Parallel sharded data -> cupynumeric load

Why this skill exists. cupynumeric mirrors NumPy's array API, including cupynumeric.load for a single .npy file. Beyond that, file loading lives in Legate, not cupynumeric:

This skill shows the canonical way to fill the gap in the last row: write a Legate Python task that calls the third-party reader the format needs (h5py, pyarrow, np.memmap, ...) inside the task body, and let Legate distribute the reads across GPUs / nodes. For the formats with a built-in loader, prefer it unless you need a custom in-task body (mmap-based loader, format-specific decoder, sidecar metadata, partial / sharded reads).

Canonical pattern: manual partition + manual task launch, sized to the machine, not the files. Only axis 0 is sharded; trailing axes ride along inside each tile. Per-shard row counts may differ across files (only dtype and trailing axes must match); the launch fills every available processor regardless of how many files there are.

.npy is the worked example because the header carries shape and dtype on disk, but the skeleton applies to any format with cheap range/slice reads (raw binary, HDF5, Parquet/Arrow — see "Other formats" below). Reference implementation: [assets/examples/parallelnpyload.py](assets/examples/parallelnpyload.py).

Data layout assumption

This skill is purely about loading — it assumes the data is already laid out on a shared filesystem in some predictable, indexable way. Producing those files is out of scope (the example ships a write subcommand for convenience, but real users bring their own).

The worked example assumes one specific layout:

• A directory containing files named shard0000.npy, shard0001.npy,

... in a contiguous integer sequence (zero-padded width 4).

• All shards share the same dtype and the same trailing axes

(shape[1:]); axis 0 (rows per shard) may differ across files — the recipe builds a cumulative row-offset table and reads each file's overlapping slice from inside the leaf task.

• The directory is visible to every rank (shared filesystem for

multi-node runs).

The example's discoverlayout() prints what it found and hard-fails with a descriptive error when the layout is wrong (missing directory, no shards, mismatched dtype / trailing axes, or a hole in the contiguous shardNNNN.npy sequence).

If your data lives in a different layout — fixed-stride raw binary, an HDF5 file with one dataset per shard, a directory tree, ... — only the glob pattern, the per-file reader (step 4 below), and the metadata discovery (step 1 below) change. The partitioning and launch machinery is layout-agnostic.

When to use

See the format table above for the routing decision (built-in loader vs. this skill). Beyond that, two additional cues that this skill is the right fit:

• Replacing sequential np.concatenate([read(f) for f in files]) with

parallel per-GPU reads.

• Demonstrating how a user-defined Legate Python task writes into a

cupynumeric output array via a manual launch.

Examples

Paths below are written relative to this skill's directory (the script ships at assets/examples/parallelnpyload.py). Adjust the prefix to match wherever your skill is installed (e.g. skills/cupynumeric-parallel-data-load/assets/... if the skill lives under a top-level skills/ directory).

# Single-node, 4 GPUs.
legate --gpus 4 --fbmem 4000 --min-gpu-chunk 1 \
    assets/examples/parallel_npy_load.py \
    read --shard-dir /shared/scratch/demo

# Multi-node, 2 nodes x 4 GPUs (slurm), shared filesystem at --shard-dir.
# Generate the shards once on rank 0, then re-run `read` at any scale.
legate --launcher srun --nodes 2 --cpus 1 \
    assets/examples/parallel_npy_load.py \
    write --shard-dir /shared/scratch/demo

legate --launcher srun --nodes 2 --ranks-per-node 4 \
    --gpus 4 --fbmem 4000 --min-gpu-chunk 1 \
    assets/examples/parallel_npy_load.py \
    read --shard-dir /shared/scratch/demo

No layout flags — the read driver walks every .npy header to recover per-file row counts, the trailing shape, and the dtype, then derives tile_rows from the available processor count.

--min-gpu-chunk 1 is only needed when the per-tile element count is below Legate's default minimum chunk size for GPU launches (e.g. the worked example's defaults — total rows split across 4 GPUs at ~1M per tile — fall below the threshold and would otherwise be folded onto a single GPU). For production-sized datasets (tens of millions of elements per tile or larger) you can drop the flag and let Legate use its default. Bumping it to a moderate value (e.g. --min-gpu-chunk 1024) is fine when each tile is large enough that per-task overhead matters more than getting every GPU a tile.

Instructions

Five steps from a .npy worked example; only step 1 (parsing the format header) and step 4 (the per-file reader inside the task body) are format-specific. The other three (allocate destination, partition, fence) are reused unchanged across formats — see "Other formats" below for the swap-points.

1. Read the metadata from every shard

Scan the directory and peek at every .npy header (mmap_mode="r" reads only the header). The header carries the per-shard shape and dtype, so the driver can recover total rows, trailing shape, and a cumulative row-offset table without ever loading the data:

paths = sorted(SHARD_DIR.glob("shard_*.npy"))

per_file_rows = []                       # rows along axis 0 per file
trailing_shape = None                    # shape[1:], must match across files
dtype = None
for p in paths:
    hdr = np.load(p, mmap_mode="r")
    if trailing_shape is None:
        trailing_shape = tuple(hdr.shape[1:])
        dtype = hdr.dtype
    elif tuple(hdr.shape[1:]) != trailing_shape or hdr.dtype != dtype:
        raise RuntimeError(
            f"{p.name}: trailing shape / dtype mismatch "
            f"({hdr.shape[1:]}/{hdr.dtype} vs {trailing_shape}/{dtype})"
        )
    per_file_rows.append(int(hdr.shape[0]))

cum_rows = np.cumsum([0] + per_file_rows, dtype=np.int64)  # length N+1
total_rows = int(cum_rows[-1])

The snippet above enforces matching dtype and trailingshape (i.e. shape[1:]) across files. Per-shard row counts may differ — the cum-rows table handles that. Production code should also verify that names form a contiguous shard0000.npy ... shardNNNN.npy sequence (omitted from the snippet for brevity; see discoverlayout() in the worked example). Discovery relies only on what the on-disk format itself exposes (the .npy header here, .shape / .dtype for HDF5, etc.); any sidecar (manifest, content hashes) is a separate verification step on top.

2. Create the cupynumeric output store from the metadata

The total array spans totalrows along axis 0; trailing axes come from trailingshape unchanged. Use cn.empty — the task overwrites every cell, zero-init would be wasted.

import cupynumeric as cn

total_shape = (total_rows,) + trailing_shape
out = cn.empty(total_shape, dtype=dtype)

3. Tile the store by processor count

The launch shape is sized to the available processors, not to the file count. Pick tilerows = ceil(totalrows / numprocessors) and partition axis 0 by that tile size. Trailing axes are not partitioned (tile spans the full extent there). The last tile is allowed to be short — that's exactly what partitionby_tiling supports — so the recipe needs no divisibility constraint.

from legate.core import TaskTarget, get_legate_runtime
from legate.core.data_interface import as_logical_array

runtime = get_legate_runtime()
machine = runtime.get_machine()
num_processors = max(
    machine.count(TaskTarget.GPU),
    machine.count(TaskTarget.OMP),
    machine.count(TaskTarget.CPU),
    1,
)

tile_rows = max(1, (total_rows + num_processors - 1) // num_processors)
tile_shape = (tile_rows,) + trailing_shape
partition = as_logical_array(out).data.partition_by_tiling(tile_shape)

num_tasks = (total_rows + tile_rows - 1) // tile_rows  # match partition tile count

4. Define the leaf task and launch it manually

PATHS and CUMROWS (the file paths and cumulative row-offset table from step 1) plus TILEROWS are populated as module globals by the driver before launching; control replication runs the driver on every rank, so every worker sees identical values.

Each task builds its consumer view first (cupy on GPU, numpy on CPU/OMP) and reads the tile's actual row count from view.shape[0] — PhysicalStore itself has no .shape attribute, so going through the view is required. It then computes its global row range from its launch coordinate and that row count, bisects cumrows for the overlapping file(s), and copies each overlapping file slice into the matching destination slice. Register CPU, OMP, and GPU variants so the same launch runs unchanged anywhere; dispatch on ctx.getvariantkind() picks the consumer matching where the OutputStore is resident (cp.fromdlpack(dst) for FBMEM, np.asarray(dst) for SYSMEM). cupy is imported inside the GPU branch only, so the task body loads on machines without cupy.

import bisect
from legate.core import TaskContext, VariantCode
from legate.core.task import OutputStore, task

@task(variants=(VariantCode.CPU, VariantCode.OMP, VariantCode.GPU))
def load_tile(ctx: TaskContext, dst: OutputStore) -> None:
    t = ctx.task_index[0]                              # tile index 0..num_tasks-1

    variant = ctx.get_variant_kind()
    if variant == VariantCode.GPU:
        import cupy as cp                              # lazy: only on GPU
        view = cp.from_dlpack(dst)
    else:
        view = np.asarray(dst)                         # zero-copy numpy view

    tile_rows_actual = view.shape[0]                   # short on the last tile
    row_start = t * TILE_ROWS                          # global axis-0 start
    row_end = row_start + tile_rows_actual

    # Find the half-open range of file indices that overlap [row_start, row_end).
    first_file = bisect.bisect_right(CUM_ROWS, row_start) - 1
    last_file = bisect.bisect_right(CUM_ROWS, row_end - 1) - 1

    for f in range(first_file, last_file + 1):
        # Intersection of tile [row_start, row_end) with file [cum[f], cum[f+1]).
        lo = max(row_start, int(CUM_ROWS[f]))
        hi = min(row_end, int(CUM_ROWS[f + 1]))
        file_lo = lo - int(CUM_ROWS[f])
        file_hi = hi - int(CUM_ROWS[f])
        dst_lo = lo - row_start
        dst_hi = hi - row_start
        chunk = np.ascontiguousarray(
            np.load(PATHS[f], mmap_mode="r")[file_lo:file_hi]
        )
        if variant == VariantCode.GPU:
            view[dst_lo:dst_hi].set(chunk)             # cudaMemcpyAsync H2D
        else:
            view[dst_lo:dst_hi] = chunk                # zero-copy numpy write

manual_task = runtime.create_manual_task(
    load_tile.library,
    load_tile.task_id,
    (num_tasks,),                                      # launch domain == tile count
)
manual_task.add_output(partition)
manual_task.execute()

Both consumers go through PhysicalStore's native producers (dlpack for cupy, __array_interface__ for np.asarray) — zero-copy views of the local tile. Bisect cost is O(log num_shards) and the inner loop typically iterates 1–2 times (tiles overlap at most a couple of files).

5. Fence and verify

get_legate_runtime().issue_execution_fence(block=True)

Hard constraints

1. All shards must share dtype and trailing axes (shape[1:]).

The recipe stacks shards along axis 0; the destination's trailing axes come from trailing_shape, which the discovery step locks to the value of the first file. Per-shard row counts (shape[0]) may freely differ — the cumulative-offset table handles them. The example rejects any shard whose dtype or trailing shape differs from the first one with a descriptive error.

1. Pick the consumer that matches the variant. cp.from_dlpack

rejects SYSMEM-resident stores; np.asarray silently returns a host view of an FBMEM-resident store you can't actually write through. Dispatch on ctx.getvariantkind() so each variant uses its own consumer — see step 4.

1. mmap views aren't always C-contiguous — wrap each per-file

slice with np.ascontiguousarray(arr[filelo:filehi]) before .set() or the numpy in-place write.

1. Multi-node: SHARD_DIR must be on a shared filesystem. Every

worker (on every rank) opens shards by path; node-local /tmp paths only work for single-node demos.

Variants

Uniform-shard fast path (one task per file)

When every shard already has the same (shape, dtype) and you happen to have numshards processors available, the cum-rows / bisect machinery is overhead. Set tilerows = shardshape[0] and numtasks = num_shards; the partition then has one tile per file and each task reads exactly one file end-to-end (no bisect, no inner loop). The driver-side switch is a one-liner:

if all(r == per_file_rows[0] for r in per_file_rows) and num_shards == num_processors:
    tile_rows = per_file_rows[0]
else:
    tile_rows = max(1, (total_rows + num_processors - 1) // num_processors)

The same load_tile task body still works in either mode — the inner loop just happens to iterate exactly once per task. There's no need for a separate task body for the fast path.

Over-decompose for better load balancing

The default tilerows = ceil(totalrows / numprocessors) gives one tile per processor. To over-decompose by a factor K (smaller tiles, more point tasks, finer-grained queueing), divide by K * numprocessors instead:

tile_rows = max(1, (total_rows + K * num_processors - 1) // (K * num_processors))

numtasks = ceil(totalrows / tilerows) then expands to roughly K * numprocessors. The same task body still works — bisect just lands on more tasks per file.

Other formats

Only the per-file reader inside loadtile changes. The reader's contract: given a file path and a half-open row range [filelo, filehi) along axis 0, return a numpy array of shape (filehi - filelo,) + trailingshape that can be made C-contiguous. Cheap range/slice reads are required — formats that only support "read the whole file" defeat the partial-overlap case (a tile that covers only part of one file).

(For built-in single-call loaders per format, see the "Why this skill exists" table at the top of this file.)

The discovery step (step 1) parses each format's metadata: .npy / HDF5 / Parquet all carry per-file row count + dtype on disk. Raw binary doesn't — sidecar or derive from file size.

Common pitfalls

cn.asarray(dst) is illegal in a leaf task

Inside a @task body, any cupynumeric op that touches the top-level runtime — cn.asarray(store), slice assignment cndst[s] = hostnp — triggers createindexspace from the wrong context and Legion aborts:

LEGION API USAGE EXCEPTION: Invalid task context passed to runtime call
create_index_space

Fix: consume the DLPack capsule with a third-party library (cupy / torch / numpy) inside leaf tasks. cn.asarray is fine in the driver, just not in leaf tasks. See examples/dlpack/leaftaskinterop.py for the torch-flavoured workaround.

In-task assert aborts the runtime

Legate treats unraised exceptions in a @task as a contract violation and aborts unless the task was registered with throws_exception(). Sanity-check on the host before launching.

Launch domain must match the partition tile count

createmanualtask(launchshape=...) and partitionbytiling(...) are independent — the runtime doesn't catch a mismatch. Larger launch domain → out-of-range tiles; smaller → unwritten tiles. Always derive both from the same (totalrows, tilerows) via two separate ceil divisions (sizing the launch domain to numprocessors directly would over-launch when numprocessors > totalrows):

tile_rows = max(1, (total_rows + num_processors - 1) // num_processors)
num_tasks = (total_rows + tile_rows - 1) // tile_rows
partition = ...partition_by_tiling((tile_rows,) + trailing_shape)
runtime.create_manual_task(load_tile.library, load_tile.task_id, (num_tasks,))