pto.tget_async¶

Introduction¶

pto.tget_async is an asynchronous remote read primitive. It starts a transfer from remote GM to local GM and returns an AsyncEvent immediately.

Data flow:

srcGlobalData (remote GM) -> DMA engine -> dstGlobalData (local GM)

Template Parameter¶

• engine:
  • DmaEngine::SDMA (default)
  • DmaEngine::URMA (Ascend950, NPU_ARCH 3510 only)

Important (SDMA path) pto.tget_async with DmaEngine::SDMA currently supports only flat contiguous logical 1D tensors. Non-1D or non-contiguous layouts are not supported by the current SDMA async implementation.

C++ Intrinsic¶

Declared in include/pto/comm/pto_comm_inst.hpp.

template <DmaEngine engine = DmaEngine::SDMA,
          typename GlobalDstData, typename GlobalSrcData, typename... WaitEvents>
PTO_INST AsyncEvent GET_ASYNC(GlobalDstData &dstGlobalData, GlobalSrcData &srcGlobalData,
                              const AsyncSession &session, WaitEvents &... events);

AsyncSession is an engine-agnostic session object. Build once with BuildAsyncSession<engine>(), then pass to all async calls and event waits. The template engine parameter selects the DMA backend at compile time, making the code forward-compatible with future engines (CCU, etc.).

AsyncSession Construction¶

Use BuildAsyncSession from include/pto/comm/async_common/async_event_impl.hpp. There are two overloads — one for SDMA and one for URMA — with different parameter lists.

SDMA Construction (default)¶

template <DmaEngine engine = DmaEngine::SDMA, typename ScratchTile>
PTO_INTERNAL bool BuildAsyncSession(ScratchTile &scratchTile,
                                   __gm__ uint8_t *workspace,
                                   AsyncSession &session,
                                   uint32_t syncId = 0,
                                   const sdma::SdmaBaseConfig &baseConfig = {sdma::kDefaultSdmaBlockBytes, 0, 1},
                                   uint32_t channelGroupIdx = sdma::kAutoChannelGroupIdx);

Parameter	Default	Description
scratchTile	—	UB scratch tile for SDMA control metadata (see scratchTile Role).
workspace	—	GM pointer allocated by host-side SdmaWorkspaceManager.
session	—	Output AsyncSession object.
syncId	0	MTE3/MTE2 pipe sync event id (0-7). Override if kernel uses other pipe barriers on the same id.
baseConfig	{kDefaultSdmaBlockBytes, 0, 1}	{block_bytes, comm_block_offset, queue_num}. Suitable for most single-queue transfers.
channelGroupIdx	kAutoChannelGroupIdx	SDMA channel group index. Default uses get_block_idx() internally, mapping to current AI core. Override for multi-block or custom channel mapping scenarios.

URMA Construction (NPU_ARCH 3510 only)¶

URMA (User-level RDMA Memory Access) is a hardware-accelerated RDMA transport available on Ascend950 (NPU_ARCH 3510).

#ifdef PTO_URMA_SUPPORTED
template <DmaEngine engine>
PTO_INTERNAL bool BuildAsyncSession(__gm__ uint8_t *workspace,
                                  uint32_t destRankId,
                                  AsyncSession &session);
#endif

Parameter	Description
workspace	GM pointer allocated by host-side UrmaWorkspaceManager.
destRankId	Remote PE rank id that this session communicates with. For pto.tget_async this is the source rank.
session	Output AsyncSession object.

URMA does not require scratchTile — polling uses ld_dev/st_dev hardware intrinsics directly.

Constraints¶

scratchTile Role¶

scratchTile is not used to hold transferred payload data. It is converted to TmpBuffer and used as temporary UB workspace for:

• writing/reading SDMA control words (flag, sq_tail, channel_info)
• polling event completion flags
• committing queue tail during completion

The real payload path remains remote GM -> DMA engine -> local GM; scratchTile is only for control/synchronization metadata.

scratchTile Type and Size Constraints¶

• must be a pto::Tile type
• must be UB/Vec tile (ScratchTile::Loc == TileType::Vec)
• available bytes must be at least sizeof(uint64_t) (8 bytes)

Recommended: Tile<TileType::Vec, uint8_t, 1, comm::sdma::UB_ALIGN_SIZE> (256B).

Completion Semantics (Quiet Semantics)¶

The completion mechanism differs by engine, but user-facing quiet semantics are identical:

• SDMA: pto.tget_async only submits data transfer SQEs. The flag SQE is deferred to Wait, which polls the flag for completion.

• URMA: pto.tget_async submits an RDMA READ WQE and rings the doorbell immediately. Wait polls the Completion Queue (CQ) until all expected CQEs have been consumed.

• event.Wait(session) — blocks until all async operations issued since the last Wait are complete

This means after multiple pto.tget_async calls, a single Wait on the last returned AsyncEvent drains all pending operations (similar to shmem's quiet semantics).

After wait succeeds, all issued reads into dstGlobalData are complete.

Example¶

Single Transfer¶

#include <pto/comm/pto_comm_inst.hpp>
#include <pto/common/pto_tile.hpp>

using namespace pto;

template <typename T>
__global__ AICORE void SimpleGet(__gm__ T *localDst, __gm__ T *remoteSrc,
                                 __gm__ uint8_t *sdmaWorkspace)
{
    using ShapeDyn = Shape<DYNAMIC, DYNAMIC, DYNAMIC, DYNAMIC, DYNAMIC>;
    using StrideDyn = Stride<DYNAMIC, DYNAMIC, DYNAMIC, DYNAMIC, DYNAMIC>;
    using GT = GlobalTensor<T, ShapeDyn, StrideDyn, Layout::ND>;
    using ScratchTile = Tile<TileType::Vec, uint8_t, 1, comm::sdma::UB_ALIGN_SIZE>;

    ShapeDyn shape(1, 1, 1, 1, 1024);
    StrideDyn stride(1024, 1024, 1024, 1024, 1);
    GT dstG(localDst, shape, stride);
    GT srcG(remoteSrc, shape, stride);

    ScratchTile scratchTile;
    TASSIGN(scratchTile, 0x0);

    comm::AsyncSession session;
    if (!comm::BuildAsyncSession<comm::DmaEngine::SDMA>(scratchTile, sdmaWorkspace, session)) {
        return;
    }

    auto event = comm::GET_ASYNC<comm::DmaEngine::SDMA>(dstG, srcG, session);
    (void)event.Wait(session);
}

Batch Transfer (Quiet Semantics)¶

template <typename T>
__global__ AICORE void BatchGet(__gm__ T *localDstBase, __gm__ T *remoteSrcBase,
                                __gm__ uint8_t *sdmaWorkspace, int nranks)
{
    using ShapeDyn = Shape<DYNAMIC, DYNAMIC, DYNAMIC, DYNAMIC, DYNAMIC>;
    using StrideDyn = Stride<DYNAMIC, DYNAMIC, DYNAMIC, DYNAMIC, DYNAMIC>;
    using GT = GlobalTensor<T, ShapeDyn, StrideDyn, Layout::ND>;
    using ScratchTile = Tile<TileType::Vec, uint8_t, 1, comm::sdma::UB_ALIGN_SIZE>;

    ShapeDyn shape(1, 1, 1, 1, 1024);
    StrideDyn stride(1024, 1024, 1024, 1024, 1);

    ScratchTile scratchTile;
    TASSIGN(scratchTile, 0x0);

    comm::AsyncSession session;
    if (!comm::BuildAsyncSession(scratchTile, sdmaWorkspace, session)) {
        return;
    }

    comm::AsyncEvent lastEvent;
    for (int rank = 0; rank < nranks; ++rank) {
        GT dstG(localDstBase + rank * 1024, shape, stride);
        GT srcG(remoteSrcBase + rank * 1024, shape, stride);
        lastEvent = comm::GET_ASYNC(dstG, srcG, session);
    }
    (void)lastEvent.Wait(session);  // single Wait drains all pending ops
}

URMA Example (NPU_ARCH 3510)¶

#include <pto/comm/pto_comm_inst.hpp>
#include <pto/common/pto_tile.hpp>

using namespace pto;

template <typename T>
__global__ AICORE void SimpleGetUrma(__gm__ T *localDst, __gm__ T *remoteSrc,
                                     __gm__ uint8_t *urmaWorkspace, uint32_t srcRankId)
{
    using ShapeDyn = Shape<DYNAMIC, DYNAMIC, DYNAMIC, DYNAMIC, DYNAMIC>;
    using StrideDyn = Stride<DYNAMIC, DYNAMIC, DYNAMIC, DYNAMIC, DYNAMIC>;
    using GT = GlobalTensor<T, ShapeDyn, StrideDyn, Layout::ND>;

    ShapeDyn shape(1, 1, 1, 1, 1024);
    StrideDyn stride(1024, 1024, 1024, 1024, 1);
    GT dstG(localDst, shape, stride);
    GT srcG(remoteSrc, shape, stride);

    comm::AsyncSession session;
    if (!comm::BuildAsyncSession<comm::DmaEngine::URMA>(urmaWorkspace, srcRankId, session)) {
        return;
    }

    auto event = comm::GET_ASYNC<comm::DmaEngine::URMA>(dstG, srcG, session);
    (void)event.Wait(session);
}