Communicator Class

class nccl.core.Communicator(ptr: int = 0)

Bases: object

NCCL communicator for collective and point-to-point operations.

A communicator represents a group of ranks that can perform collective operations (e.g. allreduce, broadcast) and point-to-point operations (send/recv). Each rank has a unique ID in [0, nranks).

Communicator instances expose a number of properties for inspection (ptr, nranks, device, rank, plus device-API related properties like cuda_dev, nvml_dev, device_api_support, multimem_support, gin_type, n_lsa_teams, host_rma_support, railed_gin_type); see the per-property documentation for details.

__init__(ptr: int = 0) → None

Initializes a communicator with a raw NCCL pointer.

Unlike the init() classmethod, this constructor allows ptr=0 for creating null communicators (e.g. when split() excludes a rank). A null communicator can later be initialized via initialize(), or used as the caller for grow() to join an existing communicator.

Parameters:

ptr – Integer representing an NCCL communicator pointer (0 for a null communicator). Defaults to 0.

Properties

Identity

Communicator.ptr

Raw pointer to the underlying ncclComm_t structure (0 if destroyed or null).

Communicator.is_valid

Whether the communicator is valid (not destroyed or null).

Communicator.nranks

Total number of ranks in the communicator.

Raises:

NcclInvalid – If the communicator is not initialized.

Communicator.device

CUDA device associated with this communicator.

Returns a cuda.core.Device providing additional functionality such as to_system_device for obtaining the NVML device, device properties, and synchronization.

Raises:

NcclInvalid – If the communicator is not initialized.

Communicator.rank

This caller’s rank within the communicator (0 to nranks - 1).

Raises:

NcclInvalid – If the communicator is not initialized.

Device-API capability

These properties reflect the underlying NCCL ncclCommProperties_t structure.

Communicator.cuda_dev

CUDA device ID associated with this communicator.

Raises:

NcclInvalid – If the communicator is not initialized.

Communicator.nvml_dev

NVML device ID for the GPU associated with this communicator.

Uses the NVML indexing space, which may differ from CUDA indexing.

Raises:

NcclInvalid – If the communicator is not initialized.

Communicator.device_api_support

Whether device-side NCCL operations are supported on this platform.

If False, a device communicator cannot be created.

Raises:

NcclInvalid – If the communicator is not initialized.

Communicator.multimem_support

Whether ranks in the same LSA team can communicate using multimem.

If False, a device communicator cannot be created with multimem resources.

Raises:

NcclInvalid – If the communicator is not initialized.

Communicator.gin_type

GPU Interconnect Network (GIN) type.

If equal to NcclGinType.NONE, a device communicator cannot be created with GIN resources.

Raises:

NcclInvalid – If the communicator is not initialized.

Communicator.n_lsa_teams

Number of Local Shared Array (LSA) teams for this communicator.

Raises:

NcclInvalid – If the communicator is not initialized.

Communicator.host_rma_support

Whether host RMA is supported on this communicator.

Raises:

NcclInvalid – If the communicator is not initialized.

Communicator.railed_gin_type

Railed GIN type supported by this communicator.

If equal to NcclGinType.NONE, a device communicator cannot be created with GIN connection type NcclGinConnectionType.RAIL.

Raises:

NcclInvalid – If the communicator is not initialized.