avoid unnecessary workers with sequential `CombinedLoader ` (#17639)

Co-authored-by: Carlos Mocholí <carlossmocholi@gmail.com>
Co-authored-by: Adrian Wälchli <aedu.waelchli@gmail.com>

src/lightning/pytorch/CHANGELOG.md

@@ -108,6 +108,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Fixed
 
+- `CombinedLoader` only starts DataLoader workers when necessary when operating in sequential mode ([#17639](https://github.com/Lightning-AI/lightning/pull/17639))
+
+
 - Fixed issue where `Model.load_from_checkpoint("checkpoint.ckpt", map_location=map_location)` would always return model on CPU ([#17308](https://github.com/Lightning-AI/lightning/pull/17308))
 
 

src/lightning/pytorch/utilities/combined_loader.py

@@ -108,7 +108,7 @@ class _Sequential(_ModeIterator[Tuple[Any, int, int]]):
         self._limits = limits
 
     def __next__(self) -> Tuple[Any, int, int]:
-        n = len(self.iterators)
+        n = len(self.iterables)
         if n == 0 or self._iterator_idx >= n:
             raise StopIteration
 
@@ -120,7 +120,7 @@ class _Sequential(_ModeIterator[Tuple[Any, int, int]]):
                     raise StopIteration
 
         try:
-            out = next(self.iterators[self._iterator_idx])
+            out = next(self.iterators[0])
             index = self._idx
             self._idx += 1
             # batch, batch_idx, dataloader_idx
@@ -131,9 +131,9 @@ class _Sequential(_ModeIterator[Tuple[Any, int, int]]):
             return self.__next__()
 
     def __iter__(self) -> Self:
-        super().__iter__()
         self._iterator_idx = 0
         self._idx = 0
+        self._load_current_iterator()
         return self
 
     def reset(self) -> None:
@@ -141,9 +141,18 @@ class _Sequential(_ModeIterator[Tuple[Any, int, int]]):
         self._iterator_idx = 0
         self._idx = 0
 
+    def _load_current_iterator(self) -> None:
+        # Load a single DataLoader, prevents multiple sets of workers from starting unnecessarily
+        if self._iterator_idx < len(self.iterables):
+            self.iterators = [iter(self.iterables[self._iterator_idx])]
+        else:
+            # No more iterables to step through, return an empty list
+            self.iterators = []
+
     def _use_next_iterator(self) -> None:
         self._iterator_idx += 1
         self._idx = 0
+        self._load_current_iterator()
 
 
 class _MaxSize(_ModeIterator[List]):

tests/tests_pytorch/loops/test_loops.py

@@ -844,8 +844,7 @@ def test_workers_are_shutdown(tmpdir, should_fail, persistent_workers):
             # iterable check
             0,
             # epoch ends
-            1,
-            # teardown
+            0,
             1,
         ]
     else:
@@ -855,9 +854,8 @@ def test_workers_are_shutdown(tmpdir, should_fail, persistent_workers):
             # iterable check
             0,
             # epoch ends
+            0,
             1,
             2,
-            # teardown
-            3,
         ]
     assert val_dataloader.shutdown_workers_epochs == expected

tests/tests_pytorch/utilities/test_combined_loader.py

@@ -305,6 +305,40 @@ def test_combined_loader_sequence_iterable_dataset(mode, use_multiple_dataloader
     assert idx == expected - 1
 
 
+@pytest.mark.parametrize("mode", ["min_size", "max_size_cycle", "max_size", "sequential"])
+def test_combined_loader_simultaneous_workers(mode):
+    """Test `CombinedLoader` to check how it initializes dataloader workers."""
+
+    class TestDataLoader(DataLoader):
+        def __init__(self, *args, **kwargs):
+            super().__init__(*args, **kwargs)
+            self.workers_active = False
+
+        def _get_iterator(self):
+            self.workers_active = True
+            return super()._get_iterator()
+
+        def _shutdown_workers(self):
+            self.workers_active = False
+            super()._shutdown_workers()
+
+    loaders = [
+        TestDataLoader(range(10), batch_size=2, num_workers=0),
+        TestDataLoader(range(20), batch_size=2, num_workers=0),
+    ]
+    combined_loader = CombinedLoader(loaders, mode)
+    # Start the dataloader
+    _ = iter(combined_loader)
+
+    workers_active = []
+    for loader in loaders:
+        workers_active.append(loader.workers_active)
+
+    # Sequential only starts the first dataloader, other modes start both
+    expected = [True, False] if mode == "sequential" else [True, True]
+    assert workers_active == expected
+
+
 @pytest.mark.parametrize(
     ("limits", "expected"),
     [