Struct SingleGpuBackend 

pub struct SingleGpuBackend;

2026-09-26: One rank: every collective and point-to-point operation returns Ok without doing anything, rank is 0 and world_size is 1.

Trait Implementations

impl CommBackend for SingleGpuBackend

fn all_reduce(&self, _ptr: u64, _bytes: usize) -> Result<()>

2026-09-26: Sum bytes at ptr across all ranks, in place; every rank gets the result. NcclBackend reduces BF16 elements.

fn all_gather( &self, _send_ptr: u64, _recv_ptr: u64, _bytes: usize, ) -> Result<()>

2026-09-26: Each rank contributes bytes from send_ptr; every rank receives all contributions, in rank order, at recv_ptr.

fn reduce_scatter( &self, _send_ptr: u64, _recv_ptr: u64, _bytes: usize, ) -> Result<()>

2026-09-26: Sum across ranks and give each rank its bytes-sized share at recv_ptr. NcclBackend sums u8 elements.

fn broadcast(&self, _ptr: u64, _bytes: usize, _root: usize) -> Result<()>

2026-09-26: Copy bytes at ptr from rank root to every rank, in place.

fn barrier(&self) -> Result<()>

2026-09-26: A barrier across ranks. NcclBackend enqueues a zero-count all-reduce on its stream and returns without waiting for it.

fn send_to( &self, _ptr: u64, _bytes: usize, _dest_rank: usize, _stream: u64, ) -> Result<()>

2026-09-26: Send bytes at ptr to dest_rank, enqueued on stream. Pairs with a recv_from of the same size on dest_rank.

fn recv_from( &self, _ptr: u64, _bytes: usize, _src_rank: usize, _stream: u64, ) -> Result<()>

2026-09-26: Receive bytes into ptr from src_rank, enqueued on stream. Pairs with a send_to of the same size on src_rank.

fn rank(&self) -> usize

2026-09-26: This rank’s index, from 0.

fn world_size(&self) -> usize

2026-09-26: Number of ranks.

fn recv_command_u32(&self, ptr: u64, root: usize) -> Result<()>

2026-09-26: On a non-root rank, receive the first u32 of the next worker command from root, the receive side of a 4-byte broadcast. Unlike broadcast, it may wait without a deadline, so an implementation must keep checking for transport errors while it waits. The command’s later words use broadcast. The default refuses root and otherwise calls broadcast(ptr, 4, root).

fn all_reduce_async( &self, ptr: u64, bytes: usize, compute_stream: u64, ) -> Result<()>

2026-09-26: All-reduce ordered against compute_stream on the device: the reduce starts after the work already queued there and later work there waits for it. The host does not wait. The default ignores compute_stream and calls all_reduce.

fn register_buffer(&self, _ptr: u64, _bytes: usize) -> Result<u64>

2026-09-26: Register a device buffer with the backend and return an opaque handle (ncclCommRegister in NcclBackend). The default registers nothing and returns 0.

fn deregister_buffer(&self, _handle: u64) -> Result<()>

2026-09-26: Deregister a handle from register_buffer.

fn symmetric_alloc(&self, _bytes: usize) -> Result<u64>

2026-09-26: Allocate bytes of device memory from the backend’s allocator (ncclMemAlloc in NcclBackend) and return the pointer. The default returns Err.

fn symmetric_free(&self, _ptr: u64) -> Result<()>

2026-09-26: Free a pointer from symmetric_alloc. The default returns Err.

fn set_add_kernel(&self, _handle: u64)

2026-09-26: Hand the backend the bf16_add_inplace kernel handle. The model engine looks it up and passes it after registering its buffers. NcclBackend takes its 2-rank send/recv all-reduce path only once it is set.

fn group_start(&self) -> Result<()>

2026-09-26: Open a group: NcclBackend calls ncclGroupStart, so the operations up to group_end are submitted together. The default does nothing.

fn group_end(&self) -> Result<()>

2026-09-26: Close the group opened by group_start.

fn is_healthy(&self) -> bool

2026-09-26: false after the backend has seen a transport error or a completion timeout, until a successful attempt_reconnect. NcclBackend also queries ncclCommGetAsyncError. The default returns true.

fn attempt_reconnect(&self) -> Result<()>

2026-09-26: Replace a failed communicator. NcclBackend aborts it and repeats the TCP bootstrap, so every rank must call this at the same time. The default does nothing.