custom_all_reduce.cuh

#pragma once

#include <cuda.h>
#include <cuda_bf16.h>
#include <cuda_fp16.h>
#include <cuda_runtime.h>

#include <iostream>
#include <limits>
#include <map>
#include <unordered_map>
#include <vector>

#define CUDACHECK(cmd)                                              \
  do {                                                              \
    cudaError_t e = cmd;                                            \
    if (e != cudaSuccess) {                                         \
      printf("Failed: Cuda error %s:%d '%s'\n", __FILE__, __LINE__, \
             cudaGetErrorString(e));                                \
      exit(EXIT_FAILURE);                                           \
    }                                                               \
  } while (0)

namespace vllm {

constexpr int kMaxBlocks = 64;
// note: we don't want to use atomics for signals because peer atomics are no
// supported on PCIe links
struct Signal {
  alignas(128) uint32_t start[kMaxBlocks][8];
  alignas(128) uint32_t end[kMaxBlocks][8];
};

struct __align__(16) RankData { const void *__restrict__ ptrs[8]; };

struct __align__(16) RankSignals { volatile Signal *signals[8]; };

// like std::array, but aligned
template <typename T, int sz>
struct __align__(alignof(T) * sz) array_t {
  T data[sz];
  using type = T;
  static constexpr int size = sz;
};

// use packed type to maximize memory efficiency
// goal: generate ld.128 and st.128 instructions
template <typename T>
struct packed_t {
  // the (P)acked type for load/store
  using P = array_t<T, 16 / sizeof(T)>;
  // the (A)ccumulator type for reduction
  using A = array_t<float, 16 / sizeof(T)>;
};

#define DINLINE __device__ __forceinline__

// scalar cast functions
DINLINE float upcast_s(half val) { return __half2float(val); }

template <typename T>
DINLINE T downcast_s(float val);
template <>
DINLINE half downcast_s(float val) {
  return __float2half(val);
}

// scalar add functions
// for some reason when compiling with Pytorch, the + operator for half and
// bfloat is disabled so we call the intrinsics directly
DINLINE half &assign_add(half &a, half b) {
  a = __hadd(a, b);
  return a;
}
DINLINE float &assign_add(float &a, float b) { return a += b; }

#if (__CUDA_ARCH__ >= 800 || !defined(__CUDA_ARCH__))
DINLINE float upcast_s(nv_bfloat16 val) { return __bfloat162float(val); }
template <>
DINLINE nv_bfloat16 downcast_s(float val) {
  return __float2bfloat16(val);
}
DINLINE nv_bfloat16 &assign_add(nv_bfloat16 &a, nv_bfloat16 b) {
  a = __hadd(a, b);
  return a;
}
#endif

template <typename T, int N>
DINLINE array_t<T, N> &packed_assign_add(array_t<T, N> &a, array_t<T, N> b) {
#pragma unroll
  for (int i = 0; i < N; i++) {
    assign_add(a.data[i], b.data[i]);
  }
  return a;
}

template <typename T, int N>
DINLINE array_t<float, N> upcast(array_t<T, N> val) {
  if constexpr (std::is_same<T, float>::value) {
    return val;
  } else {
    array_t<float, N> out;
#pragma unroll
    for (int i = 0; i < N; i++) {
      out.data[i] = upcast_s(val.data[i]);
    }
    return out;
  }
}

template <typename O>
DINLINE O downcast(array_t<float, O::size> val) {
  if constexpr (std::is_same<typename O::type, float>::value) {
    return val;
  } else {
    O out;
#pragma unroll
    for (int i = 0; i < O::size; i++) {
      out.data[i] = downcast_s<typename O::type>(val.data[i]);
    }
    return out;
  }
}

// This function is meant to be used as the first synchronization in the all
// reduce kernel. Thus, it doesn't need to make any visibility guarantees for
// prior memory accesses. Note: volatile writes will not be reordered against
// other volatile writes.
template <int ngpus>
DINLINE void start_sync(const RankSignals &sg, volatile Signal *self_sg,
                        int rank) {
  if (threadIdx.x < ngpus) {
    // reset flag for next time
    self_sg->end[blockIdx.x][threadIdx.x] = 0;
    // simultaneously write to the corresponding flag of all ranks.
    // Latency = 1 p2p write
    sg.signals[threadIdx.x]->start[blockIdx.x][rank] = 1;
    // wait until we got true from all ranks
    while (!self_sg->start[blockIdx.x][threadIdx.x])
      ;
  }
  __syncthreads();
}

// This function is meant to be used as the second or the final synchronization
// barrier in the all reduce kernel. If it's the final synchronization barrier,
// we don't need to make any visibility guarantees for prior memory accesses.
template <int ngpus, bool final_sync = false>
DINLINE void end_sync(const RankSignals &sg, volatile Signal *self_sg,
                      int rank) {
  __syncthreads();
  // eliminate the case that prior writes are not visible after signals become
  // visible. Note that I did not managed to make this happen through a lot of
  // testing. Might be the case that hardware provides stronger guarantee than
  // the memory model. 
  if constexpr (!final_sync) __threadfence_system();
  if (threadIdx.x < ngpus) {
    // reset flag for next time
    self_sg->start[blockIdx.x][threadIdx.x] = 0;
    // simultaneously write to the corresponding flag of all ranks.
    // Latency = 1 p2p write
    sg.signals[threadIdx.x]->end[blockIdx.x][rank] = 1;
    // wait until we got true from all ranks
    while (!self_sg->end[blockIdx.x][threadIdx.x])
      ;
  }
  if constexpr (!final_sync) __syncthreads();
}

template <typename P, int ngpus, typename A>
DINLINE P packed_reduce(const P *ptrs[], int idx) {
  A tmp = upcast(ptrs[0][idx]);
#pragma unroll
  for (int i = 1; i < ngpus; i++) {
    packed_assign_add(tmp, upcast(ptrs[i][idx]));
  }
  return downcast<P>(tmp);
}

template <typename T, int ngpus>
__global__ void __launch_bounds__(512, 1)
    cross_device_reduce_1stage(RankData *_dp, RankSignals sg,
                               volatile Signal *self_sg, T *__restrict__ result,
                               int rank, int size) {
  using P = typename packed_t<T>::P;
  using A = typename packed_t<T>::A;
  // note: we don't reorder the address so the accumulation order is the same
  // for all ranks, ensuring bitwise identical results
  auto dp = *_dp;
  start_sync<ngpus>(sg, self_sg, rank);
  // do the actual reduction
  for (int idx = blockIdx.x * blockDim.x + threadIdx.x; idx < size;
       idx += gridDim.x * blockDim.x) {
    ((P *)result)[idx] =
        packed_reduce<P, ngpus, A>((const P **)&dp.ptrs[0], idx);
  }
  end_sync<ngpus, true>(sg, self_sg, rank);
}

template <typename P>
DINLINE P *get_tmp_buf(volatile Signal *sg) {
  return (P *)(((Signal *)sg) + 1);
}

template <typename T, int ngpus>
__global__ void __launch_bounds__(512, 1)
    cross_device_reduce_2stage(RankData *_dp, RankSignals sg,
                               volatile Signal *self_sg, T *__restrict__ result,
                               int rank, int size) {
  int tid = blockIdx.x * blockDim.x + threadIdx.x;
  int stride = gridDim.x * blockDim.x;
  using P = typename packed_t<T>::P;
  using A = typename packed_t<T>::A;
  int part = size / ngpus;
  int start = rank * part;
  int end = rank == ngpus - 1 ? size : start + part;
  int largest_part = part + size % ngpus;
  const P *ptrs[ngpus];
  P *tmps[ngpus];
#pragma unroll
  for (int i = 0; i < ngpus; i++) {
    int target = (rank + i) % ngpus;
    ptrs[i] = (const P *)_dp->ptrs[target];
    tmps[i] = get_tmp_buf<P>(sg.signals[target]);
  }
  auto tmp_out = tmps[0];
  start_sync<ngpus>(sg, self_sg, rank);
  // stage 1: reduce scatter
  for (int idx = start + tid; idx < end; idx += stride) {
    tmp_out[idx - start] = packed_reduce<P, ngpus, A>(ptrs, idx);
  }
  end_sync<ngpus>(sg, self_sg, rank);

  // stage 2: allgather. Note: it's important to match the tid between
  // the two stages, because visibility across devices is only guaranteed
  // between threads that have the same tid. If thread i computes the sum of
  // start + i in the first stage, then thread i also gathers start + i from all
  // ranks.
  for (int idx = tid; idx < largest_part; idx += stride) {
#pragma unroll
    for (int i = 0; i < ngpus; i++) {
      int gather_from_rank = ((rank + i) % ngpus);
      if (gather_from_rank == ngpus - 1 || idx < part) {
        int dst_idx = gather_from_rank * part + idx;
        ((P *)result)[dst_idx] = tmps[i][idx];
      }
    }
  }
}

using IPC_KEY = std::array<uint8_t, sizeof(cudaIpcMemHandle_t)>;
static_assert(sizeof(IPC_KEY) == sizeof(cudaIpcMemHandle_t));
static_assert(alignof(IPC_KEY) == alignof(cudaIpcMemHandle_t));

class CustomAllreduce {
 public:
  int rank_;
  int world_size_;
  bool full_nvlink_;

  // below are device pointers
  RankSignals sg_;
  std::unordered_map<void *, RankData *> buffers_;
  Signal *self_sg_;

  // stores the registered device pointers from all ranks
  RankData *d_rank_data_base_, *d_rank_data_end_;
  std::vector<void *> graph_unreg_buffers_;
  // a map from IPC handles to opened IPC pointers
  std::map<IPC_KEY, char *> ipc_handles_;

  /**
   * meta is a pointer to device metadata and temporary buffer for allreduce.
   *
   * There's a total of sizeof(Signal) of prefix before the actual data,
   * so meta + 1 points to actual temporary buffer.
   *
   * note: this class does not own any device memory. Any required buffers
   * are passed in from the constructor
   */
  CustomAllreduce(Signal *meta, void *rank_data, size_t rank_data_sz,
                  const cudaIpcMemHandle_t *handles,
                  const std::vector<int64_t> &offsets, int rank,
                  bool full_nvlink = true)
      : rank_(rank),
        world_size_(offsets.size()),
        full_nvlink_(full_nvlink),
        self_sg_(meta),
        d_rank_data_base_(reinterpret_cast<RankData *>(rank_data)),
        d_rank_data_end_(d_rank_data_base_ + rank_data_sz / sizeof(RankData)) {
    for (int i = 0; i < world_size_; i++) {
      Signal *rank_sg;
      if (i != rank_) {
        char *handle = open_ipc_handle(&handles[i]);
        handle += offsets[i];
        rank_sg = (Signal *)handle;
      } else {
        rank_sg = self_sg_;
      }
      sg_.signals[i] = rank_sg;
    }
  }

  char *open_ipc_handle(const void *ipc_handle) {
    auto [it, new_handle] =
        ipc_handles_.insert({*((IPC_KEY *)ipc_handle), nullptr});
    if (new_handle) {
      char *ipc_ptr;
      CUDACHECK(cudaIpcOpenMemHandle((void **)&ipc_ptr,
                                     *((const cudaIpcMemHandle_t *)ipc_handle),
                                     cudaIpcMemLazyEnablePeerAccess));
      it->second = ipc_ptr;
    }
    return it->second;
  }

  std::pair<std::vector<uint8_t>, std::vector<int64_t>>
  get_graph_buffer_ipc_meta() {
    auto num_buffers = graph_unreg_buffers_.size();
    auto handle_sz = sizeof(cudaIpcMemHandle_t);
    std::vector<uint8_t> handles(handle_sz * num_buffers, 0);
    std::vector<int64_t> offsets(num_buffers);
    for (int i = 0; i < num_buffers; i++) {
      auto ptr = graph_unreg_buffers_[i];
      void *base_ptr;
      // note: must share the base address of each allocation, or we get wrong
      // address
      if (cuPointerGetAttribute(&base_ptr,
                                CU_POINTER_ATTRIBUTE_RANGE_START_ADDR,
                                (CUdeviceptr)ptr) != CUDA_SUCCESS)
        throw std::runtime_error("failed to get pointer attr");
      CUDACHECK(cudaIpcGetMemHandle(
          (cudaIpcMemHandle_t *)&handles[i * handle_sz], base_ptr));
      offsets[i] = ((char *)ptr) - ((char *)base_ptr);
    }
    return std::make_pair(handles, offsets);
  }

  void check_rank_data_capacity(size_t num = 1) {
    if (d_rank_data_base_ + num > d_rank_data_end_)
      throw std::runtime_error(
          "Rank data buffer is overflowed by " +
          std::to_string(d_rank_data_base_ + num - d_rank_data_end_));
  }

  void register_buffer(const std::vector<std::string> &handles,
                       const std::vector<int64_t> &offsets, void *self) {
    check_rank_data_capacity();
    RankData data;
    for (int i = 0; i < world_size_; i++) {
      if (i != rank_) {
        char *handle = open_ipc_handle(handles[i].data());
        handle += offsets[i];
        data.ptrs[i] = handle;
      } else {
        data.ptrs[i] = self;
      }
    }
    auto d_data = d_rank_data_base_++;
    CUDACHECK(
        cudaMemcpy(d_data, &data, sizeof(RankData), cudaMemcpyHostToDevice));
    buffers_[self] = d_data;
  }

  // note: when registering graph buffers, we intentionally choose to not
  // deduplicate the addresses. That means if the allocator reuses some
  // addresses, they will be registered again. This is to account for the remote
  // possibility of different allocation patterns between ranks. For example,
  // rank 1 may get the same input address for the second allreduce, but rank 2
  // got a different address. IPC handles have internal reference counting
  // mechanism so overhead should be small.
  void register_graph_buffers(
      const std::vector<std::string> &handles,
      const std::vector<std::vector<int64_t>> &offsets) {
    auto num_buffers = graph_unreg_buffers_.size();
    check_rank_data_capacity(num_buffers);
    std::vector<RankData> rank_data(num_buffers);
    for (int i = 0; i < num_buffers; i++) {
      auto self_ptr = graph_unreg_buffers_[i];
      auto &rd = rank_data[i];
      for (int j = 0; j < world_size_; j++) {
        if (j != rank_) {
          char *handle =
              open_ipc_handle(&handles[j][i * sizeof(cudaIpcMemHandle_t)]);
          handle += offsets[j][i];
          rd.ptrs[j] = handle;
        } else {
          rd.ptrs[j] = self_ptr;
        }
      }
    }
    CUDACHECK(cudaMemcpy(d_rank_data_base_, rank_data.data(),
                         sizeof(RankData) * num_buffers,
                         cudaMemcpyHostToDevice));
    d_rank_data_base_ += num_buffers;
    graph_unreg_buffers_.clear();
  }

  /**
   * This is the result after careful grid search. Using 36 blocks give the best
   * or close to the best runtime on the devices I tried: A100, A10, A30, T4,
   * V100. You'll notice that NCCL kernels also only take a small amount of SMs.
   * Not quite sure the underlying reason, but my guess is that too many SMs
   * will cause contention on NVLink bus.
   */
  template <typename T>
  void allreduce(cudaStream_t stream, T *input, T *output, int size,
                 int threads = 512, int block_limit = 36) {
    auto d = packed_t<T>::P::size;
    if (size % d != 0)
      throw std::runtime_error(
          "custom allreduce currently requires input length to be multiple "
          "of " +
          std::to_string(d));
    if (block_limit > kMaxBlocks)
      throw std::runtime_error("max supported block limit is " +
                               std::to_string(kMaxBlocks) + ". Got " +
                               std::to_string(block_limit));

    RankData *ptrs;
    cudaStreamCaptureStatus status;
    CUDACHECK(cudaStreamIsCapturing(stream, &status));
    if (status == cudaStreamCaptureStatusActive) {
      ptrs = d_rank_data_base_ + graph_unreg_buffers_.size();
      graph_unreg_buffers_.push_back(input);
    } else {
      auto it = buffers_.find(input);
      if (it == buffers_.end())
        throw std::runtime_error(
            "buffer address " +
            std::to_string(reinterpret_cast<uint64_t>(input)) +
            " is not registered!");
      ptrs = it->second;
    }

    size /= d;
    auto bytes = size * sizeof(typename packed_t<T>::P);
    int blocks = std::min(block_limit, (size + threads - 1) / threads);
#define KL(ngpus, name)                                                       \
  name<T, ngpus><<<blocks, threads, 0, stream>>>(ptrs, sg_, self_sg_, output, \
                                                 rank_, size);
#define REDUCE_CASE(ngpus)                            \
  case ngpus: {                                       \
    if (world_size_ == 2) {                           \
      KL(ngpus, cross_device_reduce_1stage);          \
    } else if (full_nvlink_) {                        \
      if ((world_size_ <= 4 && bytes < 512 * 1024) || \
          (world_size_ <= 8 && bytes < 256 * 1024)) { \
        KL(ngpus, cross_device_reduce_1stage);        \
      } else {                                        \
        KL(ngpus, cross_device_reduce_2stage);        \
      }                                               \
    }                                                 \
    break;                                            \
  }

    switch (world_size_) {
      REDUCE_CASE(2)
      REDUCE_CASE(4)
      REDUCE_CASE(6)
      REDUCE_CASE(8)
      default:
        throw std::runtime_error(
            "custom allreduce only supports num gpus in (2,4,6,8). Actual num "
            "gpus = " +
            std::to_string(world_size_));
    }
#undef REDUCE_CASE
#undef KL
  }

  ~CustomAllreduce() {
    for (auto [_, ptr] : ipc_handles_) {
      CUDACHECK(cudaIpcCloseMemHandle(ptr));
    }
  }
};
/**
 * To inspect PTX/SASS, copy paste this header file to compiler explorer and add
 a template instantiation:
 * template void vllm::CustomAllreduce::allreduce<half>(cudaStream_t, half *,
 half *, int, int, int);
*/
}  // namespace vllm