API library

API library#

struct ncclConfig_t#

Communicator configuration.

Users can assign value to attributes to specify the behavior of a communicator

Public Members

size_t size#: Should not be touched

unsigned int magic#: Should not be touched

unsigned int version#: Should not be touched

int blocking#: Whether or not calls should block or not

int cgaClusterSize#: Cooperative group array cluster size

int minCTAs#: Minimum number of cooperative thread arrays (blocks)

int maxCTAs#: Maximum number of cooperative thread arrays (blocks)

const char *netName#: Force NCCL to use a specfic network

int splitShare#: Allow communicators to share resources

struct ncclUniqueId#

Opaque unique id used to initialize communicators.

The ncclUniqueId must be passed to all participating ranks

Public Members

char internal[NCCL_UNIQUE_ID_BYTES]#: Opaque array>

file mainpage.txt

file nccl.h.in

#include <hip/hip_runtime.h>

#include <hip/hip_fp16.h>

#include <limits.h>

Defines

NCCL_H_#

NCCL_MAJOR#

NCCL_MINOR#

NCCL_PATCH#

NCCL_SUFFIX#

NCCL_VERSION_CODE#

NCCL_VERSION(X, Y, Z)#

RCCL_BFLOAT16#

RCCL_FLOAT8#

RCCL_GATHER_SCATTER#

RCCL_ALLTOALLV#

NCCL_COMM_NULL#

NCCL_UNIQUE_ID_BYTES#

NCCL_CONFIG_UNDEF_INT#

NCCL_CONFIG_UNDEF_PTR#

NCCL_SPLIT_NOCOLOR#

NCCL_CONFIG_INITIALIZER#

Typedefs

typedef struct ncclComm *ncclComm_t#

Opaque handle to communicator.

A communicator contains information required to facilitate collective communications calls

typedef int mscclAlgoHandle_t#: Opaque handle to MSCCL algorithm.

Enums

enum ncclResult_t#

Result type.

Return codes aside from ncclSuccess indicate that a call has failed

Values:

enumerator ncclSuccess#: No error

enumerator ncclUnhandledCudaError#: Unhandled HIP error

enumerator ncclSystemError#: Unhandled system error

enumerator ncclInternalError#: Internal Error - Please report to RCCL developers

enumerator ncclInvalidArgument#: Invalid argument

enumerator ncclInvalidUsage#: Invalid usage

enumerator ncclRemoteError#: Remote process exited or there was a network error

enumerator ncclInProgress#: RCCL operation in progress

enumerator ncclNumResults#: Number of result types

enum ncclRedOp_dummy_t#

Dummy reduction enumeration.

Dummy reduction enumeration used to determine value for ncclMaxRedOp

Values:

enumerator ncclNumOps_dummy#

enum ncclRedOp_t#

Reduction operation selector.

Enumeration used to specify the various reduction operations ncclNumOps is the number of built-in ncclRedOp_t values and serves as the least possible value for dynamic ncclRedOp_t values constructed by ncclRedOpCreate functions.

ncclMaxRedOp is the largest valid value for ncclRedOp_t and is defined to be the largest signed value (since compilers are permitted to use signed enums) that won’t grow sizeof(ncclRedOp_t) when compared to previous RCCL versions to maintain ABI compatibility.

Values:

enumerator ncclSum#: Sum

enumerator ncclProd#: Product

enumerator ncclMax#: Max

enumerator ncclMin#: Min

enumerator ncclAvg#: Average

enumerator ncclNumOps#: Number of built-in reduction ops

enumerator ncclMaxRedOp#: Largest value for ncclRedOp_t

enum ncclDataType_t#

Data types.

Enumeration of the various supported datatype

Values:

enumerator ncclInt8#

enumerator ncclChar#

enumerator ncclUint8#

enumerator ncclInt32#

enumerator ncclInt#

enumerator ncclUint32#

enumerator ncclInt64#

enumerator ncclUint64#

enumerator ncclFloat16#

enumerator ncclHalf#

enumerator ncclFloat32#

enumerator ncclFloat#

enumerator ncclFloat64#

enumerator ncclDouble#

enumerator ncclBfloat16#

enumerator ncclFp8E4M3#

enumerator ncclFp8E5M2#

enumerator ncclNumTypes#

enum ncclScalarResidence_t#

Location and dereferencing logic for scalar arguments.

Enumeration specifying memory location of the scalar argument. Based on where the value is stored, the argument will be dereferenced either while the collective is running (if in device memory), or before the ncclRedOpCreate() function returns (if in host memory).

Values:

enumerator ncclScalarDevice#: Scalar is in device-visible memory

enumerator ncclScalarHostImmediate#: Scalar is in host-visible memory

Functions

ncclResult_t ncclMemAlloc(void **ptr, size_t size)#

ncclResult_t pncclMemAlloc(void **ptr, size_t size)#

ncclResult_t ncclMemFree(void *ptr)#

ncclResult_t pncclMemFree(void *ptr)#

ncclResult_t ncclGetVersion(int *version)#

Return the RCCL_VERSION_CODE of RCCL in the supplied integer.

This integer is coded with the MAJOR, MINOR and PATCH level of RCCL.

Parameters:: version – [out] Pointer to where version will be stored
Returns:: Result code. See Result Codes for more details.

ncclResult_t ncclGetUniqueId(ncclUniqueId *uniqueId)#

Generates an ID for ncclCommInitRank.

Generates an ID to be used in ncclCommInitRank. ncclGetUniqueId should be called once by a single rank and the ID should be distributed to all ranks in the communicator before using it as a parameter for ncclCommInitRank.

Parameters:: uniqueId – [out] Pointer to where uniqueId will be stored
Returns:: Result code. See Result Codes for more details.

ncclResult_t ncclCommInitRankConfig(ncclComm_t *comm, int nranks, ncclUniqueId commId, int rank, ncclConfig_t *config)#

Create a new communicator with config.

Create a new communicator (multi thread/process version) with a configuration set by users. See Communicator Configuration for more details. Each rank is associated to a CUDA device, which has to be set before calling ncclCommInitRank.

Parameters:

comm – [out] Pointer to created communicator
nranks – [in] Total number of ranks participating in this communicator
commId – [in] UniqueId required for initialization
rank – [in] Current rank to create communicator for. [0 to nranks-1]
config – [in] Pointer to communicator configuration

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclCommInitRank(ncclComm_t *comm, int nranks, ncclUniqueId commId, int rank)#

Creates a new communicator (multi thread/process version).

Rank must be between 0 and nranks-1 and unique within a communicator clique. Each rank is associated to a CUDA device, which has to be set before calling ncclCommInitRank. ncclCommInitRank implicitly syncronizes with other ranks, so it must be called by different threads/processes or use ncclGroupStart/ncclGroupEnd.

Parameters:

comm – [out] Pointer to created communicator
nranks – [in] Total number of ranks participating in this communicator
commId – [in] UniqueId required for initialization
rank – [in] Current rank to create communicator for

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclCommInitAll(ncclComm_t *comm, int ndev, const int *devlist)#

Creates a clique of communicators (single process version).

This is a convenience function to create a single-process communicator clique. Returns an array of ndev newly initialized communicators in comm. comm should be pre-allocated with size at least ndev*sizeof(ncclComm_t). If devlist is NULL, the first ndev HIP devices are used. Order of devlist defines user-order of processors within the communicator.

Parameters:

comm – [out] Pointer to array of created communicators
ndev – [in] Total number of ranks participating in this communicator
devlist – [in] Array of GPU device indices to create for

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclCommFinalize(ncclComm_t comm)#

Finalize a communicator.

ncclCommFinalize flushes all issued communications and marks communicator state as ncclInProgress. The state will change to ncclSuccess when the communicator is globally quiescent and related resources are freed; then, calling ncclCommDestroy can locally free the rest of the resources (e.g. communicator itself) without blocking.

Parameters:: comm – [in] Communicator to finalize
Returns:: Result code. See Result Codes for more details.

ncclResult_t ncclCommDestroy(ncclComm_t comm)#

Frees local resources associated with communicator object.

Destroy all local resources associated with the passed in communicator object

Parameters:: comm – [in] Communicator to destroy
Returns:: Result code. See Result Codes for more details.

ncclResult_t ncclCommAbort(ncclComm_t comm)#

Abort any in-progress calls and destroy the communicator object.

Frees resources associated with communicator object and aborts any operations that might still be running on the device.

Parameters:: comm – [in] Communicator to abort and destroy
Returns:: Result code. See Result Codes for more details.

ncclResult_t ncclCommSplit(ncclComm_t comm, int color, int key, ncclComm_t *newcomm, ncclConfig_t *config)#

Create one or more communicators from an existing one.

Creates one or more communicators from an existing one. Ranks with the same color will end up in the same communicator. Within the new communicator, key will be used to order ranks. NCCL_SPLIT_NOCOLOR as color will indicate the rank will not be part of any group and will therefore return a NULL communicator. If config is NULL, the new communicator will inherit the original communicator’s configuration

Parameters:

comm – [in] Original communicator object for this rank
color – [in] Color to assign this rank
key – [in] Key used to order ranks within the same new communicator
newcomm – [out] Pointer to new communicator
config – [in] Config file for new communicator. May be NULL to inherit from comm

Returns:

Result code. See Result Codes for more details.

const char *ncclGetErrorString(ncclResult_t result)#

Returns a string for each result code.

Returns a human-readable string describing the given result code.

Parameters:: result – [in] Result code to get description for
Returns:: String containing description of result code.

const char *ncclGetLastError(ncclComm_t comm)#

ncclResult_t ncclCommGetAsyncError(ncclComm_t comm, ncclResult_t *asyncError)#

Checks whether the comm has encountered any asynchronous errors.

Query whether the provided communicator has encountered any asynchronous errors

Parameters:

comm – [in] Communicator to query
asyncError – [out] Pointer to where result code will be stored

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclCommCount(const ncclComm_t comm, int *count)#

Gets the number of ranks in the communicator clique.

Returns the number of ranks in the communicator clique (as set during initialization)

Parameters:

comm – [in] Communicator to query
count – [out] Pointer to where number of ranks will be stored

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclCommCuDevice(const ncclComm_t comm, int *device)#

Get the ROCm device index associated with a communicator.

Returns the ROCm device number associated with the provided communicator.

Parameters:

comm – [in] Communicator to query
device – [out] Pointer to where the associated ROCm device index will be stored

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclCommUserRank(const ncclComm_t comm, int *rank)#

Get the rank associated with a communicator.

Returns the user-ordered “rank” associated with the provided communicator.

Parameters:

comm – [in] Communicator to query
rank – [out] Pointer to where the associated rank will be stored

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclCommRegister(const ncclComm_t comm, void *buff, size_t size, void **handle)#

ncclResult_t ncclCommDeregister(const ncclComm_t comm, void *handle)#

ncclResult_t ncclRedOpCreatePreMulSum(ncclRedOp_t *op, void *scalar, ncclDataType_t datatype, ncclScalarResidence_t residence, ncclComm_t comm)#

Create a custom pre-multiplier reduction operator.

Creates a new reduction operator which pre-multiplies input values by a given scalar locally before reducing them with peer values via summation. For use only with collectives launched against comm and datatype. The residence* argument indicates how/when the memory pointed to by scalar will be dereferenced. Upon return, the newly created operator’s handle is stored in op.

Parameters:

op – [out] Pointer to where newly created custom reduction operator is to be stored
scalar – [in] Pointer to scalar value.
datatype – [in] Scalar value datatype
residence – [in] Memory type of the scalar value
comm – [in] Communicator to associate with this custom reduction operator

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclRedOpDestroy(ncclRedOp_t op, ncclComm_t comm)#

Destroy custom reduction operator.

Destroys the reduction operator op. The operator must have been created by ncclRedOpCreatePreMul with the matching communicator comm. An operator may be destroyed as soon as the last RCCL function which is given that operator returns.

Parameters:

op – [in] Custom reduction operator is to be destroyed
comm – [in] Communicator associated with this reduction operator

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclReduce(const void *sendbuff, void *recvbuff, size_t count, ncclDataType_t datatype, ncclRedOp_t op, int root, ncclComm_t comm, hipStream_t stream)#

Reduce.

Reduces data arrays of length count in sendbuff into recvbuff using op operation. recvbuff* may be NULL on all calls except for root device. root* is the rank (not the HIP device) where data will reside after the operation is complete. In-place operation will happen if sendbuff == recvbuff.

Parameters:

sendbuff – [in] Local device data buffer to be reduced
recvbuff – [out] Data buffer where result is stored (only for root rank). May be null for other ranks.
count – [in] Number of elements in every send buffer
datatype – [in] Data buffer element datatype
op – [in] Reduction operator type
root – [in] Rank where result data array will be stored
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclBcast(void *buff, size_t count, ncclDataType_t datatype, int root, ncclComm_t comm, hipStream_t stream)#

(Deprecated) Broadcast (in-place)

Copies count values from root to all other devices. root is the rank (not the CUDA device) where data resides before the operation is started. This operation is implicitly in-place.

Parameters:

buff – [inout] Input array on root to be copied to other ranks. Output array for all ranks.
count – [in] Number of elements in data buffer
datatype – [in] Data buffer element datatype
root – [in] Rank owning buffer to be copied to others
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclBroadcast(const void *sendbuff, void *recvbuff, size_t count, ncclDataType_t datatype, int root, ncclComm_t comm, hipStream_t stream)#

Broadcast.

Copies count values from sendbuff on root to recvbuff on all devices. root* is the rank (not the HIP device) where data resides before the operation is started. sendbuff* may be NULL on ranks other than root. In-place operation will happen if sendbuff == recvbuff.

Parameters:

sendbuff – [in] Data array to copy (if root). May be NULL for other ranks
recvbuff – [in] Data array to store received array
count – [in] Number of elements in data buffer
datatype – [in] Data buffer element datatype
root – [in] Rank of broadcast root
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclAllReduce(const void *sendbuff, void *recvbuff, size_t count, ncclDataType_t datatype, ncclRedOp_t op, ncclComm_t comm, hipStream_t stream)#

All-Reduce.

Reduces data arrays of length count in sendbuff using op operation, and leaves identical copies of result on each recvbuff. In-place operation will happen if sendbuff == recvbuff.

Parameters:

sendbuff – [in] Input data array to reduce
recvbuff – [out] Data array to store reduced result array
count – [in] Number of elements in data buffer
datatype – [in] Data buffer element datatype
op – [in] Reduction operator
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclReduceScatter(const void *sendbuff, void *recvbuff, size_t recvcount, ncclDataType_t datatype, ncclRedOp_t op, ncclComm_t comm, hipStream_t stream)#

Reduce-Scatter.

Reduces data in sendbuff using op operation and leaves reduced result scattered over the devices so that recvbuff on rank i will contain the i-th block of the result. Assumes sendcount is equal to nranks*recvcount, which means that sendbuff should have a size of at least nranks*recvcount elements. In-place operations will happen if recvbuff == sendbuff + rank * recvcount.

Parameters:

sendbuff – [in] Input data array to reduce
recvbuff – [out] Data array to store reduced result subarray
recvcount – [in] Number of elements each rank receives
datatype – [in] Data buffer element datatype
op – [in] Reduction operator
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclAllGather(const void *sendbuff, void *recvbuff, size_t sendcount, ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream)#

All-Gather.

Each device gathers sendcount values from other GPUs into recvbuff, receiving data from rank i at offset i*sendcount. Assumes recvcount is equal to nranks*sendcount, which means that recvbuff should have a size of at least nranks*sendcount elements. In-place operations will happen if sendbuff == recvbuff + rank * sendcount.

Parameters:

sendbuff – [in] Input data array to send
recvbuff – [out] Data array to store the gathered result
sendcount – [in] Number of elements each rank sends
datatype – [in] Data buffer element datatype
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclSend(const void *sendbuff, size_t count, ncclDataType_t datatype, int peer, ncclComm_t comm, hipStream_t stream)#

Send.

Send data from sendbuff to rank peer. Rank peer needs to call ncclRecv with the same datatype and the same count as this rank. This operation is blocking for the GPU. If multiple ncclSend and ncclRecv operations need to progress concurrently to complete, they must be fused within a ncclGroupStart / ncclGroupEnd section.

Parameters:

sendbuff – [in] Data array to send
count – [in] Number of elements to send
datatype – [in] Data buffer element datatype
peer – [in] Peer rank to send to
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclRecv(void *recvbuff, size_t count, ncclDataType_t datatype, int peer, ncclComm_t comm, hipStream_t stream)#

Receive.

Receive data from rank peer into recvbuff. Rank peer needs to call ncclSend with the same datatype and the same count as this rank. This operation is blocking for the GPU. If multiple ncclSend and ncclRecv operations need to progress concurrently to complete, they must be fused within a ncclGroupStart/ ncclGroupEnd section.

Parameters:

recvbuff – [out] Data array to receive
count – [in] Number of elements to receive
datatype – [in] Data buffer element datatype
peer – [in] Peer rank to send to
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclGather(const void *sendbuff, void *recvbuff, size_t sendcount, ncclDataType_t datatype, int root, ncclComm_t comm, hipStream_t stream)#

Gather.

Root device gathers sendcount values from other GPUs into recvbuff, receiving data from rank i at offset i*sendcount. Assumes recvcount is equal to nranks*sendcount, which means that recvbuff should have a size of at least nranks*sendcount elements. In-place operations will happen if sendbuff == recvbuff + rank * sendcount. recvbuff* may be NULL on ranks other than root.

Parameters:

sendbuff – [in] Data array to send
recvbuff – [out] Data array to receive into on root.
sendcount – [in] Number of elements to send per rank
datatype – [in] Data buffer element datatype
root – [in] Rank that receives data from all other ranks
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclScatter(const void *sendbuff, void *recvbuff, size_t recvcount, ncclDataType_t datatype, int root, ncclComm_t comm, hipStream_t stream)#

Scatter.

Scattered over the devices so that recvbuff on rank i will contain the i-th block of the data on root. Assumes sendcount is equal to nranks*recvcount, which means that sendbuff should have a size of at least nranks*recvcount elements. In-place operations will happen if recvbuff == sendbuff + rank * recvcount.

Parameters:

sendbuff – [in] Data array to send (on root rank). May be NULL on other ranks.
recvbuff – [out] Data array to receive partial subarray into
recvcount – [in] Number of elements to receive per rank
datatype – [in] Data buffer element datatype
root – [in] Rank that scatters data to all other ranks
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclAllToAll(const void *sendbuff, void *recvbuff, size_t count, ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream)#

All-To-All.

Device (i) send (j)th block of data to device (j) and be placed as (i)th block. Each block for sending/receiving has count elements, which means that recvbuff and sendbuff should have a size of nranks*count elements. In-place operation is NOT supported. It is the user’s responsibility to ensure that sendbuff and recvbuff are distinct.

Parameters:

sendbuff – [in] Data array to send (contains blocks for each other rank)
recvbuff – [out] Data array to receive (contains blocks from each other rank)
count – [in] Number of elements to send between each pair of ranks
datatype – [in] Data buffer element datatype
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t ncclAllToAllv(const void *sendbuff, const size_t sendcounts[], const size_t sdispls[], void *recvbuff, const size_t recvcounts[], const size_t rdispls[], ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream)#

All-To-Allv.

Device (i) sends sendcounts[j] of data from offset sdispls[j] to device (j). At the same time, device (i) receives recvcounts[j] of data from device (j) to be placed at rdispls[j]. sendcounts, sdispls, recvcounts and rdispls are all measured in the units of datatype, not bytes. In-place operation will happen if sendbuff == recvbuff.

Parameters:

sendbuff – [in] Data array to send (contains blocks for each other rank)
sendcounts – [in] Array containing number of elements to send to each participating rank
sdispls – [in] Array of offsets into sendbuff for each participating rank
recvbuff – [out] Data array to receive (contains blocks from each other rank)
recvcounts – [in] Array containing number of elements to receive from each participating rank
rdispls – [in] Array of offsets into recvbuff for each participating rank
datatype – [in] Data buffer element datatype
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t mscclLoadAlgo(const char *mscclAlgoFilePath, mscclAlgoHandle_t *mscclAlgoHandle, int rank)#

MSCCL Load Algorithm.

Load MSCCL algorithm file specified in mscclAlgoFilePath and return its handle via mscclAlgoHandle. This API is expected to be called by MSCCL scheduler instead of end users.

Parameters:

mscclAlgoFilePath – [in] Path to MSCCL algorithm file
mscclAlgoHandle – [out] Returned handle to MSCCL algorithm
rank – [in] Current rank

Returns:

Result code. See Result Codes for more details.

ncclResult_t mscclRunAlgo(const void *sendBuff, const size_t sendCounts[], const size_t sDisPls[], void *recvBuff, const size_t recvCounts[], const size_t rDisPls[], size_t count, ncclDataType_t dataType, int root, int peer, ncclRedOp_t op, mscclAlgoHandle_t mscclAlgoHandle, ncclComm_t comm, hipStream_t stream)#

MSCCL Run Algorithm.

Run MSCCL algorithm specified by mscclAlgoHandle. The parameter list merges all possible parameters required by different operations as this is a general-purposed API. This API is expected to be called by MSCCL scheduler instead of end users.

Parameters:

sendBuff – [in] Data array to send
sendCounts – [in] Array containing number of elements to send to each participating rank
sDisPls – [in] Array of offsets into sendbuff for each participating rank
recvBuff – [out] Data array to receive
recvCounts – [in] Array containing number of elements to receive from each participating rank
rDisPls – [in] Array of offsets into recvbuff for each participating rank
count – [in] Number of elements
dataType – [in] Data buffer element datatype
root – [in] Root rank index
peer – [in] Peer rank index
op – [in] Reduction operator
mscclAlgoHandle – [in] Handle to MSCCL algorithm
comm – [in] Communicator group object to execute on
stream – [in] HIP stream to execute collective on

Returns:

Result code. See Result Codes for more details.

ncclResult_t mscclUnloadAlgo(mscclAlgoHandle_t mscclAlgoHandle)#

MSCCL Unload Algorithm.

Deprecated:: This function has been removed from the public API.

Unload MSCCL algorithm previous loaded using its handle. This API is expected to be called by MSCCL scheduler instead of end users.

Parameters:: mscclAlgoHandle – [in] Handle to MSCCL algorithm to unload
Returns:: Result code. See Result Codes for more details.

ncclResult_t ncclGroupStart()#

Group Start.

Start a group call. All calls to RCCL until ncclGroupEnd will be fused into a single RCCL operation. Nothing will be started on the HIP stream until ncclGroupEnd.

Returns:: Result code. See Result Codes for more details.

ncclResult_t ncclGroupEnd()#

Group End.

End a group call. Start a fused RCCL operation consisting of all calls since ncclGroupStart. Operations on the HIP stream depending on the RCCL operations need to be called after ncclGroupEnd.

Returns:: Result code. See Result Codes for more details.

group rccl_result_code

The various result codes that RCCL API calls may return

Enums

enum ncclResult_t

Result type.

Return codes aside from ncclSuccess indicate that a call has failed

Values:

enumerator ncclSuccess: No error

enumerator ncclUnhandledCudaError: Unhandled HIP error

enumerator ncclSystemError: Unhandled system error

enumerator ncclInternalError: Internal Error - Please report to RCCL developers

enumerator ncclInvalidArgument: Invalid argument

enumerator ncclInvalidUsage: Invalid usage

enumerator ncclRemoteError: Remote process exited or there was a network error

enumerator ncclInProgress: RCCL operation in progress

enumerator ncclNumResults: Number of result types

group rccl_config_type

Structure that allows for customizing Communicator behavior via ncclCommInitRankConfig

Defines

NCCL_CONFIG_INITIALIZER

group rccl_api_version: API call that returns RCCL version

group rccl_api_communicator: API calls that operate on communicators. Communicators objects are used to launch collective communication operations. Unique ranks between 0 and N-1 must be assigned to each HIP device participating in the same Communicator. Using the same HIP device for multiple ranks of the same Communicator is not supported at this time.

group rccl_api_errcheck: API calls that check for errors

group rccl_api_comminfo: API calls that query communicator information

group rccl_api_enumerations

Enumerations used by collective communication calls

Enums

enum ncclRedOp_dummy_t

Dummy reduction enumeration.

Dummy reduction enumeration used to determine value for ncclMaxRedOp

Values:

enumerator ncclNumOps_dummy

enum ncclRedOp_t

Reduction operation selector.

Enumeration used to specify the various reduction operations ncclNumOps is the number of built-in ncclRedOp_t values and serves as the least possible value for dynamic ncclRedOp_t values constructed by ncclRedOpCreate functions.

ncclMaxRedOp is the largest valid value for ncclRedOp_t and is defined to be the largest signed value (since compilers are permitted to use signed enums) that won’t grow sizeof(ncclRedOp_t) when compared to previous RCCL versions to maintain ABI compatibility.

Values:

enumerator ncclSum: Sum

enumerator ncclProd: Product

enumerator ncclMax: Max

enumerator ncclMin: Min

enumerator ncclAvg: Average

enumerator ncclNumOps: Number of built-in reduction ops

enumerator ncclMaxRedOp: Largest value for ncclRedOp_t

enum ncclDataType_t

Data types.

Enumeration of the various supported datatype

Values:

enumerator ncclInt8

enumerator ncclChar

enumerator ncclUint8

enumerator ncclInt32

enumerator ncclInt

enumerator ncclUint32

enumerator ncclInt64

enumerator ncclUint64

enumerator ncclFloat16

enumerator ncclHalf

enumerator ncclFloat32

enumerator ncclFloat

enumerator ncclFloat64

enumerator ncclDouble

enumerator ncclBfloat16

enumerator ncclFp8E4M3

enumerator ncclFp8E5M2

enumerator ncclNumTypes

group rccl_api_custom_redop

API calls relating to creation/destroying custom reduction operator that pre-multiplies local source arrays prior to reduction

Enums

enum ncclScalarResidence_t

Location and dereferencing logic for scalar arguments.

Enumeration specifying memory location of the scalar argument. Based on where the value is stored, the argument will be dereferenced either while the collective is running (if in device memory), or before the ncclRedOpCreate() function returns (if in host memory).

Values:

enumerator ncclScalarDevice: Scalar is in device-visible memory

enumerator ncclScalarHostImmediate: Scalar is in host-visible memory

group rccl_collective_api

Collective communication operations must be called separately for each communicator in a communicator clique.

They return when operations have been enqueued on the HIP stream. Since they may perform inter-CPU synchronization, each call has to be done from a different thread or process, or need to use Group Semantics (see below).

group msccl_api

API calls relating to the optional MSCCL algorithm datapath

Typedefs

typedef int mscclAlgoHandle_t: Opaque handle to MSCCL algorithm.

group rccl_group_api

When managing multiple GPUs from a single thread, and since RCCL collective calls may perform inter-CPU synchronization, we need to “group” calls for different ranks/devices into a single call.

Grouping RCCL calls as being part of the same collective operation is done using ncclGroupStart and ncclGroupEnd. ncclGroupStart will enqueue all collective calls until the ncclGroupEnd call, which will wait for all calls to be complete. Note that for collective communication, ncclGroupEnd only guarantees that the operations are enqueued on the streams, not that the operation is effectively done.

Both collective communication and ncclCommInitRank can be used in conjunction of ncclGroupStart/ncclGroupEnd, but not together.

Group semantics also allow to fuse multiple operations on the same device to improve performance (for aggregated collective calls), or to permit concurrent progress of multiple send/receive operations.

page deprecated

Global mscclUnloadAlgo (mscclAlgoHandle_t mscclAlgoHandle): This function has been removed from the public API.

dir src

page index

Introduction#

RCCL (pronounced “Rickle”) is a stand-alone library of standard collective communication routines for GPUs, implementing all-reduce, all-gather, reduce, broadcast, reduce-scatter, gather, scatter, and all-to-all. There is also initial support for direct GPU-to-GPU send and receive operations. It has been optimized to achieve high bandwidth on platforms using PCIe, xGMI as well as networking using InfiniBand Verbs or TCP/IP sockets. RCCL supports an arbitrary number of GPUs installed in a single node or multiple nodes, and can be used in either single- or multi-process (e.g., MPI) applications.

The collective operations are implemented using ring and tree algorithms and have been optimized for throughput and latency. For best performance, small operations can be either batched into larger operations or aggregated through the API.

RCCL API File#

nccl.h.in