Release 3.2.1

Fix hang caused by `steal` command with empty test queue

.pre-commit-config.yaml

@@ -1,19 +1,19 @@
 repos:
 -   repo: https://github.com/PyCQA/autoflake
-    rev: v2.0.0
+    rev: v2.0.1
     hooks:
     -   id: autoflake
         args: ["--in-place", "--remove-unused-variables", "--remove-all-unused-imports"]
 -   repo: https://github.com/psf/black
-    rev: 22.10.0
+    rev: 23.1.0
     hooks:
     -   id: black
         args: [--safe, --quiet, --target-version, py35]
 -   repo: https://github.com/asottile/blacken-docs
-    rev: v1.12.1
+    rev: 1.13.0
     hooks:
     -   id: blacken-docs
-        additional_dependencies: [black==20.8b1]
+        additional_dependencies: [black==23.1.0]
 -   repo: https://github.com/pre-commit/pre-commit-hooks
     rev: v4.4.0
     hooks:
@@ -26,7 +26,7 @@ repos:
     hooks:
     -   id: flake8
 -   repo: https://github.com/asottile/pyupgrade
-    rev: v3.2.2
+    rev: v3.3.1
     hooks:
     -   id: pyupgrade
         args: [--py3-plus]
@@ -39,7 +39,7 @@ repos:
         language: python
         additional_dependencies: [pygments, restructuredtext_lint]
 -   repo: https://github.com/pre-commit/mirrors-mypy
-    rev: v0.991
+    rev: v1.0.1
     hooks:
     -   id: mypy
         files: ^(src/|testing/)

CHANGELOG.rst

@@ -1,3 +1,36 @@
+pytest-xdist 3.2.1 (2023-03-12)
+===============================
+
+Bug Fixes
+---------
+
+- `#884 <https://github.com/pytest-dev/pytest-xdist/issues/884>`_: Fixed hang in ``worksteal`` scheduler.
+
+
+pytest-xdist 3.2.0 (2023-02-07)
+===============================
+
+Improved Documentation
+----------------------
+
+- `#863 <https://github.com/pytest-dev/pytest-xdist/issues/863>`_: Document limitations for debugging due to standard I/O of workers not being forwarded. Also, mention remote debugging as a possible workaround.
+
+
+Features
+--------
+
+- `#855 <https://github.com/pytest-dev/pytest-xdist/issues/855>`_: Users can now configure ``load`` scheduling precision using ``--maxschedchunk`` command
+  line option.
+
+- `#858 <https://github.com/pytest-dev/pytest-xdist/issues/858>`_: New ``worksteal`` scheduler, based on the idea of `work stealing <https://en.wikipedia.org/wiki/Work_stealing>`_. It's similar to ``load`` scheduler, but it should handle tests with significantly differing duration better, and, at the same time, it should provide similar or better reuse of fixtures.
+
+
+Trivial Changes
+---------------
+
+- `#870 <https://github.com/pytest-dev/pytest-xdist/issues/870>`_: Make the tests pass even when ``$PYTEST_XDIST_AUTO_NUM_WORKERS`` is set.
+
+
 pytest-xdist 3.1.0 (2022-12-01)
 ===============================
 

docs/distribution.rst

@@ -82,4 +82,13 @@ The test distribution algorithm is configured with the ``--dist`` command-line o
   This will make sure ``test1`` and ``TestA::test2`` will run in the same worker.
   Tests without the ``xdist_group`` mark are distributed normally as in the ``--dist=load`` mode.
 
+* ``--dist worksteal``: Initially, tests are distributed evenly among all
+  available workers. When a worker completes most of its assigned tests and
+  doesn't have enough tests to continue (currently, every worker needs at least
+  two tests in its queue), an attempt is made to reassign ("steal") a portion
+  of tests from some other worker's queue. The results should be similar to
+  the ``load`` method, but ``worksteal`` should handle tests with significantly
+  differing duration better, and, at the same time, it should provide similar
+  or better reuse of fixtures.
+
 * ``--dist no``: The normal pytest execution mode, runs one test at a time (no distribution at all).

docs/how-to.rst

@@ -15,7 +15,7 @@ a test or fixture, you may use the ``worker_id`` fixture to do so:
 
     @pytest.fixture()
     def user_account(worker_id):
-        """ use a different account in each xdist worker """
+        """use a different account in each xdist worker"""
         return "account_%s" % worker_id
 
 When ``xdist`` is disabled (running with ``-n0`` for example), then
@@ -80,7 +80,7 @@ wanted to create a separate database for each test run:
 
     @pytest.fixture(scope="session", autouse=True)
     def create_unique_database(testrun_uid):
-        """ create a unique database for this particular test run """
+        """create a unique database for this particular test run"""
         database_url = f"psql://myapp-{testrun_uid}"
 
         with Semaphore(f"/{testrun_uid}-lock", flags=O_CREAT, initial_value=1):
@@ -90,7 +90,7 @@ wanted to create a separate database for each test run:
 
     @pytest.fixture()
     def db(testrun_uid):
-        """ retrieve unique database """
+        """retrieve unique database"""
         database_url = f"psql://myapp-{testrun_uid}"
         return database_get_instance(database_url)
 
@@ -221,7 +221,6 @@ Example:
     def pytest_configure(config):
         worker_id = os.environ.get("PYTEST_XDIST_WORKER")
         if worker_id is not None:
-            log_file = config.getini("worker_log_file")
             logging.basicConfig(
                 format=config.getini("log_file_format"),
                 filename=f"tests_{worker_id}.log",

docs/known-limitations.rst

@@ -6,7 +6,7 @@ pytest-xdist has some limitations that may be supported in pytest but can't be s
 Order and amount of test must be consistent
 -------------------------------------------
 
-Is is not possible to have tests that differ in order or their amount across workers.
+It is not possible to have tests that differ in order or their amount across workers.
 
 This is especially true with ``pytest.mark.parametrize``, when values are produced with sets or other unordered iterables/generators.
 
@@ -59,6 +59,13 @@ Output (stdout and stderr) from workers
 
 The ``-s``/``--capture=no`` option is meant to disable pytest capture, so users can then see stdout and stderr output in the terminal from tests and application code in real time.
 
-However this option does not work with ``pytest-xdist`` because `execnet <https://github.com/pytest-dev/execnet>`__ the underlying library used for communication between master and workers, does not support transferring stdout/stderr from workers.
+However, this option does not work with ``pytest-xdist`` because `execnet <https://github.com/pytest-dev/execnet>`__ the underlying library used for communication between master and workers, does not support transferring stdout/stderr from workers.
 
-Currenlty there are no plans ot support this in ``pytest-xdist``.
+Currently, there are no plans to support this in ``pytest-xdist``.
+
+Debugging
+~~~~~~~~~
+
+This also means that debugging using PDB (or any other debugger that wants to use standard I/O) will not work. The ``--pdb`` option is disabled when distributing tests with ``pytest-xdist`` for this reason.
+
+It is generally likely best to use ``pytest-xdist`` to find failing tests and then debug them without distribution; however, if you need to debug from within a worker process (for example, to address failures that only happen when running tests concurrently), remote debuggers (for example, `python-remote-pdb <https://github.com/ionelmc/python-remote-pdb>`__ or `python-web-pdb <https://github.com/romanvm/python-web-pdb>`__) have been reported to work for this purpose.

src/xdist/dsession.py

@@ -8,6 +8,7 @@ from xdist.scheduler import (
     LoadScopeScheduling,
     LoadFileScheduling,
     LoadGroupScheduling,
+    WorkStealingScheduling,
 )
 
 
@@ -100,6 +101,7 @@ class DSession:
             "loadscope": LoadScopeScheduling,
             "loadfile": LoadFileScheduling,
             "loadgroup": LoadGroupScheduling,
+            "worksteal": WorkStealingScheduling,
         }
         return schedulers[dist](config, log)
 
@@ -282,6 +284,17 @@ class DSession:
         """
         self.sched.mark_test_complete(node, item_index, duration)
 
+    def worker_unscheduled(self, node, indices):
+        """
+        Emitted when a node fires the 'unscheduled' event, signalling that
+        some tests have been removed from the worker's queue and should be
+        sent to some worker again.
+
+        This should happen only in response to 'steal' command, so schedulers
+        not using 'steal' command don't have to implement it.
+        """
+        self.sched.remove_pending_tests_from_node(node, indices)
+
     def worker_collectreport(self, node, rep):
         """Emitted when a node calls the pytest_collectreport hook.
 

src/xdist/looponfail.py

@@ -35,7 +35,6 @@ def pytest_addoption(parser):
 
 @pytest.hookimpl
 def pytest_cmdline_main(config):
-
     if config.getoption("looponfail"):
         usepdb = config.getoption("usepdb", False)  # a core option
         if usepdb:

src/xdist/plugin.py

@@ -94,7 +94,15 @@ def pytest_addoption(parser):
         "--dist",
         metavar="distmode",
         action="store",
-        choices=["each", "load", "loadscope", "loadfile", "loadgroup", "no"],
+        choices=[
+            "each",
+            "load",
+            "loadscope",
+            "loadfile",
+            "loadgroup",
+            "worksteal",
+            "no",
+        ],
         dest="dist",
         default="no",
         help=(
@@ -107,6 +115,8 @@ def pytest_addoption(parser):
             "loadfile: load balance by sending test grouped by file"
             " to any available environment.\n\n"
             "loadgroup: like load, but sends tests marked with 'xdist_group' to the same worker.\n\n"
+            "worksteal: split the test suite between available environments,"
+            " then rebalance when any worker runs out of tests.\n\n"
             "(default) no: run tests inprocess, don't distribute."
         ),
     )
@@ -153,6 +163,19 @@ def pytest_addoption(parser):
             "on every test run."
         ),
     )
+    group.addoption(
+        "--maxschedchunk",
+        action="store",
+        type=int,
+        help=(
+            "Maximum number of tests scheduled in one step for --dist=load. "
+            "Setting it to 1 will force pytest to send tests to workers one by "
+            "one - might be useful for a small number of slow tests. "
+            "Larger numbers will allow the scheduler to submit consecutive "
+            "chunks of tests to workers - allows reusing fixtures. "
+            "Unlimited if not set."
+        ),
+    )
 
     parser.addini(
         "rsyncdirs",

src/xdist/remote.py

@@ -6,6 +6,7 @@
     needs not to be installed in remote environments.
 """
 
+import contextlib
 import sys
 import os
 import time
@@ -56,14 +57,31 @@ def worker_title(title):
 
 
 class WorkerInteractor:
+    SHUTDOWN_MARK = object()
+    QUEUE_REPLACED_MARK = object()
+
     def __init__(self, config, channel):
         self.config = config
         self.workerid = config.workerinput.get("workerid", "?")
         self.testrunuid = config.workerinput["testrunuid"]
         self.log = Producer(f"worker-{self.workerid}", enabled=config.option.debug)
         self.channel = channel
+        self.torun = self._make_queue()
+        self.nextitem_index = None
         config.pluginmanager.register(self)
 
+    def _make_queue(self):
+        return self.channel.gateway.execmodel.queue.Queue()
+
+    def _get_next_item_index(self):
+        """Gets the next item from test queue. Handles the case when the queue
+        is replaced concurrently in another thread.
+        """
+        result = self.torun.get()
+        while result is self.QUEUE_REPLACED_MARK:
+            result = self.torun.get()
+        return result
+
     def sendevent(self, name, **kwargs):
         self.log("sending", name, kwargs)
         self.channel.send((name, kwargs))
@@ -92,38 +110,63 @@ class WorkerInteractor:
     def pytest_collection(self, session):
         self.sendevent("collectionstart")
 
+    def handle_command(self, command):
+        if command is self.SHUTDOWN_MARK:
+            self.torun.put(self.SHUTDOWN_MARK)
+            return
+
+        name, kwargs = command
+
+        self.log("received command", name, kwargs)
+        if name == "runtests":
+            for i in kwargs["indices"]:
+                self.torun.put(i)
+        elif name == "runtests_all":
+            for i in range(len(self.session.items)):
+                self.torun.put(i)
+        elif name == "shutdown":
+            self.torun.put(self.SHUTDOWN_MARK)
+        elif name == "steal":
+            self.steal(kwargs["indices"])
+
+    def steal(self, indices):
+        indices = set(indices)
+        stolen = []
+
+        old_queue, self.torun = self.torun, self._make_queue()
+
+        def old_queue_get_nowait_noraise():
+            with contextlib.suppress(self.channel.gateway.execmodel.queue.Empty):
+                return old_queue.get_nowait()
+
+        for i in iter(old_queue_get_nowait_noraise, None):
+            if i in indices:
+                stolen.append(i)
+            else:
+                self.torun.put(i)
+
+        self.sendevent("unscheduled", indices=stolen)
+        old_queue.put(self.QUEUE_REPLACED_MARK)
+
     @pytest.hookimpl
     def pytest_runtestloop(self, session):
         self.log("entering main loop")
-        torun = []
+        self.channel.setcallback(self.handle_command, endmarker=self.SHUTDOWN_MARK)
-        while 1:
+        self.nextitem_index = self._get_next_item_index()
-            try:
+        while self.nextitem_index is not self.SHUTDOWN_MARK:
-                name, kwargs = self.channel.receive()
+            self.run_one_test()
-            except EOFError:
-                return True
-            self.log("received command", name, kwargs)
-            if name == "runtests":
-                torun.extend(kwargs["indices"])
-            elif name == "runtests_all":
-                torun.extend(range(len(session.items)))
-            self.log("items to run:", torun)
-            # only run if we have an item and a next item
-            while len(torun) >= 2:
-                self.run_one_test(torun)
-            if name == "shutdown":
-                if torun:
-                    self.run_one_test(torun)
-                break
         return True
 
-    def run_one_test(self, torun):
+    def run_one_test(self):
+        self.item_index = self.nextitem_index
+        self.nextitem_index = self._get_next_item_index()
+
         items = self.session.items
-        self.item_index = torun.pop(0)
         item = items[self.item_index]
-        if torun:
+        if self.nextitem_index is self.SHUTDOWN_MARK:
-            nextitem = items[torun[0]]
-        else:
             nextitem = None
+        else:
+            nextitem = items[self.nextitem_index]
 
         worker_title("[pytest-xdist running] %s" % item.nodeid)
 

src/xdist/scheduler/__init__.py

@@ -3,3 +3,4 @@ from xdist.scheduler.load import LoadScheduling  # noqa
 from xdist.scheduler.loadfile import LoadFileScheduling  # noqa
 from xdist.scheduler.loadscope import LoadScopeScheduling  # noqa
 from xdist.scheduler.loadgroup import LoadGroupScheduling  # noqa
+from xdist.scheduler.worksteal import WorkStealingScheduling  # noqa

src/xdist/scheduler/load.py

@@ -64,6 +64,7 @@ class LoadScheduling:
         else:
             self.log = log.loadsched
         self.config = config
+        self.maxschedchunk = self.config.getoption("maxschedchunk")
 
     @property
     def nodes(self):
@@ -185,7 +186,9 @@ class LoadScheduling:
                     # so let's rather wait with sending new items
                     return
                 num_send = items_per_node_max - len(node_pending)
-                self._send_tests(node, num_send)
+                # keep at least 2 tests pending even if --maxschedchunk=1
+                maxschedchunk = max(2 - len(node_pending), self.maxschedchunk)
+                self._send_tests(node, min(num_send, maxschedchunk))
         else:
             node.shutdown()
 
@@ -245,6 +248,9 @@ class LoadScheduling:
         if not self.collection:
             return
 
+        if self.maxschedchunk is None:
+            self.maxschedchunk = len(self.collection)
+
         # 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.
@@ -265,7 +271,8 @@ class LoadScheduling:
             # 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)
+            node_chunksize = min(items_per_node // 4, self.maxschedchunk)
+            node_chunksize = max(node_chunksize, 2)
             # and initialize each node with a chunk of tests
             for node in self.nodes:
                 self._send_tests(node, node_chunksize)

src/xdist/scheduler/loadscope.py

@@ -213,13 +213,11 @@ class LoadScopeScheduling:
 
         # A new node has been added later, perhaps an original one died.
         if self.collection_is_completed:
-
             # Assert that .schedule() should have been called by now
             assert self.collection
 
             # Check that the new collection matches the official collection
             if collection != self.collection:
-
                 other_node = next(iter(self.registered_collections.keys()))
 
                 msg = report_collection_diff(

src/xdist/scheduler/worksteal.py

@@ -0,0 +1,325 @@
+from collections import namedtuple
+
+from _pytest.runner import CollectReport
+
+from xdist.remote import Producer
+from xdist.workermanage import parse_spec_config
+from xdist.report import report_collection_diff
+
+
+NodePending = namedtuple("NodePending", ["node", "pending"])
+
+# Every worker needs at least 2 tests in queue - the current and the next one.
+MIN_PENDING = 2
+
+
+class WorkStealingScheduling:
+    """Implement work-stealing scheduling.
+
+    Initially, tests are distributed evenly among all nodes.
+
+    When some node completes most of its assigned tests (when only one pending
+    test remains), an attempt is made to reassign ("steal") some tests from
+    other nodes to this node.
+
+    Attributes:
+
+    :numnodes: The expected number of nodes taking part.  The actual
+       number of nodes will vary during the scheduler's lifetime as
+       nodes are added by the DSession as they are brought up and
+       removed either because of a dead node or normal shutdown.  This
+       number is primarily used to know when the initial collection is
+       completed.
+
+    :node2collection: Map of nodes and their test collection.  All
+       collections should always be identical.
+
+    :node2pending: Map of nodes and the indices of their pending
+       tests.  The indices are an index into ``.pending`` (which is
+       identical to their own collection stored in
+       ``.node2collection``).
+
+    :collection: The one collection once it is validated to be
+       identical between all the nodes.  It is initialised to None
+       until ``.schedule()`` is called.
+
+    :pending: List of indices of globally pending tests.  These are
+       tests which have not yet been allocated to a chunk for a node
+       to process.
+
+    :log: A py.log.Producer instance.
+
+    :config: Config object, used for handling hooks.
+
+    :steal_requested_from_node: The node to which the current "steal" request
+       was sent. ``None`` if there is no request in progress. Only one request
+       can be in progress at any time, the scheduler doesn't send multiple
+       simultaneous requests.
+    """
+
+    def __init__(self, config, log=None):
+        self.numnodes = len(parse_spec_config(config))
+        self.node2collection = {}
+        self.node2pending = {}
+        self.pending = []
+        self.collection = None
+        if log is None:
+            self.log = Producer("workstealsched")
+        else:
+            self.log = log.workstealsched
+        self.config = config
+        self.steal_requested_from_node = None
+
+    @property
+    def nodes(self):
+        """A list of all nodes in the scheduler."""
+        return list(self.node2pending.keys())
+
+    @property
+    def collection_is_completed(self):
+        """Boolean indication initial test collection is complete.
+
+        This is a boolean indicating all initial participating nodes
+        have finished collection.  The required number of initial
+        nodes is defined by ``.numnodes``.
+        """
+        return len(self.node2collection) >= self.numnodes
+
+    @property
+    def tests_finished(self):
+        """Return True if all tests have been executed by the nodes."""
+        if not self.collection_is_completed:
+            return False
+        if self.pending:
+            return False
+        if self.steal_requested_from_node is not None:
+            return False
+        for pending in self.node2pending.values():
+            if len(pending) >= MIN_PENDING:
+                return False
+        return True
+
+    @property
+    def has_pending(self):
+        """Return True if there are pending test items
+
+        This indicates that collection has finished and nodes are
+        still processing test items, so this can be thought of as
+        "the scheduler is active".
+        """
+        if self.pending:
+            return True
+        for pending in self.node2pending.values():
+            if pending:
+                return True
+        return False
+
+    def add_node(self, node):
+        """Add a new node to the scheduler.
+
+        From now on the node will be allocated chunks of tests to
+        execute.
+
+        Called by the ``DSession.worker_workerready`` hook when it
+        successfully bootstraps a new node.
+        """
+        assert node not in self.node2pending
+        self.node2pending[node] = []
+
+    def add_node_collection(self, node, collection):
+        """Add the collected test items from a node
+
+        The collection is stored in the ``.node2collection`` map.
+        Called by the ``DSession.worker_collectionfinish`` hook.
+        """
+        assert node in self.node2pending
+        if self.collection_is_completed:
+            # A new node has been added later, perhaps an original one died.
+            # .schedule() should have
+            # been called by now
+            assert self.collection
+            if collection != self.collection:
+                other_node = next(iter(self.node2collection.keys()))
+                msg = report_collection_diff(
+                    self.collection, collection, other_node.gateway.id, node.gateway.id
+                )
+                self.log(msg)
+                return
+        self.node2collection[node] = list(collection)
+
+    def mark_test_complete(self, node, item_index, duration=None):
+        """Mark test item as completed by node
+
+        This is called by the ``DSession.worker_testreport`` hook.
+        """
+        self.node2pending[node].remove(item_index)
+        self.check_schedule()
+
+    def mark_test_pending(self, item):
+        self.pending.insert(
+            0,
+            self.collection.index(item),
+        )
+        self.check_schedule()
+
+    def remove_pending_tests_from_node(self, node, indices):
+        """Node returned some test indices back in response to 'steal' command.
+
+        This is called by ``DSession.worker_unscheduled``.
+        """
+        assert node is self.steal_requested_from_node
+        self.steal_requested_from_node = None
+
+        indices_set = set(indices)
+        self.node2pending[node] = [
+            i for i in self.node2pending[node] if i not in indices_set
+        ]
+        self.pending.extend(indices)
+        self.check_schedule()
+
+    def check_schedule(self):
+        """Reschedule tests/perform load balancing."""
+        nodes_up = [
+            NodePending(node, pending)
+            for node, pending in self.node2pending.items()
+            if not node.shutting_down
+        ]
+
+        def get_idle_nodes():
+            return [node for node, pending in nodes_up if len(pending) < MIN_PENDING]
+
+        idle_nodes = get_idle_nodes()
+        if not idle_nodes:
+            return
+
+        if self.pending:
+            # Distribute pending tests evenly among idle nodes
+            for i, node in enumerate(idle_nodes):
+                nodes_remaining = len(idle_nodes) - i
+                num_send = len(self.pending) // nodes_remaining
+                self._send_tests(node, num_send)
+
+            idle_nodes = get_idle_nodes()
+            # No need to steal anything if all nodes have enough work to continue
+            if not idle_nodes:
+                return
+
+        # Only one active stealing request is allowed
+        if self.steal_requested_from_node is not None:
+            return
+
+        # Find the node that has the longest test queue
+        steal_from = max(
+            nodes_up, key=lambda node_pending: len(node_pending.pending), default=None
+        )
+
+        if steal_from is None:
+            num_steal = 0
+        else:
+            # Steal half of the test queue - but keep that node running too.
+            # If the node has 2 or less tests queued, stealing will fail
+            # anyway.
+            max_steal = max(0, len(steal_from.pending) - MIN_PENDING)
+            num_steal = min(len(steal_from.pending) // 2, max_steal)
+
+        if num_steal == 0:
+            # Can't get more work - shutdown idle nodes. This will force them
+            # to run the last test now instead of waiting for more tests.
+            for node in idle_nodes:
+                node.shutdown()
+            return
+
+        steal_from.node.send_steal(steal_from.pending[-num_steal:])
+        self.steal_requested_from_node = steal_from.node
+
+    def remove_node(self, node):
+        """Remove a node from the scheduler
+
+        This should be called either when the node crashed or at
+        shutdown time.  In the former case any pending items assigned
+        to the node will be re-scheduled.  Called by the
+        ``DSession.worker_workerfinished`` and
+        ``DSession.worker_errordown`` hooks.
+
+        Return the item which was being executing while the node
+        crashed or None if the node has no more pending items.
+
+        """
+        pending = self.node2pending.pop(node)
+
+        # If node was removed without completing its assigned tests - it crashed
+        if pending:
+            crashitem = self.collection[pending.pop(0)]
+        else:
+            crashitem = None
+
+        self.pending.extend(pending)
+
+        # Dead node won't respond to "steal" request
+        if self.steal_requested_from_node is node:
+            self.steal_requested_from_node = None
+
+        self.check_schedule()
+        return crashitem
+
+    def schedule(self):
+        """Initiate distribution of the test collection
+
+        Initiate scheduling of the items across the nodes.  If this
+        gets called again later it behaves the same as calling
+        ``.check_schedule()`` on all nodes so that newly added nodes
+        will start to be used.
+
+        This is called by the ``DSession.worker_collectionfinish`` hook
+        if ``.collection_is_completed`` is True.
+        """
+        assert self.collection_is_completed
+
+        # Initial distribution already happened, reschedule on all nodes
+        if self.collection is not None:
+            self.check_schedule()
+            return
+
+        if not self._check_nodes_have_same_collection():
+            self.log("**Different tests collected, aborting run**")
+            return
+
+        # Collections are identical, create the index of pending items.
+        self.collection = list(self.node2collection.values())[0]
+        self.pending[:] = range(len(self.collection))
+        if not self.collection:
+            return
+
+        self.check_schedule()
+
+    def _send_tests(self, node, num):
+        tests_per_node = self.pending[:num]
+        if tests_per_node:
+            del self.pending[:num]
+            self.node2pending[node].extend(tests_per_node)
+            node.send_runtest_some(tests_per_node)
+
+    def _check_nodes_have_same_collection(self):
+        """Return True if all nodes have collected the same items.
+
+        If collections differ, this method returns False while logging
+        the collection differences and posting collection errors to
+        pytest_collectreport hook.
+        """
+        node_collection_items = list(self.node2collection.items())
+        first_node, col = node_collection_items[0]
+        same_collection = True
+        for node, collection in node_collection_items[1:]:
+            msg = report_collection_diff(
+                col, collection, first_node.gateway.id, node.gateway.id
+            )
+            if msg:
+                same_collection = False
+                self.log(msg)
+                if self.config is not None:
+                    rep = CollectReport(
+                        node.gateway.id, "failed", longrepr=msg, result=[]
+                    )
+                    self.config.hook.pytest_collectreport(report=rep)
+
+        return same_collection

src/xdist/workermanage.py

@@ -300,6 +300,9 @@ class WorkerController:
     def send_runtest_all(self):
         self.sendcommand("runtests_all")
 
+    def send_steal(self, indices):
+        self.sendcommand("steal", indices=indices)
+
     def shutdown(self):
         if not self._down:
             try:
@@ -359,6 +362,8 @@ class WorkerController:
                 self.notify_inproc(eventname, node=self, ids=kwargs["ids"])
             elif eventname == "runtest_protocol_complete":
                 self.notify_inproc(eventname, node=self, **kwargs)
+            elif eventname == "unscheduled":
+                self.notify_inproc(eventname, node=self, **kwargs)
             elif eventname == "logwarning":
                 self.notify_inproc(
                     eventname,

testing/test_dsession.py

@@ -1,6 +1,6 @@
 from xdist.dsession import DSession, get_default_max_worker_restart
 from xdist.report import report_collection_diff
-from xdist.scheduler import EachScheduling, LoadScheduling
+from xdist.scheduler import EachScheduling, LoadScheduling, WorkStealingScheduling
 from typing import Optional
 
 import pytest
@@ -17,6 +17,7 @@ class MockGateway:
 class MockNode:
     def __init__(self) -> None:
         self.sent = []  # type: ignore[var-annotated]
+        self.stolen = []  # type: ignore[var-annotated]
         self.gateway = MockGateway()
         self._shutdown = False
 
@@ -26,6 +27,9 @@ class MockNode:
     def send_runtest_all(self) -> None:
         self.sent.append("ALL")
 
+    def send_steal(self, indices) -> None:
+        self.stolen.extend(indices)
+
     def shutdown(self) -> None:
         self._shutdown = True
 
@@ -129,30 +133,79 @@ class TestLoadScheduling:
         assert node1.sent == [0, 1, 4, 5]
         assert not sched.pending
 
-    def test_schedule_fewer_tests_than_nodes(self, pytester: pytest.Pytester) -> None:
+    def test_schedule_maxchunk_none(self, pytester: pytest.Pytester) -> None:
         config = pytester.parseconfig("--tx=2*popen")
         sched = LoadScheduling(config)
         sched.add_node(MockNode())
         sched.add_node(MockNode())
+        node1, node2 = sched.nodes
+        col = [f"test{i}" for i in range(16)]
+        sched.add_node_collection(node1, col)
+        sched.add_node_collection(node2, col)
+        sched.schedule()
+        assert node1.sent == [0, 1]
+        assert node2.sent == [2, 3]
+        assert sched.pending == list(range(4, 16))
+        assert sched.node2pending[node1] == node1.sent
+        assert sched.node2pending[node2] == node2.sent
+        sched.mark_test_complete(node1, 0)
+        assert node1.sent == [0, 1, 4, 5]
+        assert sched.pending == list(range(6, 16))
+        sched.mark_test_complete(node1, 1)
+        assert node1.sent == [0, 1, 4, 5]
+        assert sched.pending == list(range(6, 16))
+
+        for i in range(7, 16):
+            sched.mark_test_complete(node1, i - 3)
+            assert node1.sent == [0, 1] + list(range(4, i))
+            assert node2.sent == [2, 3]
+            assert sched.pending == list(range(i, 16))
+
+    def test_schedule_maxchunk_1(self, pytester: pytest.Pytester) -> None:
+        config = pytester.parseconfig("--tx=2*popen", "--maxschedchunk=1")
+        sched = LoadScheduling(config)
+        sched.add_node(MockNode())
+        sched.add_node(MockNode())
+        node1, node2 = sched.nodes
+        col = [f"test{i}" for i in range(16)]
+        sched.add_node_collection(node1, col)
+        sched.add_node_collection(node2, col)
+        sched.schedule()
+        assert node1.sent == [0, 1]
+        assert node2.sent == [2, 3]
+        assert sched.pending == list(range(4, 16))
+        assert sched.node2pending[node1] == node1.sent
+        assert sched.node2pending[node2] == node2.sent
+
+        for complete_index, first_pending in enumerate(range(5, 16)):
+            sched.mark_test_complete(node1, node1.sent[complete_index])
+            assert node1.sent == [0, 1] + list(range(4, first_pending))
+            assert node2.sent == [2, 3]
+            assert sched.pending == list(range(first_pending, 16))
+
+    def test_schedule_fewer_tests_than_nodes(self, pytester: pytest.Pytester) -> None:
+        config = pytester.parseconfig("--tx=3*popen")
+        sched = LoadScheduling(config)
+        sched.add_node(MockNode())
+        sched.add_node(MockNode())
         sched.add_node(MockNode())
         node1, node2, node3 = sched.nodes
         col = ["xyz"] * 2
         sched.add_node_collection(node1, col)
         sched.add_node_collection(node2, col)
+        sched.add_node_collection(node3, col)
+        assert sched.collection_is_completed
         sched.schedule()
         # assert not sched.tests_finished
-        sent1 = node1.sent
+        assert node1.sent == [0]
-        sent2 = node2.sent
+        assert node2.sent == [1]
-        sent3 = node3.sent
+        assert node3.sent == []
-        assert sent1 == [0]
-        assert sent2 == [1]
-        assert sent3 == []
         assert not sched.pending
 
     def test_schedule_fewer_than_two_tests_per_node(
         self, pytester: pytest.Pytester
     ) -> None:
-        config = pytester.parseconfig("--tx=2*popen")
+        config = pytester.parseconfig("--tx=3*popen")
         sched = LoadScheduling(config)
         sched.add_node(MockNode())
         sched.add_node(MockNode())
@@ -161,14 +214,13 @@ class TestLoadScheduling:
         col = ["xyz"] * 5
         sched.add_node_collection(node1, col)
         sched.add_node_collection(node2, col)
+        sched.add_node_collection(node3, col)
+        assert sched.collection_is_completed
         sched.schedule()
         # assert not sched.tests_finished
-        sent1 = node1.sent
+        assert node1.sent == [0, 3]
-        sent2 = node2.sent
+        assert node2.sent == [1, 4]
-        sent3 = node3.sent
+        assert node3.sent == [2]
-        assert sent1 == [0, 3]
-        assert sent2 == [1, 4]
-        assert sent3 == [2]
         assert not sched.pending
 
     def test_add_remove_node(self, pytester: pytest.Pytester) -> None:
@@ -217,6 +269,169 @@ class TestLoadScheduling:
         assert "Different tests were collected between" in rep.longrepr
 
 
+class TestWorkStealingScheduling:
+    def test_ideal_case(self, pytester: pytest.Pytester) -> None:
+        config = pytester.parseconfig("--tx=2*popen")
+        sched = WorkStealingScheduling(config)
+        sched.add_node(MockNode())
+        sched.add_node(MockNode())
+        node1, node2 = sched.nodes
+        collection = [f"test_workstealing.py::test_{i}" for i in range(16)]
+        assert not sched.collection_is_completed
+        sched.add_node_collection(node1, collection)
+        assert not sched.collection_is_completed
+        sched.add_node_collection(node2, collection)
+        assert sched.collection_is_completed
+        assert sched.node2collection[node1] == collection
+        assert sched.node2collection[node2] == collection
+        sched.schedule()
+        assert not sched.pending
+        assert not sched.tests_finished
+        assert node1.sent == list(range(0, 8))
+        assert node2.sent == list(range(8, 16))
+        for i in range(8):
+            sched.mark_test_complete(node1, node1.sent[i])
+            sched.mark_test_complete(node2, node2.sent[i])
+        assert sched.tests_finished
+        assert node1.stolen == []
+        assert node2.stolen == []
+
+    def test_stealing(self, pytester: pytest.Pytester) -> None:
+        config = pytester.parseconfig("--tx=2*popen")
+        sched = WorkStealingScheduling(config)
+        sched.add_node(MockNode())
+        sched.add_node(MockNode())
+        node1, node2 = sched.nodes
+        collection = [f"test_workstealing.py::test_{i}" for i in range(16)]
+        sched.add_node_collection(node1, collection)
+        sched.add_node_collection(node2, collection)
+        assert sched.collection_is_completed
+        sched.schedule()
+        assert node1.sent == list(range(0, 8))
+        assert node2.sent == list(range(8, 16))
+        for i in range(8):
+            sched.mark_test_complete(node1, node1.sent[i])
+        assert node2.stolen == list(range(12, 16))
+        sched.remove_pending_tests_from_node(node2, node2.stolen)
+        for i in range(4):
+            sched.mark_test_complete(node2, node2.sent[i])
+        assert node1.stolen == [14, 15]
+        sched.remove_pending_tests_from_node(node1, node1.stolen)
+        sched.mark_test_complete(node1, 12)
+        sched.mark_test_complete(node2, 14)
+        assert node2.stolen == list(range(12, 16))
+        assert node1.stolen == [14, 15]
+        assert sched.tests_finished
+
+    def test_steal_on_add_node(self, pytester: pytest.Pytester) -> None:
+        node = MockNode()
+        config = pytester.parseconfig("--tx=popen")
+        sched = WorkStealingScheduling(config)
+        sched.add_node(node)
+        collection = [f"test_workstealing.py::test_{i}" for i in range(5)]
+        sched.add_node_collection(node, collection)
+        assert sched.collection_is_completed
+        sched.schedule()
+        assert not sched.pending
+        sched.mark_test_complete(node, 0)
+        node2 = MockNode()
+        sched.add_node(node2)
+        sched.add_node_collection(node2, collection)
+        assert sched.collection_is_completed
+        sched.schedule()
+        assert node.stolen == [3, 4]
+        sched.remove_pending_tests_from_node(node, node.stolen)
+        sched.mark_test_complete(node, 1)
+        sched.mark_test_complete(node2, 3)
+        assert sched.tests_finished
+        assert node2.stolen == []
+
+    def test_schedule_fewer_tests_than_nodes(self, pytester: pytest.Pytester) -> None:
+        config = pytester.parseconfig("--tx=3*popen")
+        sched = WorkStealingScheduling(config)
+        sched.add_node(MockNode())
+        sched.add_node(MockNode())
+        sched.add_node(MockNode())
+        node1, node2, node3 = sched.nodes
+        col = ["xyz"] * 2
+        sched.add_node_collection(node1, col)
+        sched.add_node_collection(node2, col)
+        sched.add_node_collection(node3, col)
+        sched.schedule()
+        assert node1.sent == []
+        assert node1.stolen == []
+        assert node2.sent == [0]
+        assert node2.stolen == []
+        assert node3.sent == [1]
+        assert node3.stolen == []
+        assert not sched.pending
+        assert sched.tests_finished
+
+    def test_schedule_fewer_than_two_tests_per_node(
+        self, pytester: pytest.Pytester
+    ) -> None:
+        config = pytester.parseconfig("--tx=3*popen")
+        sched = WorkStealingScheduling(config)
+        sched.add_node(MockNode())
+        sched.add_node(MockNode())
+        sched.add_node(MockNode())
+        node1, node2, node3 = sched.nodes
+        col = ["xyz"] * 5
+        sched.add_node_collection(node1, col)
+        sched.add_node_collection(node2, col)
+        sched.add_node_collection(node3, col)
+        sched.schedule()
+        assert node1.sent == [0]
+        assert node2.sent == [1, 2]
+        assert node3.sent == [3, 4]
+        assert not sched.pending
+        assert not sched.tests_finished
+        sched.mark_test_complete(node1, node1.sent[0])
+        sched.mark_test_complete(node2, node2.sent[0])
+        sched.mark_test_complete(node3, node3.sent[0])
+        sched.mark_test_complete(node3, node3.sent[1])
+        assert sched.tests_finished
+        assert node1.stolen == []
+        assert node2.stolen == []
+        assert node3.stolen == []
+
+    def test_add_remove_node(self, pytester: pytest.Pytester) -> None:
+        node = MockNode()
+        config = pytester.parseconfig("--tx=popen")
+        sched = WorkStealingScheduling(config)
+        sched.add_node(node)
+        collection = ["test_file.py::test_func"]
+        sched.add_node_collection(node, collection)
+        assert sched.collection_is_completed
+        sched.schedule()
+        assert not sched.pending
+        crashitem = sched.remove_node(node)
+        assert crashitem == collection[0]
+
+    def test_different_tests_collected(self, pytester: pytest.Pytester) -> None:
+        class CollectHook:
+            def __init__(self):
+                self.reports = []
+
+            def pytest_collectreport(self, report):
+                self.reports.append(report)
+
+        collect_hook = CollectHook()
+        config = pytester.parseconfig("--tx=2*popen")
+        config.pluginmanager.register(collect_hook, "collect_hook")
+        node1 = MockNode()
+        node2 = MockNode()
+        sched = WorkStealingScheduling(config)
+        sched.add_node(node1)
+        sched.add_node(node2)
+        sched.add_node_collection(node1, ["a.py::test_1"])
+        sched.add_node_collection(node2, ["a.py::test_2"])
+        sched.schedule()
+        assert len(collect_hook.reports) == 1
+        rep = collect_hook.reports[0]
+        assert "Different tests were collected between" in rep.longrepr
+
+
 class TestDistReporter:
     @pytest.mark.xfail
     def test_rsync_printing(self, pytester: pytest.Pytester, linecomp) -> None:

testing/test_plugin.py

@@ -54,6 +54,8 @@ def test_auto_detect_cpus(
 ) -> None:
     from xdist.plugin import pytest_cmdline_main as check_options
 
+    monkeypatch.delenv("PYTEST_XDIST_AUTO_NUM_WORKERS", raising=False)
+
     with suppress(ImportError):
         import psutil
 
@@ -101,6 +103,7 @@ def test_auto_detect_cpus_psutil(
 
     psutil = pytest.importorskip("psutil")
 
+    monkeypatch.delenv("PYTEST_XDIST_AUTO_NUM_WORKERS", raising=False)
     monkeypatch.setattr(psutil, "cpu_count", lambda logical=True: 84 if logical else 42)
 
     config = pytester.parseconfigure("-nauto")
@@ -117,6 +120,8 @@ def test_auto_detect_cpus_os(
 ) -> None:
     from xdist.plugin import pytest_cmdline_main as check_options
 
+    monkeypatch.delenv("PYTEST_XDIST_AUTO_NUM_WORKERS", raising=False)
+
     config = pytester.parseconfigure("-nauto")
     check_options(config)
     assert config.getoption("numprocesses") == 3
@@ -178,6 +183,8 @@ def test_hook_auto_num_workers_none(
     # but we document it so let's test it.
     from xdist.plugin import pytest_cmdline_main as check_options
 
+    monkeypatch.delenv("PYTEST_XDIST_AUTO_NUM_WORKERS", raising=False)
+
     pytester.makeconftest(
         """
         def pytest_xdist_auto_num_workers():

testing/test_remote.py

@@ -220,6 +220,91 @@ class TestWorkerInteractor:
         ev = worker.popevent()
         assert ev.name == "errordown"
 
+    def test_steal_work(self, worker: WorkerSetup, unserialize_report) -> None:
+        worker.pytester.makepyfile(
+            """
+            import time
+            def test_func(): time.sleep(1)
+            def test_func2(): pass
+            def test_func3(): pass
+            def test_func4(): pass
+        """
+        )
+        worker.setup()
+        ev = worker.popevent("collectionfinish")
+        ids = ev.kwargs["ids"]
+        assert len(ids) == 4
+        worker.sendcommand("runtests_all")
+
+        # wait for test_func setup
+        ev = worker.popevent("testreport")
+        rep = unserialize_report(ev.kwargs["data"])
+        assert rep.nodeid.endswith("::test_func")
+        assert rep.when == "setup"
+
+        worker.sendcommand("steal", indices=[1, 2])
+        ev = worker.popevent("unscheduled")
+        assert ev.kwargs["indices"] == [2]
+
+        reports = [
+            ("test_func", "call"),
+            ("test_func", "teardown"),
+            ("test_func2", "setup"),
+            ("test_func2", "call"),
+            ("test_func2", "teardown"),
+        ]
+
+        for func, when in reports:
+            ev = worker.popevent("testreport")
+            rep = unserialize_report(ev.kwargs["data"])
+            assert rep.nodeid.endswith(f"::{func}")
+            assert rep.when == when
+
+        worker.sendcommand("shutdown")
+
+        for when in ["setup", "call", "teardown"]:
+            ev = worker.popevent("testreport")
+            rep = unserialize_report(ev.kwargs["data"])
+            assert rep.nodeid.endswith("::test_func4")
+            assert rep.when == when
+
+        ev = worker.popevent("workerfinished")
+        assert "workeroutput" in ev.kwargs
+
+    def test_steal_empty_queue(self, worker: WorkerSetup, unserialize_report) -> None:
+        worker.pytester.makepyfile(
+            """
+            def test_func(): pass
+            def test_func2(): pass
+        """
+        )
+        worker.setup()
+        ev = worker.popevent("collectionfinish")
+        ids = ev.kwargs["ids"]
+        assert len(ids) == 2
+        worker.sendcommand("runtests_all")
+
+        for when in ["setup", "call", "teardown"]:
+            ev = worker.popevent("testreport")
+            rep = unserialize_report(ev.kwargs["data"])
+            assert rep.nodeid.endswith("::test_func")
+            assert rep.when == when
+
+        worker.sendcommand("steal", indices=[0, 1])
+        ev = worker.popevent("unscheduled")
+        assert ev.kwargs["indices"] == []
+
+        worker.sendcommand("shutdown")
+
+        for when in ["setup", "call", "teardown"]:
+            ev = worker.popevent("testreport")
+            rep = unserialize_report(ev.kwargs["data"])
+            assert rep.nodeid.endswith("::test_func2")
+            assert rep.when == when
+
+        ev = worker.popevent("workerfinished")
+        assert "workeroutput" in ev.kwargs
+
 
 def test_remote_env_vars(pytester: pytest.Pytester) -> None:
     pytester.makepyfile(