mtlpy

Python bindings for GPU compute on Apple Metal, built on nanobind and Apple's metal-cpp. Write a Metal compute kernel as a string, dispatch it over NumPy arrays, get the result back as a NumPy array — no separate build step, no manual buffer plumbing.

import numpy as np
import mtlpy

device = mtlpy.Device()
a = device.buffer(np.array([1.0, 2.0, 3.0], dtype=np.float32))
b = device.buffer(np.array([10.0, 20.0, 30.0], dtype=np.float32))

print((a + b).contents)  # [11. 22. 33.]

Why this exists

mtlpy is a from-scratch rewrite of themetalgpu project. Rather than build on top of that codebase's ctypes-based bindings and global singleton state, mtlpy starts over with a few deliberate improvements:

• nanobind instead of ctypes — real type safety across the Python/C++ boundary, and Metal errors propagate as Python exceptions instead of silent failures.
• No global singleton state — each Device owns its own command queue and pipeline cache; nothing is saved/restored behind your back.
• Pipeline compile caching — a compute pipeline is compiled once per (shader source, function name) and reused, both within a process and (via an on-disk Metal binary archive) across process launches.
• Async dispatch — Pipeline.run(..., wait=False) lets you batch work without stalling on every call.

Status

Alpha, but built, tested, and benchmarked on real Apple Silicon hardware — see Building from source and the test suite for current coverage.

Architecture

metal-cpp/          Apple's C++ Metal headers (git submodule)
csrc/                C++ extension (nanobind + metal-cpp)
  device.{h,cpp}       MTL::Device + MTL::CommandQueue owner
  buffer.{h,cpp}       MTL::Buffer wrapper (shared-storage, CPU/GPU unified memory)
  texture.{h,cpp}      MTL::Texture wrapper (1D/2D/3D)
  sampler.{h,cpp}      MTL::SamplerState wrapper
  pipeline.{h,cpp}     Dispatches a compiled MTL::ComputePipelineState
  command_buffer.{h,cpp}  Batches multiple Pipeline::run() dispatches into
                          one MTL::CommandBuffer submission
  pipeline_cache.{h,cpp}  Compiles-once cache, keyed on (source, function name),
                          backed by an on-disk MTL::BinaryArchive
  metal_impl.mm        Single Obj-C++ translation unit providing the
                        NS::/CA::/MTL:: private implementations
  bindings.cpp         nanobind module definition (`_mtlpy`)
src/mtlpy/          Python package (src layout, for PyPI)
  device.py            Device: buffer/empty/texture/sampler/compile, list_devices(), wraps _mtlpy.Device
  buffer.py             Buffer: NumPy-backed contents, arithmetic/comparison/in-place operators
  texture.py             Texture, Sampler: wrap _mtlpy.Texture/_mtlpy.Sampler
  pipeline.py           Pipeline, CommandBuffer: thin wrappers over _mtlpy.Pipeline/CommandBuffer
  operators.py          sqrt/cos/sin/tan/exp/log, sum/max/min/mean reductions
  shader.py             Generates Metal Shading Language source per dtype/texture type
  utils.py              NumPy dtype <-> Metal type/pixel format mapping
tests/               pytest suite
benchmarks/          Standalone performance baseline scripts
examples/            Runnable usage examples

Each Device in Python owns exactly one MTL::Device, one MTL::CommandQueue, and one PipelineCache. Buffers use MTL::ResourceStorageModeShared, so on Apple Silicon's unified memory there's no copy between CPU and GPU views of the same allocation — Buffer.contents is a NumPy array backed directly by GPU-visible memory (accessing .contents is a true zero-copy view; writing new data into it via buf.contents[:] = arr is still a real memcpy from arr's own memory, same as it would be for any destination).

Features

• Elementwise operators: +, -, *, /, unary -, and in-place +=/-=/*=//= (which dispatch in-place, into the same Buffer, with no extra allocation) on Buffer — each also works with a NumPy/Python scalar on either side (buf + 5.0, 5.0 - buf), not just Buffer op Buffer. Plus sqrt, cos, sin, tan, exp, log, and astype for dtype conversion.
• Comparisons: ==, !=, <, <=, >, >= (against another Buffer or a scalar) return a bool Buffer, matching NumPy's ndarray convention — which also makes Buffer unhashable, same tradeoff NumPy makes.
• Reductions: operators.sum/max/min/mean — an O(log n) multi-pass tree reduction returning a plain Python scalar.
• Custom kernels: compile and dispatch arbitrary Metal Shading Language source directly (see Custom kernels below). Pipeline.run validates the buffer/texture/sampler counts against the kernel's own argument reflection, so passing too few of any of them raises a clear Python exception instead of leaving a Metal argument unbound (undefined behavior).
• Textures: 1D/2D/3D Textures (device.texture(array, pixel_format) / device.empty_texture(shape, pixel_format)) and Samplers for kernels written against texture2d<...>/etc. rather than raw buffers -- see Textures below.
• Shapes: Buffer.shape tracks the logical shape a buffer was created or reshape()d with (elementwise ops preserve it); Buffer.numpy() / np.asarray(buf) return contents in that shape. Buffer.contents itself stays flat regardless — see Shapes and NumPy interop.
• Zero-copy interop with other GPU libraries: every Buffer implements the DLPack protocol, tagged Metal-backed rather than CPU — mx.asarray(buf, copy=False) (MLX, or any other DLPack-aware library) shares the same id<MTLBuffer> with no copy. Raw mtl_ptr handles are also available for non-DLPack native interop — see Zero-copy interop with other GPU libraries.
• Dtype support: float32, float16, int32, uint32, int16, uint16, int64, uint64, bool — mapped to their Metal equivalents (float, half, int, uint, short, ushort, long, ulong, bool) in src/mtlpy/utils.py. float64 has no Metal equivalent (no Apple GPU supports double precision), so it's silently downcast to float32 at buffer creation. Note that Buffer / Buffer uses Metal's native / for the shared dtype (truncating for integers), not NumPy's always-promote-to-float64 semantics.
• Pipeline caching: identical (source, function name) pairs are compiled once per process and reused; a binary archive on disk (~/Library/Caches/mtlpy/pipelines.metallib) carries compiled pipelines across process launches too. Device.flush_cache() (or using Device as a context manager: with mtlpy.Device() as d:) serializes it on demand, rather than only when the Device is garbage collected.
• Async dispatch: wait=False commits work without blocking; Metal retires command buffers on a queue in commit order, so a later wait=True dispatch that reads the result is enough to synchronize (see examples/async_dispatch.py). Pipeline.run releases the GIL for the whole call, so other Python threads keep running during the GPU wait instead of being blocked for its full duration.
• Batched dispatches: Device.command_buffer() batches multiple Pipeline.run() calls into one MTLCommandBuffer submission instead of one per dispatch -- see Batching dispatches below.
• Multi-GPU support: mtlpy.list_devices() lists every Metal-capable GPU on the machine; mtlpy.Device(index=...) selects one (the default targets the system default GPU).
• Errors as exceptions: shader compile failures, missing kernel functions, mismatched buffer counts, mismatched-Device operands, and GPU execution errors all raise Python exceptions with a clear message, instead of failing silently or invoking undefined behavior.

Building from source

Requires macOS with Metal support, Xcode (for the Metal/Objective-C++ toolchain), CMake, and Python 3.9+.

git clone --recursive git@github.com:peyton-howe/mtlpy.git
cd mtlpy
pip install -e ".[dev]"

If you already cloned without --recursive:

git submodule update --init

Quick start

import numpy as np
import mtlpy

device = mtlpy.Device()

a = device.buffer(np.array([1.0, 2.0, 3.0, 4.0], dtype=np.float32))
b = device.buffer(np.array([10.0, 20.0, 30.0, 40.0], dtype=np.float32))

c = a + b
print(c.contents)          # numpy.ndarray([11. 22. 33. 44.])

d = mtlpy.operators.sqrt(a)
print(d.contents)

e = a.astype(np.int32)
print(e.dtype, e.contents)

Custom kernels

Device.compile(source, function_name) compiles arbitrary Metal Shading Language and returns a Pipeline you can dispatch directly:

source = """
#include <metal_stdlib>
using namespace metal;
kernel void square(
    device const float *a [[buffer(0)]],
    device       float *b [[buffer(1)]],
    uint id [[thread_position_in_grid]])
{
    b[id] = a[id] * a[id];
}
"""
pipeline = device.compile(source, "square")

a = device.buffer(np.array([1.0, 2.0, 3.0, 4.0], dtype=np.float32))
b = device.empty(4, np.float32)
pipeline.run([a, b], grid=4)

print(b.contents)  # [1. 4. 9. 16.]

grid may be an int (1D dispatch) or a 3-tuple/list for 2D/3D dispatch. Threadgroup sizing is computed automatically from the pipeline's thread_execution_width and max_threads_per_threadgroup.

Shapes and NumPy interop

Buffer.contents is deliberately always flat (see Custom kernels and the note above on unified memory) — but every Buffer also tracks a logical .shape, set when you create it from an ndarray or via device.empty(shape, dtype), and preserved by the elementwise operators:

img = np.arange(24, dtype=np.float32).reshape(4, 6)
buf = device.buffer(img)

buf.shape          # (4, 6)
buf.contents.shape # (24,)  -- always flat
buf.numpy().shape   # (4, 6) -- contents reshaped to buf.shape, still zero-copy

np.asarray(buf)     # same as buf.numpy() -- Buffer implements __array__
np.array(buf, dtype=np.float64)  # dtype conversion via the same protocol

grid = buf.reshape(2, 12)  # new Buffer, same underlying Metal allocation

.numpy() and __array__ are both zero-copy views, same as .contents — reshaping a flat contiguous array is always a view in NumPy, never a copy, so none of this allocates or duplicates GPU memory. .reshape() similarly shares the same MTL::Buffer rather than reallocating.

Elementwise operators (+, -, *, comparisons, ...) check .size, not .shape — a Metal buffer has no shape of its own (it's just bytes), so two buffers with equal flat size but different declared .shape are still valid operands; the result takes the first operand's .shape. .shape is purely Python-side bookkeeping layered on top, not something Metal itself knows about.

Zero-copy interop with other GPU libraries

Every Buffer implements the DLPack protocol (__dlpack__/__dlpack_device__), tagged as Metal-backed (kDLMetal) rather than plain CPU memory. Any DLPack-aware library gets a genuine zero-copy view over the same id<MTLBuffer> — no allocation, no memcpy — just by taking the Buffer directly:

import mlx.core as mx

buf = device.buffer(np.arange(8, dtype=np.float32))
arr = mx.asarray(buf, copy=False)  # shares this Buffer's live memory

buf.contents[0] = 111.0
arr[0].item()  # 111.0 -- same underlying allocation, not a copy

Measured at 4K/8K image sizes: ~15-25µs regardless of resolution (just wrapping the existing handle), versus hundreds of µs to a few ms for the .contents()-view-then-copy alternative most non-DLPack-aware code falls back to — that path scales linearly with buffer size since it's a real memcpy, while the DLPack path doesn't move any bytes at all.

mtlpy itself never imports MLX (or any other consumer) anywhere — the dunder protocol methods are the complete surface; this is standard DLPack, not an MLX-specific integration, so any other DLPack-aware library works the same way.

For interop with something that isn't DLPack-aware (e.g. hand-written PyObjC/Metal bridging code), Device.mtl_ptr / Buffer.mtl_ptr / Texture.mtl_ptr expose the raw id<MTLDevice>/id<MTLBuffer>/ id<MTLTexture> handle as a plain integer. Unlike the DLPack path (which retains the underlying Metal object for as long as the consumer holds it), these are bare pointers with no lifetime management — valid only as long as you keep the owning mtlpy object referenced yourself.

Textures

Texture wraps MTL::Texture (1D/2D/3D) for kernels that want texture2d<...>-style access instead of raw device float* buffers -- useful for image-processing-style kernels, and for sampling (bilinear filtering, addressing modes) rather than plain indexing.

img = np.random.rand(64, 64).astype(np.float32)          # (height, width)
tex = device.texture(img, "r32Float")                      # uploads in one call

invert_source = """
#include <metal_stdlib>
using namespace metal;
kernel void invert(
    texture2d<float, access::read_write> tex [[texture(0)]],
    uint2 gid [[thread_position_in_grid]])
{
    float4 c = tex.read(gid);
    tex.write(1.0 - c, gid);
}
"""
pipeline = device.compile(invert_source, "invert")
pipeline.run([], grid=(64, 64, 1), textures=[tex])   # no buffers, one texture

result = tex.download()  # or np.asarray(tex) / tex.numpy()

Pipeline.run takes buffers, textures, and samplers as separate lists because Metal Shading Language gives each its own independent binding namespace ([[buffer(n)]] / [[texture(n)]] / [[sampler(n)]]) -- list position i binds to index i in that namespace, same convention buffers already use. A sampling kernel needs a Sampler too:

sample_source = """
#include <metal_stdlib>
using namespace metal;
kernel void downscale(
    texture2d<float, access::sample> src [[texture(0)]],
    texture2d<float, access::write>  dst [[texture(1)]],
    sampler                          smp [[sampler(0)]],
    uint2 gid [[thread_position_in_grid]])
{
    float2 uv = (float2(gid) + 0.5) / float2(dst.get_width(), dst.get_height());
    dst.write(src.sample(smp, uv), gid);
}
"""
small = device.empty_texture((32, 32), "r32Float")
smp   = device.sampler(linear=True)   # linear=False for nearest-neighbor
downscale = device.compile(sample_source, "downscale")
downscale.run([], grid=(32, 32, 1), textures=[tex, small], samplers=[smp])

A few things that differ from Buffer:

• No zero-copy .contents. Metal doesn't guarantee a texture's CPU-visible memory is a tightly packed array the way a Shared-storage Buffer's is (rows can be padded/tiled internally), so there's no Buffer.contents equivalent. .upload()/.download() (and .numpy()/ __array__, which call .download()) are genuine copies via MTL::Texture's replaceRegion/getBytes.
• shape excludes the channel count on input, includes it on output. device.empty_texture(shape, pixel_format) takes a spatial shape only -- (width,), (height, width), or (depth, height, width) -- with channel count implied by pixel_format ("rgba8Unorm" is 4-channel, "r32Float" is 1-channel). Texture.shape (and what .download() returns) appends a trailing channel dim when channels > 1, matching common image-array conventions. empty_texture raises if a 3-element shape looks like it still has that channel axis attached (its last element equals the format's channel count) -- if you have an (H, W, C) array, either use device.texture(data, pixel_format) (which strips it for you) or pass shape[:-1].
• Pixel formats, not dtypes. A small, deliberately non-exhaustive set covering 8-bit and float image data (Metal defines 100+ pixel formats, most for graphics rather than compute): r8Unorm, rgba8Unorm, r16Float, rgba16Float, r32Float, rgba32Float, r32Uint, rgba32Uint (src/mtlpy/utils.py). Unorm formats store small integers but kernels read/write them as float in [0, 1] -- Metal normalizes automatically; shader.texture_type(dims, msl_scalar_type, access) generates the right MSL type string (texture2d<float, access::sample>, etc.) if you don't want to hand-write it.

Moving data in and out of a Texture

device.texture(data, pixel_format) and .upload()/.download() (above) are the simple default path -- always available, but CPU-side copies (replaceRegion/getBytes) that only work on the default Shared storage mode. There's a faster, GPU-side path for every CPU/Buffer/Texture direction, each suited to a different data source and each working regardless of storage mode (including private=True, see below) -- see benchmarks/README.md for the measurements behind these:

empty_texture(..., readable=, writable=, private=) controls usage flags and storage mode at creation. private=True (MTLStorageModePrivate, GPU-only memory) requires the GPU-side methods above -- .upload()/ .download() raise a clear error on a private texture, since Metal itself rejects replaceRegion/getBytes on Private storage.

Reusing buffers in a hot loop

Buffer.contents is a live NumPy view over the same underlying Metal allocation, not a copy — writing buf.contents[:] = ... updates GPU-visible memory in place, and reading it back after a wait=True dispatch needs no reallocation either. For a kernel dispatched repeatedly (e.g. in a while loop), compile the pipeline and allocate buffers once, then just write/read .contents each iteration:

pipeline = device.compile(source, "square")

a   = device.buffer(np.zeros(4, dtype=np.float32))  # allocated once
out = device.empty(4, np.float32)                    # allocated once

while running:
    a.contents[:] = get_next_input()   # in-place write, no realloc
    pipeline.run([a, out], grid=4)     # wait=True by default
    consume(out.contents)              # in-place read, no realloc

The out-of-place convenience operators (a + b, operators.sqrt(a), astype, etc.) don't follow this pattern — each call allocates a fresh output Buffer internally, which is fine for one-off use but wasteful in a tight loop. The in-place operators (a += b, a *= 2.0, ...) do reuse a's own buffer with no extra allocation, if that fits your loop. See examples/reuse_buffers.py.

Batching dispatches

Each Pipeline.run() call submits its own MTLCommandBuffer by default — one command-buffer-create + commit + (if wait=True) waitUntilCompleted() round trip per dispatch. For a fixed sequence of dispatches that always run together (a multi-pass kernel, or any "run these N things then read the result" pattern), Device.command_buffer() batches them into one submission instead, as a context manager:

with device.command_buffer() as cb:
    horizontal_pass.run([], grid, textures=[src, mid], cb=cb)
    vertical_pass.run([], grid, textures=[mid, dst], cb=cb)
# one submit, one wait, covering both dispatches

Pipeline.run(..., cb=cb) encodes into cb's shared encoder instead of committing its own command buffer — wait is ignored in that case (you can't partially wait on part of a not-yet-committed command buffer), and it always returns (0.0, 0.0): per-dispatch GPU timing isn't meaningful once dispatches share a command buffer, only cb.commit()'s combined timing is. The with block commits (and waits, by default) on normal exit; if the block raises, it does not commit — a partially-encoded batch is discarded rather than submitted, the same way a database transaction rolls back on exception instead of committing a partial write. Measured ~2x faster than two separate wait=True dispatches for a two-pass texture kernel.

Without a context manager: cb = device.command_buffer(), encode dispatches into it the same way, then cb.commit(wait=True) (the default) or cb.commit(wait=False) to defer waiting the same way Pipeline.run(..., wait=False) does — a later wait=True dispatch on the same queue still guarantees this batch finished first (Metal retires command buffers on a queue in commit order). Calling .commit() more than once, or encoding into a CommandBuffer after it's committed, raises RuntimeError.

CommandBuffer vs. a plain wait=False chain (see Async dispatch above): both avoid stalling between dependent dispatches, but they're not interchangeable —

• Back-to-back dispatches with no CPU-side work between them: roughly tied either way.
• CPU-side work between dispatches (e.g. computing the next dispatch's arguments): a wait=False chain wins, measured ~1.2x faster for a 4K two-pass kernel with 2ms of CPU work in between. wait=False submits immediately, so the GPU starts executing that dispatch while the CPU is still busy; CommandBuffer batching defers all submission until commit(), so the GPU sits idle until the whole batch has been encoded.
• Sequence not known upfront (the next dispatch depends on inspecting something first): only a wait=False chain fits -- nothing in a CommandBuffer batch executes until it's fully encoded and committed, so you can't make encoding decisions based on an earlier batched dispatch's result without breaking the batch anyway.

CommandBuffer is the better fit for a fixed, known-upfront sequence with little CPU work in between (its original motivating case: a multi-pass kernel). A wait=False chain remains the right tool when there's real work to overlap with GPU execution, or the sequence is decided dynamically.

Testing

pytest tests/

• test_basic.py / test_operators.py — correctness for every operator (arithmetic, scalar broadcasting, comparisons, in-place, reductions), dtype, and astype conversion, plus error handling for mismatched buffer sizes/dtypes/devices and wrong kernel argument counts.
• test_async.py — wait=False dispatch ordering.
• test_buffer_reuse.py — in-place .contents writes and repeated dispatch against the same buffers, without reallocation.
• test_shapes.py — .shape, .reshape(), .numpy(), and __array__, including that .contents stays flat and elementwise ops preserve shape.
• test_texture.py — texture creation/shape across pixel formats and dimensionalities, upload/download roundtrips, a read-write compute kernel, and a sampling kernel (multi-texture + sampler binding).
• test_stability.py — repeated-dispatch and object-lifetime stress tests (regression coverage for the Metal object-ownership rules in csrc/), plus multi-threaded dispatch/compilation tests (Pipeline.run releases the GIL, so this exercises genuinely concurrent Metal calls).
• test_pipeline_persistence.py — spawns separate processes to verify the on-disk pipeline binary archive is actually written and read back, and that Device.flush_cache() writes it on demand.

Benchmarking

python benchmarks/bench.py

Measures first-dispatch (compile-included) and steady-state warm-dispatch latency/throughput for every operator across a range of buffer sizes, with NumPy CPU timings alongside for context. Each run is saved as JSON (timestamped, tagged with the git commit) under benchmarks/results/ so you can baseline future changes:

python benchmarks/bench.py --baseline benchmarks/results/<earlier-run>.json

License

MIT