Fix consecutive same sampler selection in round robin sampler with nu…

…m_workers>1

Signed-off-by: Piotr Żelasko <petezor@gmail.com>

lhotse/dataset/sampling/round_robin.py

@@ -3,6 +3,7 @@
 from typing import Any, Callable, Dict, List, Optional, Tuple, Union
 
 import numpy as np
+import torch
 
 from lhotse import CutSet
 from lhotse.cut import Cut
@@ -171,7 +172,18 @@ def __iter__(self):
         if self._just_restored_state:
             return self
         self._nondepleted_samplers_indices = list(range(len(self.samplers)))
+        # In case this sampler lives in the dataloading worker subprocess,
+        # set the starting index to a different value on each dataloading worker.
+        # This helps avoid situations where the round robin sampler chooses
+        # the same underlying sampler for N consecutive mini-batches, where N = num_workers (>1).
         self._cur_sampler_idx = 0
+        self._num_dl_workers = 1
+        worker_info = torch.utils.data.get_worker_info()
+        if worker_info is not None:
+            self._cur_sampler_idx = worker_info.id % len(
+                self._nondepleted_samplers_indices
+            )
+            self._num_dl_workers = worker_info.num_workers
         return self
 
     def _next_batch(self) -> Union[CutSet, Tuple[CutSet]]:
@@ -202,9 +214,9 @@ def _set_next_idx(self) -> None:
             p = [x / sum(p) for x in p]
             self._cur_sampler_idx = self.rng.choice(N, size=1, replace=False, p=p)[0]
         else:
-            self._cur_sampler_idx = (self._cur_sampler_idx + 1) % len(
-                self._nondepleted_samplers_indices
-            )
+            self._cur_sampler_idx = (
+                self._cur_sampler_idx + self._num_dl_workers
+            ) % len(self._nondepleted_samplers_indices)
 
     def set_epoch(self, epoch: int) -> None:
         """

test/dataset/sampling/test_sampling.py

@@ -1,4 +1,3 @@
-import math
 import random
 import re
 from collections import Counter
@@ -856,6 +855,27 @@ def test_round_robin_sampler(randomize):
     # ... and so on
 
 
+@pytest.mark.parametrize("num_workers", [0, 1, 2, 3])
+def test_nonrandomized_round_robin_sampler_keeps_round_robin_property_in_iterable_dataset(
+    num_workers,
+):
+    cuts1 = DummyManifest(CutSet, begin_id=0, end_id=100)
+    cuts2 = DummyManifest(CutSet, begin_id=500, end_id=600)
+    cuts3 = DummyManifest(CutSet, begin_id=1000, end_id=1100)
+    sampler = RoundRobinSampler(
+        SimpleCutSampler(cuts1, max_cuts=1, shuffle=False),
+        SimpleCutSampler(cuts2, max_cuts=2, shuffle=False),
+        SimpleCutSampler(cuts3, max_cuts=3, shuffle=False),
+    )
+    dloader = DataLoader(
+        dataset=IterableDatasetWrapper(IdentityDataset(), sampler),
+        batch_size=None,
+        num_workers=num_workers,
+    )
+    lens = [len(b) for idx, b in zip(range(15), dloader)]
+    assert lens == [1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3]
+
+
 @pytest.mark.parametrize("sampler_cls", [SimpleCutSampler, DynamicCutSampler])
 def test_single_cut_sampler_drop_last(sampler_cls):
     # The dummy cuts have a duration of 1 second each