python

nccl-extensions (Python)

Python bindings for the nccl-extensions communication libraries.

Package layout

This package installs into the nccl namespace, so the import paths are nccl.ep and nccl.m2n:

import nccl.ep as ep
import nccl.m2n as m2n

It contributes exactly three directories to that namespace, and no nccl/__init__.py:

path	contents
nccl/ep/	public facade for nccl_ep, plus CUDA-specific native libraries and headers
nccl/m2n/	public facade for NCCL M2N, plus CUDA-specific native libraries and headers. See the M2N Python guide for API usage and examples.
nccl/_extensions/	internals shared by every extension library — the Cython bindings, binding_dataclass, the distribution version

Install

CUDA_HOME=/usr/local/cuda pip install -e python/

Building requires a CUDA toolkit and a Cython toolchain.

Stage native artifacts before building a distributable wheel:

python/nccl/ep/lib/cu{12,13}/libnccl_ep.so.0
python/nccl/ep/include/**
python/nccl/m2n/lib/cu{12,13}/libnccl_m2n.so
python/nccl/m2n/include/**

EP wheels include one regular SONAME file (currently libnccl_ep.so.0) in each nccl/ep/lib/cu{12,13}/ directory. The Python loader opens this file directly. Native consumers can link with -l:libnccl_ep.so.0 (or its full path) and set RPATH/RUNPATH to the matching CUDA variant directory. Wheels do not include the unversioned linker name or the full release-version name.

For a complete Linux wheel build, the internal build_assets/build_wheels.sh script builds and stages these artifacts in a temporary project copy before running cibuildwheel. It does not stage artifacts into the source package tree.

Missing shared libraries emit explicit build warnings by default. Set NCCL_EXTENSIONS_REQUIRE_NATIVE_LIBS=1 to turn a missing library into a build error; the production wheel script sets this automatically. With the default value 0, the resulting wheel is not self-contained and needs compatible external libraries at runtime.

The sdist is source-only and excludes native shared libraries. Building a wheel from it must stage the native libraries at the paths above to bundle them, or provide compatible external libraries for runtime loading.

Pick a CUDA-variant extra to pull in the matching runtime stack (they forward to nccl4py's cu12 / cu13 extras, and are mutually exclusive):

pip install -e 'python/[cu13]'

At runtime, the installed cuda.bindings major selects the matching bundled lib/cu12 or lib/cu13 native libraries. NCCL EP compiles kernels at runtime with NCCL_EP_JIT_NVCC, NVCC, or the compiler under CUDA_HOME; that compiler and its headers must match the selected CUDA major.

Do not run Python from inside python/. There is no nccl/__init__.py there, so that directory resolves only as a namespace portion and these modules become invisible. Always go through the editable install.

NCCL EP handle lifecycle

Group.create_handle(...) remains the fused route. The v0.2 facade also exposes the split and imported-handle C APIs, which let applications allocate routing memory before CUDA graph capture or restore a prepared HT routing buffer:

import nccl.ep as ep
from nccl.core.typing import NcclDataType

# device_uint8_buffer must provide at least this many bytes.
num_bytes = group.handle_mem_size(ep.Layout.EXPERT_MAJOR, num_topk=-1)
handle_mem = ep.Tensor(device_uint8_buffer)

handle = group.init_handle(
    ep.Layout.EXPERT_MAJOR,
    num_topk=-1,
    handle_mem=handle_mem,
)
handle.update(topk_idx, layout_info=layout_info, stream=stream)

# After copying a prepared HT handle buffer, bind the snapshot locally.
state = ep.HandleState(
    num_tokens=num_tokens,
    num_topk=num_topk,
    topk_idx_datatype=NcclDataType.INT32,
)
restored = group.import_handle(
    ep.Layout.EXPERT_MAJOR,
    state,
    restored_handle_mem,
)

Combine-side MXFP8 is selected with CombineConfig(quantization_recipe=CombineQuantizationRecipe.MXFP8). It is an HT expert-major, single-LSA-team path; input tokens are BF16 and CombineInputs.scales must be None because NCCL EP generates E8M0 scales internally.

NCCL EP active-mask fault tolerance

Create an LL group with GroupConfig(enable_mask=True) to enable rank masking. The public group methods expose the mask and the separate host timeout flag:

import numpy as np

# mask_status is a preallocated ep.Tensor over contiguous GPU int32[num_ranks]
# memory on the group's device. Allocate it before CUDA graph capture.
group.mask_query(mask_status, stream=stream)

# Update takes CPU memory; keep it alive and unchanged until stream completion.
mask = np.ones(num_ranks, dtype=np.int32)
mask[-1] = 0  # 1 = active, 0 = masked
group.mask_update(mask, stream=stream)

error = group.get_async_error()  # 0 = no error, 1 = a rank timed out

# After at least one LL handle has been initialized, all surviving ranks
# call clean together to reset RDMA buffers and re-admit delayed ranks.
group.mask_clean(stream=stream)
stream.sync()
group.error_clear()  # Clean does not clear the timeout flag.

mask_query fills the supplied output without allocating or synchronizing and supports CUDA graph capture. Keep the output buffer alive through completion of all graph replays. Read it in stream order, or establish a dependency before using it on another stream. Reading on the CPU requires a device-to-host copy and synchronization. mask_update does not promise graph capture support. Error polling and clearing operate on the host without GPU synchronization. These methods are not supported for HT groups. Mask cleaning re-admits delayed ranks within the same communicator; replacing a failed rank requires a new communicator and EP group.

Generated bindings

Everything under nccl/_extensions/bindings/ is generated and checked in. Do not edit it by hand. Public builds use these checked-in sources directly.