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Internal #1143

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2 changes: 1 addition & 1 deletion grain/_src/python/dataset/transformations/BUILD
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Original file line number Diff line number Diff line change
Expand Up @@ -53,7 +53,7 @@ py_test(

py_test(
name = "prefetch_test",
timeout = "long",
timeout = "eternal",
srcs = ["prefetch_test.py"],
shard_count = 50,
srcs_version = "PY3",
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23 changes: 15 additions & 8 deletions grain/_src/python/dataset/transformations/prefetch.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1017,8 +1017,8 @@ def __str__(self) -> str:

def multithread_prefetch(
ds: dataset.IterDataset[T],
num_threads: int,
buffer_size: int,
num_threads: int = 0,
buffer_size: int = 1,
sequential_slice: bool = False,
) -> dataset.IterDataset[T]:
"""Uses a pool of threads to prefetch elements ahead of time.
Expand All @@ -1043,14 +1043,17 @@ def multithread_prefetch(
if num_threads == 0:
return ds

_validate_no_double_prefetch(ds)
dataset_options = _get_dataset_options(ds)

shards = []
for i in range(num_threads):
worker_ds = copy.deepcopy(ds)
_set_slice_iter_dataset(
worker_ds, slice(i, None, num_threads), sequential_slice
)
if num_threads == 1:
worker_ds = ds
else:
worker_ds = copy.deepcopy(ds)
_set_slice_iter_dataset(
worker_ds, slice(i, None, num_threads), sequential_slice
)
shards.append(
_MpContextIterDataset(
worker_ds,
Expand All @@ -1061,6 +1064,10 @@ def multithread_prefetch(
)
)

return interleave.InterleaveIterDataset(
ds = interleave.InterleaveIterDataset(
shards, cycle_length=num_threads, iter_buffer_size=buffer_size
)
# Apply options from parent dataset because interleave dataset does not
# propagate options.
ds = dataset.WithOptionsIterDataset(ds, dataset_options)
return ds
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