API¶
This section provides details of the library API
Communicator Functions¶
-
ncclResult_t
ncclGetUniqueId
(ncclUniqueId *uniqueId) Generates an ID for ncclCommInitRank.
Generates an ID to be used in ncclCommInitRank. ncclGetUniqueId should be called once and the Id should be distributed to all ranks in the communicator before calling ncclCommInitRank.
- Parameters
[in] uniqueId
: ncclUniqueId* pointer to uniqueId
-
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
[in] comm
: ncclComm_t* communicator struct pointer
-
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.
-
ncclResult_t
ncclCommDestroy
(ncclComm_t comm) Frees resources associated with communicator object, but waits for any operations that might still be running on the device.
-
ncclResult_t
ncclCommAbort
(ncclComm_t comm) Frees resources associated with communicator object and aborts any operations that might still be running on the device.
-
ncclResult_t
ncclCommCount
(const ncclComm_t comm, int *count) Gets the number of ranks in the communicator clique.
-
ncclResult_t
ncclCommCuDevice
(const ncclComm_t comm, int *device) Returns the rocm device number associated with the communicator.
-
ncclResult_t
ncclCommUserRank
(const ncclComm_t comm, int *rank) Returns the user-ordered “rank” associated with the communicator.
Collective Communication Operations¶
Collective communication operations must be called separately for each communicator in a communicator clique.
They return when operations have been enqueued on the hipstream.
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).
-
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 CUDA device) where data will reside after the operation is complete.
In-place operation will happen if sendbuff == recvbuff.
-
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 implicitely in place.
-
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 root to all other devices. root is the rank (not the HIP device) where data resides before the operation is started.
In-place operation will happen if sendbuff == recvbuff.
-
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.
-
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.
-
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.
-
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 from 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.
-
ncclResult_t
ncclRecv
(void *recvbuff, size_t count, ncclDataType_t datatype, int peer, ncclComm_t comm, hipStream_t stream)
-
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.
-
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.
-
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 will happen if sendbuff == recvbuff.
Group Semantics¶
When managing multiple GPUs from a single thread, and since NCCL collective calls may perform inter-CPU synchronization, we need to “group” calls for different ranks/devices into a single call.
Grouping NCCL 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.
-
ncclResult_t
ncclGroupStart
() Group Start.
Start a group call. All calls to NCCL until ncclGroupEnd will be fused into a single NCCL operation. Nothing will be started on the CUDA stream until ncclGroupEnd.
-
ncclResult_t
ncclGroupEnd
() Group End.
End a group call. Start a fused NCCL operation consisting of all calls since ncclGroupStart. Operations on the CUDA stream depending on the NCCL operations need to be called after ncclGroupEnd.
Library Functions¶
-
ncclResult_t
ncclGetVersion
(int *version) Return the NCCL_VERSION_CODE of the NCCL library in the supplied integer.
This integer is coded with the MAJOR, MINOR and PATCH level of the NCCL library
-
const char *
ncclGetErrorString
(ncclResult_t result) Returns a human-readable error message.
Types¶
There are few data structures that are internal to the library. The pointer types to these structures are given below. The user would need to use these types to create handles and pass them between different library functions.
-
typedef struct ncclComm *
ncclComm_t
Opaque handle to communicator.
-
struct
ncclUniqueId
Enumerations¶
This section provides all the enumerations used.
-
enum
ncclResult_t
Error type.
Values:
-
enumerator
ncclSuccess
= 0
-
enumerator
ncclUnhandledCudaError
= 1
-
enumerator
ncclSystemError
= 2
-
enumerator
ncclInternalError
= 3
-
enumerator
ncclInvalidArgument
= 4
-
enumerator
ncclInvalidUsage
= 5
-
enumerator
ncclNumResults
= 6
-
enumerator
-
enum
ncclRedOp_t
Reduction operation selector.
Values:
-
enumerator
ncclSum
= 0
-
enumerator
ncclProd
= 1
-
enumerator
ncclMax
= 2
-
enumerator
ncclMin
= 3
-
enumerator
ncclNumOps
= 4
-
enumerator
-
enum
ncclDataType_t
Data types.
Values:
-
enumerator
ncclInt8
= 0
-
enumerator
ncclChar
= 0
-
enumerator
ncclUint8
= 1
-
enumerator
ncclInt32
= 2
-
enumerator
ncclInt
= 2
-
enumerator
ncclUint32
= 3
-
enumerator
ncclInt64
= 4
-
enumerator
ncclUint64
= 5
-
enumerator
ncclFloat16
= 6
-
enumerator
ncclHalf
= 6
-
enumerator
ncclFloat32
= 7
-
enumerator
ncclFloat
= 7
-
enumerator
ncclFloat64
= 8
-
enumerator
ncclDouble
= 8
-
enumerator
ncclBfloat16
= 9
-
enumerator
ncclNumTypes
= 10
-
enumerator