Allow threads to exit when CheckpointManager is freed

test/spmd/test_xla_distributed_checkpoint.py

@@ -4,6 +4,7 @@
 import tempfile
 import unittest
 import test_xla_sharding_base
+import threading
 
 import torch
 import torch.distributed as dist
@@ -413,7 +414,35 @@ def test_manager_max_to_keep(self, tmpdir):
     self.assertEqual(set(chkpt_mgr.all_steps()), {20, 10})
 
   @run_with_tmpdir
-  def test_manager_async_checkpoint(self, tmpdir):
+  def test_manager_async(self, tmpdir):
+    chkpt_mgr = CheckpointManager(tmpdir, save_period=10)
+    state_dict = self._get_sharded_model().state_dict()
+
+    # Patch the manager's save method to block until this thread signals.
+    cond = threading.Condition()
+    old_save = chkpt_mgr.save
+
+    def patched_save(*args, **kwargs):
+      cond.wait()
+      old_save(*args, **kwargs)
+
+    with unittest.mock.patch.object(chkpt_mgr, 'save', patched_save):
+      chkpt_mgr.save_async(10, state_dict)
+
+    # No new steps should be tracked immediately after calling save_async
+    self.assertEqual(chkpt_mgr.all_steps(), [])
+
+    # Trigger the actual checkpoint in the background thread and wait for
+    # completion.
+    with cond:
+      cond.notify()
+    chkpt_mgr.join()
+
+    # The manager should track all steps which were asynchronously saved.
+    self.assertEqual(set(chkpt_mgr.all_steps()), {10})
+
+  @run_with_tmpdir
+  def test_manager_async_step_tracking(self, tmpdir):
     chkpt_mgr = CheckpointManager(tmpdir, save_period=10)
     state_dict = self._get_sharded_model().state_dict()
 
@@ -430,12 +459,10 @@ def test_manager_async_checkpoint(self, tmpdir):
       self.assertTrue(chkpt_mgr.save_async(step, state_dict))
       saved.add(step)
 
-    # Delete the checkpoint manager to block this thread until all pending
-    # async checkpoints are complete.
-    del chkpt_mgr
+    # Join to allow pending async checkpoints to complete
+    chkpt_mgr.join()
 
     # The manager should track all steps which were asynchronously saved.
-    chkpt_mgr = CheckpointManager(tmpdir, save_period=10)
     self.assertEqual(set(chkpt_mgr.all_steps()), saved)
 
     # Load a checkpoint into a new state_dict

torch_xla/experimental/distributed_checkpoint/manager.py

@@ -98,7 +98,8 @@ def __init__(self,
                path: str,
                save_period: int,
                max_to_keep: Optional[int] = 0,
-               async_queue_size: Optional[int] = 1):
+               async_queue_size: Optional[int] = 1,
+               process_group: dist.ProcessGroup = None):
     """
     Create a checkpoint manager that reads and writes checkpoints into
     the provided directory.
@@ -117,6 +118,9 @@ def __init__(self,
             network issues which slow down the active checkpoint.
             Default: 1, which only allows a single async checkpoint to be
             pending at a time.
+      process_group: The process group to use when coordinating the checkpoint.
+            Default: None, in which case a subgroup of the default process
+            group will be created.
     """
     assert dist.is_initialized(), "A process group is required."
     assert save_period > 0, "save_period must be positive"
@@ -128,13 +132,15 @@ def __init__(self,
     self.max_to_keep = max_to_keep
 
     self._tracked_chkpts = self._load_tracked_chkpts()
-
     self._async_queue = queue.Queue(maxsize=async_queue_size)
-    self._chkpt_thread = threading.Thread(target=self._async_worker, daemon=True)
+    self._alive = threading.Event()
+    self._alive.set()
+    self._chkpt_thread = threading.Thread(
+        target=self._async_worker, daemon=True)
     self._chkpt_thread.start()
 
-    # Create a CPU process group to coordinate the checkpoint.
-    self.pg = dist.new_group(backend='gloo')
+    # Create a new group if none is provided
+    self.pg = process_group or dist.new_group()
 
   def _load_tracked_chkpts(self) -> Deque[_CheckpointMetadata]:
     """
@@ -155,14 +161,19 @@ def _load_tracked_chkpts(self) -> Deque[_CheckpointMetadata]:
     return deque(sorted(all_chkpts, key=lambda m: m.ts))
 
   def __del__(self):
-    # Ensure pending checkpoints are finished
-    self._async_queue.join()
+    self._alive.clear()
+    # Send a sentinel value to tell the worker to exit, and wait for pending
+    # checkpoints to complete.
+    self._async_queue.put((None, None))
+    self._chkpt_thread.join()
 
   def _async_worker(self):
-    while True:
+    while self._alive.is_set():
       try:
-        step, state_dict = self._async_queue.get()
-        self.save(step, state_dict, force=True)
+        item = self._async_queue.get()
+        if item:
+          step, state_dict = item
+          self.save(step, state_dict, force=True)
       except:
         traceback.print_exc()
       finally:
@@ -284,3 +295,7 @@ def all_steps(self) -> List[int]:
     List all steps tracked by the CheckpointManager.
     """
     return sorted(x.step for x in self._tracked_chkpts)
+
+  def join(self):
+    """ Wait for all pending async checkpoints to complete. """
+    self._async_queue.join()