[Performance] Accelerate slice sampler on GPU

torchrl/data/replay_buffers/samplers.py

@@ -24,7 +24,7 @@
 
 from torchrl._utils import _replace_last, logger
 from torchrl.data.replay_buffers.storages import Storage, StorageEnsemble, TensorStorage
-from torchrl.data.replay_buffers.utils import _is_int, unravel_index
+from torchrl.data.replay_buffers.utils import _auto_device, _is_int, unravel_index
 
 try:
     from torchrl._torchrl import (
@@ -726,6 +726,10 @@ class SliceSampler(Sampler):
     This class samples sub-trajectories with replacement. For a version without
     replacement, see :class:`~torchrl.data.replay_buffers.samplers.SliceSamplerWithoutReplacement`.
 
+    .. note:: `SliceSampler` can be slow to retrieve the trajectory indices. To accelerate
+        its execution, prefer using `end_key` over `traj_key`, and consider the following
+        keyword arguments: :attr:`compile`, :attr:`cache_values` and :attr:`use_gpu`.
+
     Keyword Args:
         num_slices (int): the number of slices to be sampled. The batch-size
             must be greater or equal to the ``num_slices`` argument. Exclusive
@@ -796,6 +800,10 @@ class SliceSampler(Sampler):
             that at least `slice_len - i` samples will be gathered for each sampled trajectory.
             Using tuples allows a fine grained control over the span on the left (beginning
             of the stored trajectory) and on the right (end of the stored trajectory).
+        use_gpu (bool or torch.device): if ``True`` (or is a device is passed), an accelerator
+            will be used to retrieve the indices of the trajectory starts. This can significanlty
+            accelerate the sampling when the buffer content is large.
+            Defaults to ``False``.
 
     .. note:: To recover the trajectory splits in the storage,
         :class:`~torchrl.data.replay_buffers.samplers.SliceSampler` will first
@@ -985,6 +993,7 @@ def __init__(
         strict_length: bool = True,
         compile: bool | dict = False,
         span: bool | int | Tuple[bool | int, bool | int] = False,
+        use_gpu: torch.device | bool = False,
     ):
         self.num_slices = num_slices
         self.slice_len = slice_len
@@ -995,6 +1004,14 @@ def __init__(
         self._fetch_traj = True
         self.strict_length = strict_length
         self._cache = {}
+        self.use_gpu = bool(use_gpu)
+        self._gpu_device = (
+            None
+            if not self.use_gpu
+            else torch.device(use_gpu)
+            if not isinstance(use_gpu, bool)
+            else _auto_device()
+        )
 
         if isinstance(span, (bool, int)):
             span = (span, span)
@@ -1086,9 +1103,8 @@ def __repr__(self):
             f"strict_length={self.strict_length})"
         )
 
-    @classmethod
     def _find_start_stop_traj(
-        cls, *, trajectory=None, end=None, at_capacity: bool, cursor=None
+        self, *, trajectory=None, end=None, at_capacity: bool, cursor=None
     ):
         if trajectory is not None:
             # slower
@@ -1141,10 +1157,15 @@ def _find_start_stop_traj(
             raise RuntimeError(
                 "Expected the end-of-trajectory signal to be at least 1-dimensional."
             )
-        return cls._end_to_start_stop(length=length, end=end)
-
-    @staticmethod
-    def _end_to_start_stop(end, length):
+        return self._end_to_start_stop(length=length, end=end)
+
+    def _end_to_start_stop(self, end, length):
+        device = None
+        if self.use_gpu:
+            gpu_device = self._gpu_device
+            if end.device != gpu_device:
+                device = end.device
+                end = end.to(self._gpu_device)
         # Using transpose ensures the start and stop are sorted the same way
         stop_idx = end.transpose(0, -1).nonzero()
         stop_idx[:, [0, -1]] = stop_idx[:, [-1, 0]].clone()
@@ -1171,6 +1192,8 @@ def _end_to_start_stop(end, length):
             pass
         lengths = stop_idx[:, 0] - start_idx[:, 0] + 1
         lengths[lengths <= 0] = lengths[lengths <= 0] + length
+        if device is not None:
+            return start_idx.to(device), stop_idx.to(device), lengths.to(device)
         return start_idx, stop_idx, lengths
 
     def _start_to_end(self, st: torch.Tensor, length: int):
@@ -1547,6 +1570,10 @@ class SliceSamplerWithoutReplacement(SliceSampler, SamplerWithoutReplacement):
     the sampler, and continuous sampling without replacement is currently not
     allowed.
 
+    .. note:: `SliceSamplerWithoutReplacement` can be slow to retrieve the trajectory indices. To accelerate
+        its execution, prefer using `end_key` over `traj_key`, and consider the following
+        keyword arguments: :attr:`compile`, :attr:`cache_values` and :attr:`use_gpu`.
+
     Keyword Args:
         drop_last (bool, optional): if ``True``, the last incomplete sample (if any) will be dropped.
             If ``False``, this last sample will be kept.
@@ -1589,6 +1616,10 @@ class SliceSamplerWithoutReplacement(SliceSampler, SamplerWithoutReplacement):
             the :meth:`~sample` method will be compiled with :func:`~torch.compile`.
             Keyword arguments can also be passed to torch.compile with this arg.
             Defaults to ``False``.
+        use_gpu (bool or torch.device): if ``True`` (or is a device is passed), an accelerator
+            will be used to retrieve the indices of the trajectory starts. This can significanlty
+            accelerate the sampling when the buffer content is large.
+            Defaults to ``False``.
 
     .. note:: To recover the trajectory splits in the storage,
         :class:`~torchrl.data.replay_buffers.samplers.SliceSamplerWithoutReplacement` will first
@@ -1693,7 +1724,6 @@ class SliceSamplerWithoutReplacement(SliceSampler, SamplerWithoutReplacement):
         tensor([[0., 0., 0., 0., 0.],
                 [1., 1., 1., 1., 1.]])
 
-
     """
 
     def __init__(
@@ -1710,6 +1740,7 @@ def __init__(
         strict_length: bool = True,
         shuffle: bool = True,
         compile: bool | dict = False,
+        use_gpu: bool | torch.device = False,
     ):
         SliceSampler.__init__(
             self,
@@ -1723,6 +1754,7 @@ def __init__(
             ends=ends,
             trajectories=trajectories,
             compile=compile,
+            use_gpu=use_gpu,
         )
         SamplerWithoutReplacement.__init__(self, drop_last=drop_last, shuffle=shuffle)
 

torchrl/data/replay_buffers/utils.py

@@ -1034,3 +1034,11 @@ def tree_iter(pytree):  # noqa: F811
 def tree_iter(pytree):  # noqa: F811
     """A version-compatible wrapper around tree_iter."""
     yield from torch.utils._pytree.tree_iter(pytree)
+
+
+def _auto_device() -> torch.device:
+    if torch.cuda.is_available():
+        return torch.device("cuda:0")
+    elif torch.mps.is_available():
+        return torch.device("mps:0")
+    return torch.device("cpu")