Using NCCL Net Plugin

NCCL provides a way to use external plugins to let NCCL run on many network types. This topic describes the NCCL Net plugin API and how to implement a network plugin for NCCL.

Plugins implement the NCCL network API, and decouple NCCL binary builds which are built against a particular version of the GPU stack (i.e. CUDA) from the network code which is built against a particular version of the networking stack. That way, you can easily integrate any CUDA version with any network stack version.

NCCL network plugins come as a shared library called libnccl-net.so. That shared library contains one or more implementations of the NCCL NET API, in the form of versioned structs, filled with pointers to all required functions.

Plugin architecture

When NCCL is initialized, it will look for a libnccl-net.so library and dynamically load it, then look for symbols inside the library.

The NCCL_NET_PLUGIN environment variable allows multiple plugins to coexist. If set, NCCL will look for a library with a name of libnccl-net-${NCCL_NET_PLUGIN}.so. It is therefore advised to name the library following that pattern, with a symlink pointing libnccl-net.so to libnccl-net-${NCCL_NET_PLUGIN}.so. That way, if there are multiple plugins in the path, setting NCCL_NET_PLUGIN will allow users to select the right plugin.

Struct versioning

Once a library is found, NCCL will look for a symbol named ncclNet_vX, with X increasing over time. The versioning ensures that the plugin and the NCCL core are compatible.

Plugins are encouraged to provide multiple of those symbols, implementing multiple versions of the NCCL NET API, so that the same plugin can be compiled and support a wide range of NCCL versions.

Conversely, and to ease transition, NCCL can choose to support different plugin versions, looking for the latest ncclNet struct version, but also looking for older ones so that older plugins would still work.

In-network collective operations (collNet)

Additionally to the ncclNet structure, network plugins can provide a collNet structure which implements in-network collective operations, if supported. That can be used by the NCCL collNet algorithm to accelerate inter-node reductions in allReduce.

The collNet struct is a different, optional struct provided by the network plugin, but its versioning is tied to the ncclNet struct and many functions are common between the two to ease the implementation.

Headers management

To help users build plugins effortlessly, plugins should copy the ncclNet_vX definitions they support to their internal includes. An example is shown in ext-net/example/ where we keep all headers in the nccl/ directory and provide thin layers to implement old versions on top of newer ones.

The nccl/ directory is populated with net_vX.h files extracting all relevant definitions from old API versions. It also provides error codes in err.h.

API (v6)

Below is the main ncclNet_v6 struct. Each function is explained in later sections.

typedef struct {
// Name of the network (mainly for logs)
const char* name;
// Initialize the network.
ncclResult_t (*init)(ncclDebugLogger_t logFunction);
// Return the number of adapters.
ncclResult_t (*devices)(int* ndev);
// Get various device properties.
ncclResult_t (*getProperties)(int dev, ncclNetProperties_v6_t* props);
// Create a receiving object and provide a handle to connect to it. The
// handle can be up to NCCL_NET_HANDLE_MAXSIZE bytes and will be exchanged
// between ranks to create a connection.
ncclResult_t (*listen)(int dev, void* handle, void** listenComm);
// Connect to a handle and return a sending comm object for that peer.
// This call must not block for the connection to be established, and instead
// should return successfully with sendComm == NULL with the expectation that
// it will be called again until sendComm != NULL.
ncclResult_t (*connect)(int dev, void* handle, void** sendComm);
// Finalize connection establishment after remote peer has called connect.
// This call must not block for the connection to be established, and instead
// should return successfully with recvComm == NULL with the expectation that
// it will be called again until recvComm != NULL.
ncclResult_t (*accept)(void* listenComm, void** recvComm);
// Register/Deregister memory. Comm can be either a sendComm or a recvComm.
// Type is either NCCL_PTR_HOST or NCCL_PTR_CUDA.
ncclResult_t (*regMr)(void* comm, void* data, int size, int type, void** mhandle);
/* DMA-BUF support */
ncclResult_t (*regMrDmaBuf)(void* comm, void* data, size_t size, int type, uint64_t offset, int fd, void** mhandle);
ncclResult_t (*deregMr)(void* comm, void* mhandle);
// Asynchronous send to a peer.
// May return request == NULL if the call cannot be performed (or would block)
ncclResult_t (*isend)(void* sendComm, void* data, int size, int tag, void* mhandle, void** request);
// Asynchronous recv from a peer.
// May return request == NULL if the call cannot be performed (or would block)
ncclResult_t (*irecv)(void* recvComm, int n, void** data, int* sizes, int* tags, void** mhandles, void** request);
// Perform a flush/fence to make sure all data received with NCCL_PTR_CUDA is
// visible to the GPU
ncclResult_t (*iflush)(void* recvComm, int n, void** data, int* sizes, void** mhandles, void** request);
// Test whether a request is complete. If size is not NULL, it returns the
// number of bytes sent/received.
ncclResult_t (*test)(void* request, int* done, int* sizes);
// Close and free send/recv comm objects
ncclResult_t (*closeSend)(void* sendComm);
ncclResult_t (*closeRecv)(void* recvComm);
ncclResult_t (*closeListen)(void* listenComm);
} ncclNet_v6_t;

Error codes

All plugins functions use NCCL error codes as return value. ncclSuccess should be returned upon success.

Otherwise, plugins can return one of the following:

• ncclSystemError is the most common error for network plugins, when a call to the linux kernel or a system library fails. This typically includes all network/hardware errors.

• ncclInternalError is returned when the NCCL core code is using the network plugin in an incorrect way, for example allocating more requests than it should, or passing an invalid argument to calls.

• ncclInvalidUsage should be returned when the error is most likely a user error. This can include misconfiguration, but also sizes mismatch.

• ncclInvalidArgument should usually not be used by plugins since arguments should be checked by the NCCL core layer.

• ncclUnhandledCudaError is returned when an error comes from CUDA. Since network plugins should not need to rely on CUDA, this should not be common.

Operation overview

NCCL will call the init function first, then query the number of network devices with the devices function, getting each network device properties with getProperties.

To establish a connection between two network devices, NCCL will first call listen on the receiving side, pass the returned handle to the sender side of the connection, and call connect with that handle. Finally, accept will be called on the receiving side to finalize the connection establishment.

Once the connection is established, communication will be done using the functions isend, irecv and test. Prior to calling isend or irecv, NCCL will call the regMr function on all buffers to allow RDMA NICs to prepare buffers. deregMr will be used to unregister buffers.

In certain conditions, iflush will be called after a receive calls completes to allow the network plugin to flush data and ensure the GPU will observe the newly written data.

To close the connections NCCL will call closeListen to close the object returned by listen, closeSend to close the object returned by connect and closeRecv to close the object returned by accept.

API Functions

Initialization

• name - The name field should point to a character string with the name of the network plugin. This will be used for all logging, especially when NCCL_DEBUG=INFO is set.

Note

Setting NCCL_NET=<plugin name> will ensure a specific network implementation is used, with a matching name. This is not to be confused with NCCL_NET_PLUGIN which defines a suffix to the libnccl-net.so library name to load.

• init - As soon as NCCL finds the plugin and the correct ncclNet symbol, it will call the init function. This will allow the plugin to discover network devices and make sure they are usable. If the init function does not return ncclSuccess, then NCCL will not use the plugin and fall back on internal ones.

  To allow the plugin logs to integrate into the NCCL logs seemlessly, NCCL provides a logging function to init. This function is typically used to allow for INFO and WARN macros within the plugin code adding the following definitions:

#define WARN(...) logFunction(NCCL_LOG_WARN, NCCL_ALL, __FILE__, __LINE__, __VA_ARGS__)
#define INFO(FLAGS, ...) logFunction(NCCL_LOG_INFO, (FLAGS), __func__, __LINE__, __VA_ARGS__)

• devices - Once the plugin is initialized, NCCL will query the number of devices available. It should not be zero, otherwise NCCL initialization will fail. If no device is present or usable, the init function should not return ncclSuccess.

• getProperties - Right after getting the number of devices, NCCL will query properties for each available network device. These properties are critical when multiple adapters are present to ensure NCCL uses each adapter in the most optimized way.

The name is only used for logging.

The pciPath is the base for all topology detection and should point to the PCI device directory in /sys. This is typically the directory pointed by /sys/class/net/eth0/device or /sys/class/infiniband/mlx5_0/device. If the network interface is virtual, then pciPath should be NULL.

The guid field is used to determine when network adapters are connected to multiple PCI endpoints. For normal cases, it can be set to the device number. If multiple network devices have the same guid, then NCCL will consider the are sharing the same network port to the fabric, hence it will not use the port multiple times.

The ptrSupport field indicates whether or not CUDA pointers are supported. If so, it should be set to NCCL_PTR_HOST``|``NCCL_PTR_CUDA, otherwise it should be set to NCCL_PTR_HOST. If the plugin supports dmabuf, it should set ptrSupport to NCCL_PTR_HOST``|``NCCL_PTR_CUDA``|``NCCL_PTR_DMABUF and provide a regMrDmaBuf function.

The speed field indicates the speed of the network port in Mbps (10^6 bits per second). This is important to ensure proper optimization of flows within the node.

The port field indicates the port number. This is important again for topology detection and flow optimization within the node when a NIC with a single PCI connection is connected to the fabric with multiple ports.

The latency field indicates the network latency in microseconds. This can be useful to improve the NCCL tuning and make sure NCCL switches from tree to ring at the right size.

The maxComms field indicates the maximum number of connections we can create.

The maxRecvs field indicates the maximum number for grouped receive operations (see grouped receive).

Connection establishment

Connections are used in an unidirectional manner. There is therefore a sender side and a receiver side.

• listen - To create a connection, NCCL will start by calling listen on the receiver side. This function takes a device number as input argument, and should return a local listenComm object, and a handle to pass to the other side, so that the sender side can connect to the receiver.

  The handle is a buffer of size NCCL_NET_HANDLE_MAXSIZE and is provided by NCCL.

  This call should never block, but contrary to connect and accept, listenComm should never be NULL if the call succeeds.

• connect - NCCL will use its bootstrap infrastructure to provide the handle to the sender side, then call connect on the sender side on a given device index dev, providing the handle. connect should not block either, and instead set sendComm to NULL and return ncclSuccess. In that case, NCCL will call accept again until it succeeds.

• accept - To finalize the connection, the receiver side will call accept on the listenComm returned by the listen call previously. If the sender did not connect yet, accept should not block. It should return ncclSuccess, setting recvComm to NULL. NCCL will call accept again until it succeeds.

• closeListen/closeSend/closeRecv - Once a listenComm/sendComm/recvComm is no longer needed, NCCL will call closeListen/closeSend/closeRecv to free the associated resources.

Communication

Communication is done using asynchronous send and receive operations: isend, irecv and test. To support RDMA capabilities, buffer registration and flush functions are provided.

To keep track of asynchronous send, receive and flush operations, requests are returned to NCCL, then queried with test. Each sendComm or recvComm must be able to handle NCCL_NET_MAX_REQUESTS requests in parallel.

Note

That value should be multiplied by the multi-receive capability of the plugin for the sender side, so that we can effectively have NCCL_NET_MAX_REQUESTS multi-receive operations happening in parallel. So, if we have a maxRecvs`value of 8 and ``NCCL_NET_MAX_REQUESTS` is 8, then each sendComm must be able to handle up to 8x8=64 concurrent isend operations.

• regMr - Prior to sending or receiving data, NCCL will call regMr with any buffers later used for communication. It will provide a sendComm or recvComm as comm argument, then the buffer pointer data, size, and type being either NCCL_PTR_HOST, or NCCL_PTR_CUDA if the network supports CUDA pointers.

  The network plugin can use the output argument mhandle to keep any reference to that memory registration, as this mhandle will be passed back for all isend, irecv, iflush and deregMr calls.

• regMrDmaBuf - If the plugin has set the NCCL_PTR_DMABUF property in ptrSupport, NCCL will use regMrDmaBuf instead of regMr. If the property was not set, regMrDmaBuf can be set to NULL.

• deregMr - When buffers will no longer be used for communication, NCCL will call deregMr to let the plugin free resources. This function is used to deregister handles returned by both regMr and regMrDmaBuf.

• isend - Data will be sent through the connection using isend, passing the sendComm previously created by connect, and the buffer described by data, size, and mhandle. A tag must be used if the network supports multi-receive operations (see irecv) to distinguish between different sends matching the same multi-receive. Otherwise it can be set to 0.

  The isend operation returns a handle in the request argument for further calls to test. If the isend operation cannot be initiated, request can be set to NULL and NCCL will call isend again later.

• irecv - To receive data, NCCL will call irecv with the recvComm returned by accept. The argument n will allow NCCL to perform a multi-receive, to allow grouping of multiple sends through a single network connection. Each buffer will be described by the data, sizes, and mhandles arrays. tags will specify a tag for each receive so that each of the n independent isend operations is received into the right buffer.

  If all receive operations can be initiated, irecv will return a handle in the request pointer, otherwise it will set it to NULL. In the case of multi-receive, all n receive operations are handled by a single request handle.

  The sizes provided to irecv can (and will) be larger than the size of the isend operation. However, if the receive size is smaller than the send size this is an error.

Note

For a given connection, send/receive operations should always match in the order they were posted. Tags provided for receive operations are only used to assign a given send operation to one of the buffers of the first (multi-)receive in the queue, not to allow for out-of-order tag matching on any receive operation posted.

• test - After an isend or irecv operation is initiated, NCCL will call test on the request handles until they complete. When that happens, done will be set to 1 and sizes will be set to the real size sent or received, the latter being potentially lower than the size passed to irecv.

  In the case of a multi-receive, all receives will be considered as done as a single operation (the goal being to allow aggregation), hence they share a single request and a single done status. However, they can have different sizes, so when done is non-zero, the sizes array should contain the n sizes corresponding to the buffers passed to irecv.

  Once test returns 1 in done, the request handle can be freed, meaning that NCCL will never call test again on that request (until it is reallocated by another call to isend or irecv).

• iflush - After a receive operation completes, if the operation was targeting GPU memory and received a non-zero number of bytes, NCCL will call iflush to let the network flush any buffer and ensure the GPU can read it right after without seeing stale data. This flush operation is decoupled from the test code to improve latency of LL* protocols, as those are capable of determining when data is valid or not.

  iflush returns a request which needs to be queried with test until it completes.