 09d79ac

changelog/812.feature

@@ -0,0 +1 @@
+Partially restore old initial batch distribution algorithm in ``LoadScheduling``.

src/xdist/scheduler/load.py

@@ -248,13 +248,19 @@ def schedule(self):
         # Send a batch of tests to run. If we don't have at least two
         # tests per node, we have to send them all so that we can send
         # shutdown signals and get all nodes working.
-        initial_batch = max(len(self.pending) // 4, 2 * len(self.nodes))
-
-        # distribute tests round-robin up to the batch size
-        # (or until we run out)
-        nodes = cycle(self.nodes)
-        for i in range(initial_batch):
-            self._send_tests(next(nodes), 1)
+        if len(self.pending) < 2 * len(self.nodes):
+            # distribute tests round-robin
+            nodes = cycle(self.nodes)
+            for i in range(len(self.pending)):
+                self._send_tests(next(nodes), 1)
+        else:
+            # how many items per node do we have about?
+            items_per_node = len(self.collection) // len(self.node2pending)
+            # take a fraction of tests for initial distribution
+            node_chunksize = max(items_per_node // 4, 2)
+            # and initialize each node with a chunk of tests
+            for node in self.nodes:
+                self._send_tests(node, node_chunksize)
 
         if not self.pending:
             # initial distribution sent all tests, start node shutdown

testing/test_dsession.py

@@ -115,18 +115,18 @@ def test_schedule_batch_size(self, pytester: pytest.Pytester) -> None:
         # assert not sched.tests_finished
         sent1 = node1.sent
         sent2 = node2.sent
-        assert sent1 == [0, 2]
-        assert sent2 == [1, 3]
+        assert sent1 == [0, 1]
+        assert sent2 == [2, 3]
         assert sched.pending == [4, 5]
         assert sched.node2pending[node1] == sent1
         assert sched.node2pending[node2] == sent2
         assert len(sched.pending) == 2
         sched.mark_test_complete(node1, 0)
-        assert node1.sent == [0, 2, 4]
+        assert node1.sent == [0, 1, 4]
         assert sched.pending == [5]
-        assert node2.sent == [1, 3]
-        sched.mark_test_complete(node1, 2)
-        assert node1.sent == [0, 2, 4, 5]
+        assert node2.sent == [2, 3]
+        sched.mark_test_complete(node1, 1)
+        assert node1.sent == [0, 1, 4, 5]
         assert not sched.pending
 
     def test_schedule_fewer_tests_than_nodes(self, pytester: pytest.Pytester) -> None: