try this

src/levanter/data/loader.py

@@ -2,7 +2,7 @@
 import logging
 import time
 from collections import defaultdict
-from typing import Iterable, Iterator, Optional, Tuple, TypeVar
+from typing import AsyncIterator, Callable, Iterable, Iterator, Optional, Tuple, TypeVar
 
 import jax
 from jax import Array
@@ -20,7 +20,7 @@
 from levanter.data.dataset import AsyncDataset
 from levanter.data.utils import batched
 from levanter.shapes import NamedShapeSpec, ShapeSpec, to_raw_shape
-from levanter.utils.background_iterable import BackgroundIterable
+from levanter.utils.background_iterable import BackgroundIterator
 from levanter.utils.jax_utils import local_cpu_mesh
 from levanter.utils.thread_utils import AsyncIteratorWrapper, blocking_wait
 
@@ -113,10 +113,11 @@ def __init__(self, data_loader: DataLoader, start_from_batch: Optional[int] = No
             self.mapping = hax.partitioning.current_thread_local_mapping()
 
         buffered_batches = self.dl.max_buffered_batches
+        self._batches: Iterator[Ex]
         if buffered_batches == 0:
             self._batches = AsyncIteratorWrapper(self._produce_batches())
         else:
-            self._batches = iter(BackgroundIterable(self._produce_batches, max_capacity=buffered_batches))
+            self._batches = _JaxCpuBackgroundIterator(self._produce_batches, max_capacity=buffered_batches)
 
     def __next__(self):
         time_start = time.time()
@@ -246,25 +247,26 @@ def _pspec_for(self, shape_spec: ShapeSpec | NamedShapeSpec) -> PartitionSpec:
             return hax.partitioning.pspec_for_axis(shape_spec.shape, self.dl.axis_resources)  # type: ignore
 
 
-def _abstractify(x):
-    def _abstractify_array(x):
-        if isinstance(x, jax.numpy.ndarray):
-            return ShapeSpec(x.shape, x.dtype)
-        elif isinstance(x, hax.NamedArray):
-            return NamedShapeSpec(x.axes, x.dtype)
-
-        return x
-
-    return hax.tree_util.tree_map(_abstractify_array, x)
-
-
 def _batchified_shape(Batch, leaf: hax.NamedArray | Array) -> ShapeSpec | NamedShapeSpec:
     if is_named_array(leaf):
         return NamedShapeSpec((Batch,) + leaf.axes, leaf.dtype)
     else:
         return ShapeSpec((Batch.size,) + leaf.shape, leaf.dtype)
 
 
+class _JaxCpuBackgroundIterator(BackgroundIterator[Ex]):
+    """
+    We want the thread to only use the CPU device.
+    """
+
+    def __init__(self, producer_fn: Callable[[], Iterator[Ex] | AsyncIterator[Ex]], max_capacity: Optional[int]):
+        super().__init__(producer_fn, max_capacity)
+
+    def _fill_queue_with_batches(self):
+        with local_cpu_mesh():
+            super()._fill_queue_with_batches()
+
+
 @functools.partial(jax.jit, static_argnums=(0,))
 def _stack_tree(batch_name, individual_datums):
     def _stack_leaves_unchecked(*leaves):