Device API – GIN

GIN

Device functions. The following are callable from device (GPU) code only. GIN is supported since NCCL 2.28.7.

ncclGin

class ncclGin

A class encompassing major elements of the GIN support.

ncclGin(ncclDevComm const &comm, int contextIndex)

Initializes a new ncclGin object. comm is the device communicator created using ncclDevCommCreate(). contextIndex is the index of the GIN context – a network communication channel. Using multiple GIN contexts allows the implementation to spread traffic onto multiple connections, avoiding locking and bottlenecks. Therefore, performance-oriented kernels should cycle among the available contexts to improve resource utilization (the number of available contexts is available via ginContextCount).

void put(ncclTeam team, int peer, ncclWindow_t dstWnd, size_t dstOffset, ncclWindow_t srcWnd, size_t srcOffset, size_t bytes, RemoteAction remoteAction, LocalAction localAction, Coop coop, DescriptorSmem descriptor, cuda::thread_scope alreadyReleased, cuda::thread_scope expected_scope)

Schedules a device-initiated, one-sided data transfer operation from a local buffer to a remote buffer on a peer.

peer is a rank within team (see Teams); it may refer to the local rank (a loopback). The destination and source buffers are each specified using the window (dstWnd, srcWnd) and a byte-based offset (dstOffset, srcOffset). bytes specifies the data transfer count in bytes. If GIN is initialized with connection type NCCL_GIN_CONNECTION_RAIL, peer must be within the same rail team as the local rank.

Arguments beyond the first seven are optional. remoteAction and localAction specify actions to undertake on the destination peer and on the local rank when the payload has been settled and the input has been consumed (respectively). They default to ncclGin_None (no action); other options include ncclGin_Signal{Inc|Add} (for remoteAction) and ncclGin_CounterInc (for localAction); see Signals and Counters below for more details. coop indicates the set of threads participating in this operation (see Thread Groups); it defaults to ncclCoopThread (a single device thread), which is the recommended model.

The visibility of the signal on the destination peer implies the visibility of the put data it is attached to and all the preceding puts to the same peer, provided that they were issued using the same GIN context.

The API also defines an alternative, “convenience” variant of this method that uses ncclSymPtr types to specify the buffers and expects size to be conveyed in terms of the number of elements instead of the byte count. There are also two putValue variants that take a single element at a time (no greater than eight bytes), passed by value.

void flush(Coop coop, cuda::memory_order ord = cuda::memory_order_acquire)

Ensures that all the pending transfer operations scheduled by any threads of coop are locally consumed, meaning that their source buffers are safe to reuse. Makes no claims regarding the completion status on the remote peer(s).

Signals and Counters

type ncclGinSignal_t

Signals are used to trigger actions on remote peers, most commonly on the completion of a ncclGin::put() operation. They each have a 64-bit integer value associated with them that can be manipulated atomically.

struct ncclGin_SignalAdd

ncclGinSignal_t signal

uint64_t value

struct ncclGin_SignalInc

ncclGinSignal_t signal

These objects can be passed as the remoteAction arguments of methods such as ncclGin::put() and ncclGin::signal() to describe the actions to perform on the peer on receipt – in this case, increase the value of a signal specified by index. ncclGin_SignalInc{signalIdx} is functionally equivalent to ncclGin_SignalAdd{signalIdx, 1}; however, it may not be mixed with other signal-modifying operations without an intervening signal reset (see below). Signal values use “rolling” comparison logic to ensure that an unsigned overflow maintains the property of x < x + 1.

struct ncclGin_VASignalInc

ncclWindow_t signalWindow

size_t signalOffset

struct ncclGin_VASignalAdd

ncclWindow_t signalWindow

size_t signalOffset

uint64_t value

These objects represent “VA signals”: signals that are located at an arbitrary VA (window and offset pair) instead of a pre-allocated signal index. Like the ncclGin_SignalInc and ncclGinSignalAdd objects, these objects can be passed as the remoteAction arguments of methods such as ncclGin::put() and ncclGin::signal() to increment a signal on the peer. To use a VA signal, the window must be registered with flags NCCL_WIN_COLL_STRICT_ORDERING. When an address is used as a signal, all reads and writes to the address must be issued via GIN (i.e., a RemoteAction or GIN signal method).

Signal methods of ncclGin:

void ncclGin::signal(ncclTeam team, int peer, RemoteAction remoteAction, Coop coop, DescriptorSmem descriptor, cuda::thread_scope alreadyReleased, cuda::thread_scope expected_scope)

uint64_t ncclGin::readSignal(ncclGinSignal_t signal, int bits = 64, cuda::memory_order ord = cuda::memory_order_acquire)

void ncclGin::waitSignal(Coop coop, ncclGinSignal_t signal, uint64_t least, int bits = 64, cuda::memory_order ord = cuda::memory_order_acquire)

void ncclGin::resetSignal(ncclGinSignal_t signal)

These are signal-specific methods of ncclGin. ncclGin::signal() implements an explicit signal notification without an accompanying data transfer operation; it takes a subset of arguments of ncclGin::put(). ncclGin::readSignal() returns the bottom bits of the value of the signal. ncclGin::waitSignal() waits for the bottom bits of the signal value to meet or exceed least. Finally, ncclGin::resetSignal() resets the signal value to 0 (this method may not race with concurrent modifications to the signal).

uint64_t ncclGin::readSignal(ncclWindow_t signalWindow, size_t signalOffset, int bits = 64, cuda::memory_order ord = cuda::memory_order_acquire)

void ncclGin::waitSignal(Coop coop, ncclWindow_t signalWindow, size_t signalOffset, uint64_t least, int bits = 64, cuda::memory_order ord = cuda::memory_order_acquire)

void ncclGin::resetSignal(ncclWindow_t signalWindow, size_t signalOffset)

These are VA signal-specific methods of ncclGin.

type ncclGinCounter_t

Counters are used to trigger actions on the local rank; as such, they are complementary to signals, which are meant for remote actions. Like signals, they use “rolling” comparison logic, but they are limited to storing values of at most 56 bits.

struct ncclGin_CounterInc

ncclGinCounter_t counter

This object can be passed as the localAction argument of methods such as ncclGin::put(). It is the only action defined for counters.

Counter methods of ncclGin:

uint64_t ncclGin::readCounter(ncclGinCounter_t counter, int bits = 56, cuda::memory_order ord = cuda::memory_order_acquire)

void ncclGin::waitCounter(Coop coop, ncclGinCounter_t counter, uint64_t least, int bits = 56, cuda::memory_order ord = cuda::memory_order_acquire)

void ncclGin::resetCounter(ncclGinCounter_t counter)

These are counter-specific methods of ncclGin and they are functionally equivalent to their signal counterparts discussed above.

ncclGinBarrierSession

template<typename Coop>
class ncclGinBarrierSession

A class representing a network barrier session.

ncclGinBarrierSession(Coop coop, ncclGin gin, ncclTeamTagRail tag, uint32_t index)

Initializes a new network barrier session. coop represents a cooperative group (see Thread Groups). gin is a previously initialized ncclGin object. ncclTeamTagRail indicates that the barrier will apply to all peers on the same rail as the local rank (see Teams). index identifies the underlying barrier to use (it should be different for each coop; typically set to blockIdx.x to ensure uniqueness between CTAs).

ncclGinBarrierSession(Coop coop, ncclGin gin, ncclTeam team, ncclGinBarrierHandle handle, uint32_t index)

Initializes a new network barrier session. This is the general-purpose variant to be used, e.g., when communicating with ranks from the world team (see Teams), whereas the previous variant was specific to the rail team. This variant expects team to be passed as an argument, and also takes an extra handle argument indicating the location of the underlying barriers (typically set to the railGinBarrier field of the device communicator).

void sync(Coop coop, cuda::memory_order order, ncclGinFenceLevel fence)

Synchronizes all threads of all team members that participate in the barrier session. ncclGinFenceLevel::Relaxed is the only defined value for fence for now.