[源码解析] NVIDIA HugeCTR，GPU版本参数服务器--- (2)

[源码解析] NVIDIA HugeCTR，GPU版本参数服务器--- (2)

0x00 摘要

在这篇文章中，我们介绍了 HugeCTR，这是一个面向行业的推荐系统训练框架，针对具有模型并行嵌入和数据并行密集网络的大规模 CTR 模型进行了优化。

本文以GitHub 源码文档 https://github.com/NVIDIA-Merlin/HugeCTR/blob/master/docs/python_interface.md 的翻译为基础，并且结合源码进行分析。其中借鉴了HugeCTR源码阅读 这篇大作，特此感谢。

为了更好的说明，下面类定义之中，只保留其成员变量，成员函数会等到分析时候才会给出。

本系列其他代码为：

[源码解析] NVIDIA HugeCTR，GPU 版本参数服务器 --(1)

0x01 总体流程

1.1 概述

HugeCTR 训练的过程可以看作是数据并行+模型并行。

• 数据并行是：每张 GPU卡可以同时读取不同的数据来做训练。
• 模型并行是：Sparse 参数可以被分布式存储到不同 GPU，不同 Node 之上，每个 GPU 分配部分 Sparse 参数。

训练流程如下：

• 首先构建三级流水线，初始化模型网络。初始化参数和优化器状态。

• Reader 会从数据集加载一个 batch 的数据，放入 Host 内存之中。

• 开始解析数据，得到 sparse 参数，dense 参数，label 等等。

• 嵌入层进行前向传播，即从参数服务器读取 embedding，进行处理。

• 对于网络层进行前向传播和后向传播，具体区分是多卡，单卡，多机，单机等。

• 嵌入层反向操作。

• 多卡之间交换 dense 参数的梯度。

• 嵌入层更新 sparse 参数。就是把反向计算得到的参数梯度推送到参数服务器，由参数服务器根据梯度更新参数。

1.2 如何调用

我们从一个例子中可以看到，总体逻辑和单机很像，就是解析配置，使用 session 来读取数据，训练等等，其中 vvgpu 是 device map。

# train.py
import sys
import hugectr
from mpi4py import MPI

def train(json_config_file):
  solver_config = hugectr.solver_parser_helper(batchsize = 16384,
                                               batchsize_eval = 16384,
                                               vvgpu = [[0,1,2,3,4,5,6,7]],
                                               repeat_dataset = True)
  sess = hugectr.Session(solver_config, json_config_file)
  sess.start_data_reading()
  for i in range(10000):
    sess.train()
    if (i % 100 == 0):
      loss = sess.get_current_loss()

if __name__ == "__main__":
  json_config_file = sys.argv[1]
  train(json_config_file)

0x02 Session

既然知道了 Session 是核心，我们就通过 Session 看看如何构建 HugeCTR。

2.1 Session 定义

我们首先看看Session的定义，只保留其成员变量，可以看到其主要是：

• networks_ ：模型网络信息。
• embeddings_ ：模型嵌入层信息。
• ExchangeWgrad ：交换梯度的类。
• evaluate_data_reader_ ： 读取 evalution。
• train_data_reader_ ：读取训练数据到嵌入层。
• resource_manager_ ：GPU 资源，比如 handle 和 Stream。

class Session {
 public:
  Session(const SolverParser& solver_config, const std::string& config_file);
  Session(const Session&) = delete;
  Session& operator=(const Session&) = delete;

 private:
  std::vector> networks_;      /**< networks (dense) used in training. */
  std::vector> embeddings_; /**< embedding */
  std::shared_ptr init_data_reader_;
  std::shared_ptr
      train_data_reader_; /**< data reader to reading data from data set to embedding. */
  std::shared_ptr evaluate_data_reader_; /**< data reader for evaluation. */
  std::shared_ptr
      resource_manager_; /**< GPU resources include handles and streams etc.*/
  std::shared_ptr parser_;
  std::shared_ptr exchange_wgrad_;

  metrics::Metrics metrics_;
  SolverParser solver_config_;

  struct HolisticCudaGraph {
    std::vector initialized;
    std::vector instance;
    std::vector fork_event;
  } train_graph_;

  // TODO: these two variables for export_predictions.
  // There may be a better place for them.
  bool use_mixed_precision_;
  size_t batchsize_eval_;
};

2.2 构造函数

构造函数大致分为以下步骤：

• 使用 create_pipeline 创建流水线。
• 初始化模型网络。
• 初始化参数和优化器状态。

Session::Session(const SolverParser& solver_config, const std::string& config_file)
    : resource_manager_(ResourceManagerExt::create(solver_config.vvgpu, solver_config.seed,
                                                   solver_config.device_layout)),
      solver_config_(solver_config) {
        
  // 检查设备      
  for (auto dev : resource_manager_->get_local_gpu_device_id_list()) {
    if (solver_config.use_mixed_precision) {
      check_device(dev, 7,
                   0);  // to support mixed precision training earliest supported device is CC=70
    } else {
      check_device(dev, 6, 0);  // earliest supported device is CC=60
    }
  }

  // 生成 Parser，用来解析配置      
  parser_.reset(new Parser(config_file, solver_config.batchsize, solver_config.batchsize_eval,
                           solver_config.num_epochs < 1, solver_config.i64_input_key,
                           solver_config.use_mixed_precision, solver_config.enable_tf32_compute,
                           solver_config.scaler, solver_config.use_algorithm_search,
                           solver_config.use_cuda_graph));

  // 建立流水线      
  parser_->create_pipeline(init_data_reader_, train_data_reader_, evaluate_data_reader_,
                           embeddings_, networks_, resource_manager_, exchange_wgrad_);

#ifndef DATA_READING_TEST
#pragma omp parallel num_threads(networks_.size())
  {
    // 多线程并行初始化模型
    size_t id = omp_get_thread_num();
    networks_[id]->initialize();
    if (solver_config.use_algorithm_search) {
      networks_[id]->search_algorithm();
    }
    CK_CUDA_THROW_(cudaStreamSynchronize(resource_manager_->get_local_gpu(id)->get_stream()));
  }
#endif

  // 加载dense feature需要的参数      
  init_or_load_params_for_dense_(solver_config.model_file);
  // 加载sparse feature需要的参数
  init_or_load_params_for_sparse_(solver_config.embedding_files);

  // 加载信息      
  load_opt_states_for_sparse_(solver_config.sparse_opt_states_files);
  load_opt_states_for_dense_(solver_config.dense_opt_states_file);

  int num_total_gpus = resource_manager_->get_global_gpu_count();
  for (const auto& metric : solver_config.metrics_spec) {
    metrics_.emplace_back(
        std::move(metrics::Metric::Create(metric.first, solver_config.use_mixed_precision,
                                          solver_config.batchsize_eval / num_total_gpus,
                                          solver_config.max_eval_batches, resource_manager_)));
  }

  if (solver_config_.use_holistic_cuda_graph) {
    train_graph_.initialized.resize(networks_.size(), false);
    train_graph_.instance.resize(networks_.size());
    for (size_t i = 0; i < resource_manager_->get_local_gpu_count(); i++) {
      auto& gpu_resource = resource_manager_->get_local_gpu(i);
      CudaCPUDeviceContext context(gpu_resource->get_device_id());
      cudaEvent_t event;
      CK_CUDA_THROW_(cudaEventCreateWithFlags(&event, cudaEventDisableTiming));
      train_graph_.fork_event.push_back(event);
    }
  }

  if (embeddings_.size() == 1) {
    auto lr_scheds = embeddings_[0]->get_learning_rate_schedulers();
    for (size_t i = 0; i < lr_scheds.size(); i++) {
      networks_[i]->set_learning_rate_scheduler(lr_scheds[i]);
    }
  }
}

这里有几个相关类需要注意一下。

2.2.1 ResourceManager

我们首先看看 ResourceManager。

2.2.1.1 接口

首先是两个接口，ResourceManagerBase 是最顶层接口，ResourceManager 进行了扩展。

/**
 * @brief Top-level ResourceManager interface
 *
 * The top level resource manager interface shared by various components
 */
class ResourceManagerBase {
 public:
  virtual void set_local_gpu(std::shared_ptr gpu_resource, size_t local_gpu_id) = 0;
  virtual const std::shared_ptr& get_local_gpu(size_t local_gpu_id) const = 0;
  virtual size_t get_local_gpu_count() const = 0;
  virtual size_t get_global_gpu_count() const = 0;
};

/**
 * @brief Second-level ResourceManager interface
 *
 * The second level resource manager interface shared by training and inference
 */
class ResourceManager : public ResourceManagerBase {
   // 省略了函数定义
}

2.2.1.2 Core

然后是核心实现：ResourceManagerCore，这里记录了各种资源。

/**
 * @brief GPU resources manager which holds the minimal, essential set of resources
 *
 * A core GPU Resource manager
 */
class ResourceManagerCore : public ResourceManager {
 private:
  int num_process_;
  int process_id_;
  DeviceMap device_map_;
  std::shared_ptr cpu_resource_;
  std::vector> gpu_resources_; /**< GPU resource vector */
  std::vector> p2p_matrix_;

  std::vector> base_cuda_mr_;
  std::vector> memory_resource_;
}

2.2.1.3 拓展

ResourceManagerExt 是在 ResourceManagerCore 基础之上进行再次封装，其核心就是 core_，这是一个 ResourceManagerCore 类型。我们用 ResourceManagerExt 来分析。

/**
 * @brief GPU resources manager which holds all the resources required by training
 *
 * An extended GPU Resource manager
 */
class ResourceManagerExt : public ResourceManager {
  std::shared_ptr core_;

#ifdef ENABLE_MPI
  std::unique_ptr ib_comm_ = NULL;
#endif
  std::shared_ptr ar_comm_ = NULL;
};

其创建代码如下，可以看到其利用 MPI 做了一些通信上的配置：

std::shared_ptr ResourceManagerExt::create(
    const std::vector>& visible_devices, unsigned long long seed,
    DeviceMap::Layout layout) {
  
  int size = 1, rank = 0;

#ifdef ENABLE_MPI
  HCTR_MPI_THROW(MPI_Comm_size(MPI_COMM_WORLD, &size));
  HCTR_MPI_THROW(MPI_Comm_rank(MPI_COMM_WORLD, &rank));
#endif

  DeviceMap device_map(visible_devices, rank, layout);

  std::random_device rd;
  if (seed == 0) {
    seed = rd();
  }

#ifdef ENABLE_MPI
  HCTR_MPI_THROW(MPI_Bcast(&seed, 1, MPI_UNSIGNED_LONG_LONG, 0, MPI_COMM_WORLD));
#endif

  std::shared_ptr core(
      new ResourceManagerCore(size, rank, std::move(device_map), seed));

  return std::shared_ptr(new ResourceManagerExt(core));
}

ResourceManagerExt::ResourceManagerExt(std::shared_ptr core) : core_(core) {
#ifdef ENABLE_MPI
  int num_process = get_num_process();
  if (num_process > 1) {
    int process_id = get_process_id();
    ib_comm_ = std::make_unique();
    ib_comm_->init(num_process, get_local_gpu_count(), process_id, get_local_gpu_device_id_list());
  }
#endif
}

void ResourceManagerExt::set_ar_comm(AllReduceAlgo algo, bool use_mixed_precision) {
  int num_process = get_num_process();
#ifdef ENABLE_MPI
  ar_comm_ = AllReduceInPlaceComm::create(num_process, algo, use_mixed_precision, get_local_gpus(),
                                          ib_comm_.get());
#else
  ar_comm_ = AllReduceInPlaceComm::create(num_process, algo, use_mixed_precision, get_local_gpus());
#endif
}

具体资源上的配置还是调用了 core_ 来完成。

// from ResourceManagerBase
void set_local_gpu(std::shared_ptr gpu_resource, size_t local_gpu_id) override {
  core_->set_local_gpu(gpu_resource, local_gpu_id);
}
const std::shared_ptr& get_local_gpu(size_t local_gpu_id) const override {
  return core_->get_local_gpu(local_gpu_id);
}
size_t get_local_gpu_count() const override { return core_->get_local_gpu_count(); }
size_t get_global_gpu_count() const override { return core_->get_global_gpu_count(); }

// from ResourceManager
int get_num_process() const override { return core_->get_num_process(); }
int get_process_id() const override { return core_->get_process_id(); }

0x03 Parser

前面提到了Parser，我们接下来就看看。Parser 负责解析配置文件，建立流水线。其类似的支撑文件还有 SolverParser，Solver，InferenceParser 等等。可以说，Parser 是自动化运作的关键，是支撑系统的灵魂。

3.1 定义

/**
 * @brief The parser of configure file (in json format).
 *
 * The builder of each layer / optimizer in HugeCTR.
 * Please see User Guide to learn how to write a configure file.
 * @verbatim
 * Some Restrictions:
 *  1. Embedding should be the first element of layers.
 *  2. layers should be listed from bottom to top.
 * @endverbatim
 */
class Parser {
 private:
  nlohmann::json config_;  /**< configure file. */
  size_t batch_size_;      /**< batch size. */
  size_t batch_size_eval_; /**< batch size. */
  const bool repeat_dataset_;
  const bool i64_input_key_{false};
  const bool use_mixed_precision_{false};
  const bool enable_tf32_compute_{false};

  const float scaler_{1.f};
  const bool use_algorithm_search_;
  const bool use_cuda_graph_;
  bool grouped_all_reduce_ = false;
}

我们接下来的这些分析，其实都是调用了 Parser 或者其相关类。

3.2 如何组织网络

我们首先看看配置文件，看看其中是如何组织一个模型网络，这里以 test/scripts/deepfm_8gpu.json 为例。

这里需要说明一下 json 字段的作用：

• bottom_names: 本层的输入张量名字。
• top_names: 本层的输出张量名字。

所以，模型就是通过 bottom 和 top 从下往上组织起来的。

3.2.1 输入

输入层如下，dense 是 slice 层的输入，Sparse 是sparse_embedding1 的输入，其中包含了 26 个 slots。

{
  "name": "data",
  "type": "Data",
  "source": "./file_list.txt",
  "eval_source": "./file_list_test.txt",
  "check": "Sum",
  "label": {
    "top": "label",
    "label_dim": 1
  },
  "dense": {
    "top": "dense",
    "dense_dim": 13
  },
  "sparse": [
    {
      "top": "data1",
      "slot_num": 26,
      "is_fixed_length": false,
      "nnz_per_slot": 2
    }
  ]
},

此时模型图如下：

3.2.2 嵌入层

我们看看其定义：

• embedding_vec_size 是向量维度。

• combiner ：查找得到向量之后，如何做pooling，是做sum还是avg。

• workspace_size_per_gpu_in_mb ：每个GPU之上的内存大小。

{
  "name": "sparse_embedding1",
  "type": "DistributedSlotSparseEmbeddingHash",
  "bottom": "data1",
  "top": "sparse_embedding1",
  "sparse_embedding_hparam": {
    "embedding_vec_size": 11,
    "combiner": "sum",
    "workspace_size_per_gpu_in_mb": 10
  }
},

此时模型如下：

3.2.3 其它层

这里我们把其它层也包括进来，就是目前输入数据和嵌入层的再上一层，我们省略了很多层，这里只是给大家一个大致的逻辑。

3.2.3.1 Reshape层

Reshape 层把一个 3D 输入转换为 2D 形状。此层是嵌入层的消费者。

{
  "name": "reshape1",
  "type": "Reshape",
  "bottom": "sparse_embedding1",
  "top": "reshape1",
  "leading_dim": 11
},

3.2.3.2 Slice 层

Slice 层把一个bottom分解成多个top。

{
  "name": "slice2",
  "type": "Slice",
  "bottom": "dense",
  "ranges": [
    [
      0,
      13
    ],
    [
      0,
      13
    ]
  ],
  "top": [
    "slice21",
    "slice22"
  ]
},

3.2.3.3 Loss

这就是我们最终的损失层，label直接会输出到这里。

{
  "name": "loss",
  "type": "BinaryCrossEntropyLoss",
  "bottom": [
    "add",
    "label"
  ],
  "top": "loss"
}

3.2.3.4 简略模型图

目前逻辑如下，是从下往上组织的模型，我们省略了其他部分：

3.3 全貌

我们对每个层进行精简，省略内部标签，把配置文件中所有层都整理出来，看看一个DeepFM在HugeCTR之中的整体架构。

0x04 建立流水线

我们接着看如何建立流水线。Create_pipeline 函数是用来构建流水线的，其就是转移给了create_pipeline_internal 方法。

void Parser::create_pipeline(std::shared_ptr& init_data_reader,
                             std::shared_ptr& train_data_reader,
                             std::shared_ptr& evaluate_data_reader,
                             std::vector>& embeddings,
                             std::vector>& networks,
                             const std::shared_ptr& resource_manager,
                             std::shared_ptr& exchange_wgrad) {
  if (i64_input_key_) {
    create_pipeline_internal(init_data_reader, train_data_reader, evaluate_data_reader,
                                        embeddings, networks, resource_manager, exchange_wgrad);
  } else {
    create_pipeline_internal(init_data_reader, train_data_reader,
                                           evaluate_data_reader, embeddings, networks,
                                           resource_manager, exchange_wgrad);
  }
}

4.3.1 create_pipeline_internal

create_pipeline_internal 主要包含了四步：

• create_allreduce_comm ：建立allreduce通信相关机制。
• 建立 Data Reader。
• 建立 嵌入层相关机制。
• 建立 网络相关机制，在每张GPU卡之中构建一个network副本。
• 对梯度交换类进行分配。

template 
void Parser::create_pipeline_internal(std::shared_ptr& init_data_reader,
                                      std::shared_ptr& train_data_reader,
                                      std::shared_ptr& evaluate_data_reader,
                                      std::vector>& embeddings,
                                      std::vector>& networks,
                                      const std::shared_ptr& resource_manager,
                                      std::shared_ptr& exchange_wgrad) {
  try {
    // 建立allreduce通信相关
    create_allreduce_comm(resource_manager, exchange_wgrad);

    std::map> sparse_input_map;
    std::vector train_tensor_entries_list[resource_manager->get_local_gpu_count()];
    std::vector evaluate_tensor_entries_list[resource_manager->get_local_gpu_count()];
    {
      if (!networks.empty()) {
        CK_THROW_(Error_t::WrongInput, "vector network is not empty");
      }

      // 校验网络
      auto j_layers_array = get_json(config_, "layers");
      auto j_optimizer = get_json(config_, "optimizer");
      check_graph(tensor_active_, j_layers_array);

      // Create Data Reader
      // 建立 Data Reader
      {
        // TODO: In using AsyncReader, if the overlap is disabled,
        // scheduling the data reader should be off.
        // THe scheduling needs to be generalized.
        auto j_solver = get_json(config_, "solver");
        auto enable_overlap = get_value_from_json_soft(j_solver, "enable_overlap", false);

        const nlohmann::json& j = j_layers_array[0];
        create_datareader()(j, sparse_input_map, train_tensor_entries_list,
                                     evaluate_tensor_entries_list, init_data_reader,
                                     train_data_reader, evaluate_data_reader, batch_size_,
                                     batch_size_eval_, use_mixed_precision_, repeat_dataset_,
                                     enable_overlap, resource_manager);
      }  // Create Data Reader

      // Create Embedding
      {
        for (unsigned int i = 1; i < j_layers_array.size(); i++) {
          // 网路配置的每层是从底到上，因此只要遇到非嵌入层，就不检查其后的层了          
          // if not embedding then break
          const nlohmann::json& j = j_layers_array[i];
          auto embedding_name = get_value_from_json(j, "type");
          Embedding_t embedding_type;
          if (!find_item_in_map(embedding_type, embedding_name, EMBEDDING_TYPE_MAP)) {
            Layer_t layer_type;
            if (!find_item_in_map(layer_type, embedding_name, LAYER_TYPE_MAP) &&
                !find_item_in_map(layer_type, embedding_name, LAYER_TYPE_MAP_MP)) {
              CK_THROW_(Error_t::WrongInput, "No such layer: " + embedding_name);
            }
            break;
          }

          // 建立嵌入层
          if (use_mixed_precision_) {
            create_embedding()(
                sparse_input_map, train_tensor_entries_list, evaluate_tensor_entries_list,
                embeddings, embedding_type, config_, resource_manager, batch_size_,
                batch_size_eval_, exchange_wgrad, use_mixed_precision_, scaler_, j, use_cuda_graph_,
                grouped_all_reduce_);
          } else {
            create_embedding()(
                sparse_input_map, train_tensor_entries_list, evaluate_tensor_entries_list,
                embeddings, embedding_type, config_, resource_manager, batch_size_,
                batch_size_eval_, exchange_wgrad, use_mixed_precision_, scaler_, j, use_cuda_graph_,
                grouped_all_reduce_);
          }
        }  // for ()
      }    // Create Embedding

      // 建立网络层
      // create network
      int total_gpu_count = resource_manager->get_global_gpu_count();
      if (0 != batch_size_ % total_gpu_count) {
        CK_THROW_(Error_t::WrongInput, "0 != batch_size\%total_gpu_count");
      }
      
      // create network，在每张GPU卡之中构建一个network副本
      for (size_t i = 0; i < resource_manager->get_local_gpu_count(); i++) {
        networks.emplace_back(Network::create_network(
            j_layers_array, j_optimizer, train_tensor_entries_list[i],
            evaluate_tensor_entries_list[i], total_gpu_count, exchange_wgrad,
            resource_manager->get_local_cpu(), resource_manager->get_local_gpu(i),
            use_mixed_precision_, enable_tf32_compute_, scaler_, use_algorithm_search_,
            use_cuda_graph_, false, grouped_all_reduce_));
      }
    }
    exchange_wgrad->allocate(); // 建立梯度交换类

  } catch (const std::runtime_error& rt_err) {
    std::cerr << rt_err.what() << std::endl;
    throw;
  }
}

4.3.2 create_allreduce_comm

create_allreduce_comm 的功能是设置通信算法，比如建立 AllReduceInPlaceComm，构建 GroupedExchangeWgrad。

void Parser::create_allreduce_comm(const std::shared_ptr& resource_manager,
                                   std::shared_ptr& exchange_wgrad) {
  auto ar_algo = AllReduceAlgo::NCCL;
  bool grouped_all_reduce = false;
  
  // 获取通信算法配置
  if (has_key_(config_, "all_reduce")) {
    auto j_all_reduce = get_json(config_, "all_reduce");
    std::string ar_algo_name = "Oneshot";
    if (has_key_(j_all_reduce, "algo")) {
      ar_algo_name = get_value_from_json(j_all_reduce, "algo");
    }
    if (has_key_(j_all_reduce, "grouped")) {
      grouped_all_reduce = get_value_from_json(j_all_reduce, "grouped");
    }
    if (!find_item_in_map(ar_algo, ar_algo_name, ALLREDUCE_ALGO_MAP)) {
      CK_THROW_(Error_t::WrongInput, "All reduce algo unknown: " + ar_algo_name);
    }
  }

  // 设置通信算法，比如建立 AllReduceInPlaceComm
  resource_manager->set_ar_comm(ar_algo, use_mixed_precision_);

  // 构建 GroupedExchangeWgrad
  grouped_all_reduce_ = grouped_all_reduce;
  if (grouped_all_reduce_) {
    if (use_mixed_precision_) {
      exchange_wgrad = std::make_shared>(resource_manager);
    } else {
      exchange_wgrad = std::make_shared>(resource_manager);
    }
  } else {
    if (use_mixed_precision_) {
      exchange_wgrad = std::make_shared>(resource_manager);
    } else {
      exchange_wgrad = std::make_shared>(resource_manager);
    }
  }
}

其中 GroupedExchangeWgrad 是用来交换梯度的。

template 
class GroupedExchangeWgrad : public ExchangeWgrad {
 public:
  const BuffPtrs& get_network_wgrad_buffs() const { return network_wgrad_buffs_; }
  const BuffPtrs& get_embed_wgrad_buffs() const { return embed_wgrad_buffs_; }
  void allocate() final;
  void update_embed_wgrad_size(size_t size) final;
  void allreduce(size_t device_id, cudaStream_t stream);
  GroupedExchangeWgrad(const std::shared_ptr& resource_manager);
  ~GroupedExchangeWgrad() = default;

 private:
  BuffPtrs network_wgrad_buffs_;
  BuffPtrs embed_wgrad_buffs_;
  std::vector>> bufs_;
  std::shared_ptr resource_manager_;

  AllReduceInPlaceComm::Handle ar_handle_;

  size_t network_wgrad_size_ = 0;
  size_t embed_wgrad_size_ = 0;
  size_t num_gpus_ = 0;
};

比如通过allreduce进行交换：

template 
void GroupedExchangeWgrad::allreduce(size_t device_id, cudaStream_t stream) {
  auto ar_comm = resource_manager_->get_ar_comm();
  ar_comm->all_reduce(ar_handle_, stream, device_id);
}

4.3.3 create_datareader

DataReader 是流水线的主体，它实际包含了流水线的前两级：data reader worker 与 data collector。

4.3.3.1 建立哪些内容

create_datareader 的调用如下，

create_datareader()(j, sparse_input_map, train_tensor_entries_list,
                                 evaluate_tensor_entries_list, init_data_reader,
                                 train_data_reader, evaluate_data_reader, batch_size_,
                                 batch_size_eval_, use_mixed_precision_, repeat_dataset_,
                                 enable_overlap, resource_manager);

回忆一下，在下面代码之中会调用到create_datareader创建了几个 reader。

parser_->create_pipeline(init_data_reader_, train_data_reader_, evaluate_data_reader_,
                         embeddings_, networks_, resource_manager_, exchange_wgrad_);

其实就是 Session 之中的几个成员变量，比如：

std::shared_ptr init_data_reader_;
std::shared_ptr train_data_reader_; /**< data reader to reading data from data set to embedding. */
std::shared_ptr evaluate_data_reader_; /**< data reader for evaluation. */

分别用于训练，评估。

4.3.3.2 建立reader

因为代码太长，我们只保留部分关键代码。我们先看create_datareader里面做了什么：这里有两个 reader，一个train_data_reader和一个evaluate_data_reader，也就是一个用于训练，一个用于评估。然后会为他们建立workgroup。

对于Reader，HugeCTR 提供了三种实现：

• Norm：普通文件读取。
• Parquet ：parquet格式的文件。
• Raw：Raw 数据集格式与 Norm 数据集格式的不同之处在于训练数据出现在一个二进制文件中。

template 
void create_datareader::operator()(

    switch (format) {
      case DataReaderType_t::Norm: {
        bool start_right_now = repeat_dataset;
        train_data_reader->create_drwg_norm(source_data, check_type, start_right_now);
        evaluate_data_reader->create_drwg_norm(eval_source, check_type, start_right_now);
        break;
      }
      case DataReaderType_t::Raw: {
        const auto num_samples = get_value_from_json(j, "num_samples");
        const auto eval_num_samples = get_value_from_json(j, "eval_num_samples");
        std::vector slot_offset = f();
        bool float_label_dense = get_value_from_json_soft(j, "float_label_dense", false);
        train_data_reader->create_drwg_raw(source_data, num_samples, float_label_dense, true,
                                           false);
        evaluate_data_reader->create_drwg_raw(eval_source, eval_num_samples, float_label_dense,
                                              false, false);

        break;
      }
      case DataReaderType_t::Parquet: {
        // @Future: Should be slot_offset here and data_reader ctor should
        // be TypeKey not long long
        std::vector slot_offset = f();
        train_data_reader->create_drwg_parquet(source_data, slot_offset, true);
        evaluate_data_reader->create_drwg_parquet(eval_source, slot_offset, true);
        break;
      }
    }
}

我们以 norm 为例进行解析，首先提一下，其内部建立了 WorkerGroup。

void create_drwg_norm(std::string file_name, Check_t check_type,
                      bool start_reading_from_beginning = true) override {
  source_type_ = SourceType_t::FileList;
  worker_group_.reset(new DataReaderWorkerGroupNorm(
      thread_buffers_, resource_manager_, file_name, repeat_, check_type, params_,
      start_reading_from_beginning));
  file_name_ = file_name;
}

4.3.3.3 DataReaderWorkerGroupNorm

在 DataReaderWorkerGroupNorm 之中，建立了DataReaderWorker，其中 file_list_ 是需要读取的数据文件。

template 
class DataReaderWorkerGroupNorm : public DataReaderWorkerGroup {
  
  std::string file_list_; /**< file list of data set */

  std::shared_ptr create_source(size_t worker_id, size_t num_worker,
                                        const std::string &file_name, bool repeat) override {
    return std::make_shared(worker_id, num_worker, file_name, repeat);
  }

 public:
  // Ctor
  DataReaderWorkerGroupNorm(const std::vector> &output_buffers,
                            const std::shared_ptr &resource_manager_,
                            std::string file_list, bool repeat, Check_t check_type,
                            const std::vector ¶ms,
                            bool start_reading_from_beginning = true)
      : DataReaderWorkerGroup(start_reading_from_beginning, DataReaderType_t::Norm) {

    int num_threads = output_buffers.size();
    size_t local_gpu_count = resource_manager_->get_local_gpu_count();

    // create data reader workers
    int max_feature_num_per_sample = 0;
    for (auto ¶m : params) {
      max_feature_num_per_sample += param.max_feature_num;
    }

    set_resource_manager(resource_manager_);
    for (int i = 0; i < num_threads; i++) {
      std::shared_ptr data_reader(new DataReaderWorker(
          i, num_threads, resource_manager_->get_local_gpu(i % local_gpu_count),
          &data_reader_loop_flag_, output_buffers[i], file_list, max_feature_num_per_sample, repeat,
          check_type, params));
      data_readers_.push_back(data_reader);
    }
    create_data_reader_threads();
  }
};

然后创建了多个线程 data_reader_threads_ 分别运行这些 woker。

  /**
   * Create threads to run data reader workers
>>>>>>> v3.1_preview
   */
  void create_data_reader_threads() {

    size_t local_gpu_count = resource_manager_->get_local_gpu_count();

    for (size_t i = 0; i < data_readers_.size(); ++i) {
      auto local_gpu = resource_manager_->get_local_gpu(i % local_gpu_count);
      data_reader_threads_.emplace_back(data_reader_thread_func_, data_readers_[i],
                                        &data_reader_loop_flag_, local_gpu->get_device_id());
    }
  }

4.3.4 小结

我们总结一下。DataReader 包含了流水线的前两级，目前分析之涉及到了第一级。在 Reader之中，有一个 worker group，里面包含了若干worker，也有若干对应线程来运行这些 worker， Data Reader worker 就是流水线第一级。第二级 collecotr 我们会暂时跳过去在下一章进行介绍。

4.4 建立嵌入

我们直接调过来看流水线第三级，如下代码建立了嵌入。

create_embedding()(
            sparse_input_map, train_tensor_entries_list, evaluate_tensor_entries_list,
            embeddings, embedding_type, config_, resource_manager, batch_size_,
            batch_size_eval_, exchange_wgrad, use_mixed_precision_, scaler_, j, use_cuda_graph_,
            grouped_all_reduce_);

这里建立了一些embedding，比如DistributedSlotSparseEmbeddingHash。

如前文所述，HugeCTR 包含了若干 Hash，比如：

• LocalizedSlotEmbeddingHash：同一个槽（特征域）中的特征会存储在一个GPU中，这就是为什么它被称为“本地化槽”，根据槽的索引号，不同的槽可能存储在不同的GPU中。

• DistributedSlotEmbeddingHash：所有特征都存储于不同特征域/槽上，不管槽索引号是多少，这些特征都根据特征的索引号分布到不同的GPU上。这意味着同一插槽中的特征可能存储在不同的 GPU 中，这就是将其称为“分布式插槽”的原因。

以下代码省略了很多，有兴趣的读者可以深入源码进行阅读。

template 
void create_embedding::operator()(
    std::map>& sparse_input_map,
    std::vector* train_tensor_entries_list,
    std::vector* evaluate_tensor_entries_list,
    std::vector>& embeddings, Embedding_t embedding_type,
    const nlohmann::json& config, const std::shared_ptr& resource_manager,
    size_t batch_size, size_t batch_size_eval, std::shared_ptr& exchange_wgrad,
    bool use_mixed_precision, float scaler, const nlohmann::json& j_layers, bool use_cuda_graph,
    bool grouped_all_reduce) {
  
#ifdef ENABLE_MPI
  int num_procs = 1, pid = 0; // 建立 MPI相关
  MPI_Comm_rank(MPI_COMM_WORLD, &pid);
  MPI_Comm_size(MPI_COMM_WORLD, &num_procs);
#endif

  // 从配置文件之中读取
  auto j_optimizer = get_json(config, "optimizer");
  auto embedding_name = get_value_from_json(j_layers, "type");
  auto bottom_name = get_value_from_json(j_layers, "bottom");
  auto top_name = get_value_from_json(j_layers, "top");
  auto j_hparam = get_json(j_layers, "sparse_embedding_hparam");
  size_t workspace_size_per_gpu_in_mb =
      get_value_from_json_soft(j_hparam, "workspace_size_per_gpu_in_mb", 0);
  auto embedding_vec_size = get_value_from_json(j_hparam, "embedding_vec_size");
  size_t max_vocabulary_size_per_gpu =
      (workspace_size_per_gpu_in_mb * 1024 * 1024) / (sizeof(float) * embedding_vec_size);
  auto combiner_str = get_value_from_json(j_hparam, "combiner");

  int combiner; // 设定combiner方法
  if (combiner_str == "sum") {
    combiner = 0;
  } else if (combiner_str == "mean") {
    combiner = 1;
  } else {
    CK_THROW_(Error_t::WrongInput, "No such combiner type: " + combiner_str);
  }

  // 设定slot配置
  std::vector slot_size_array;
  if (has_key_(j_hparam, "slot_size_array")) {
    auto slots = get_json(j_hparam, "slot_size_array");
    assert(slots.is_array());
    for (auto slot : slots) {
      slot_size_array.emplace_back(slot.get());
    }
  }

  SparseInput sparse_input;
  
  // 设定优化器配置
  OptParams embedding_opt_params;
  if (has_key_(j_layers, "optimizer")) {
    embedding_opt_params = get_optimizer_param(get_json(j_layers, "optimizer"));
  } else {
    embedding_opt_params = get_optimizer_param(j_optimizer);
  }
  embedding_opt_params.scaler = scaler;

  // 建立不同的hash
  switch (embedding_type) {
    case Embedding_t::DistributedSlotSparseEmbeddingHash: {
      const SparseEmbeddingHashParams embedding_params = {batch_size,
                                                          batch_size_eval,
                                                          max_vocabulary_size_per_gpu,
                                                          {},
                                                          embedding_vec_size,
                                                          sparse_input.max_feature_num_per_sample,
                                                          sparse_input.slot_num,
                                                          combiner,  // combiner: 0-sum, 1-mean
                                                          embedding_opt_params};

      embeddings.emplace_back(new DistributedSlotSparseEmbeddingHash(
          sparse_input.train_sparse_tensors, sparse_input.evaluate_sparse_tensors, embedding_params,
          resource_manager));
      break;
    }
    case Embedding_t::LocalizedSlotSparseEmbeddingHash: {
      const SparseEmbeddingHashParams embedding_params = {batch_size,
                                                          batch_size_eval,
                                                          max_vocabulary_size_per_gpu,
                                                          slot_size_array,
                                                          embedding_vec_size,
                                                          sparse_input.max_feature_num_per_sample,
                                                          sparse_input.slot_num,
                                                          combiner,  // combiner: 0-sum, 1-mean
                                                          embedding_opt_params};

      embeddings.emplace_back(new LocalizedSlotSparseEmbeddingHash(
          sparse_input.train_sparse_tensors, sparse_input.evaluate_sparse_tensors, embedding_params,
          resource_manager));

      break;
    }
    case Embedding_t::LocalizedSlotSparseEmbeddingOneHot: {
      const SparseEmbeddingHashParams embedding_params = {...};
      embeddings.emplace_back(new LocalizedSlotSparseEmbeddingOneHot(
          sparse_input.train_sparse_tensors, sparse_input.evaluate_sparse_tensors, embedding_params,
          resource_manager));

      break;
    }
    case Embedding_t::HybridSparseEmbedding: {
      const HybridSparseEmbeddingParams embedding_params = {...};
      embeddings.emplace_back(new HybridSparseEmbedding(
          sparse_input.train_sparse_tensors, sparse_input.evaluate_sparse_tensors, embedding_params,
          embed_wgrad_buff, get_gpu_learning_rate_schedulers(config, resource_manager), graph_mode,
          resource_manager));
      break;
    }

  }  // switch
  
}

4.5 建立网络

接下来是建立网络环节，这部分过后，hugeCTR系统就正式建立起来，可以进行训练了，大体逻辑是：

• 进行GPU内存分配，这里大量使用了 create_block，其中就是 BufferBlockImpl。
• 建立训练网络层。
• 建立评估网络层。
• 建立优化器。
• 初始化网络其他信息。

Network* Network::create_network(const nlohmann::json& j_array, const nlohmann::json& j_optimizer,
                                 std::vector& train_tensor_entries,
                                 std::vector& evaluate_tensor_entries,
                                 int num_networks_in_global,
                                 std::shared_ptr& exchange_wgrad,
                                 const std::shared_ptr& cpu_resource,
                                 const std::shared_ptr& gpu_resource,
                                 bool use_mixed_precision, bool enable_tf32_compute, float scaler,
                                 bool use_algorithm_search, bool use_cuda_graph,
                                 bool inference_flag, bool grouped_all_reduce) {
  Network* network = new Network(cpu_resource, gpu_resource, use_mixed_precision, use_cuda_graph);

  auto& train_layers = network->train_layers_;
  auto* bottom_layers = &network->bottom_layers_;
  auto* top_layers = &network->top_layers_;
  auto& evaluate_layers = network->evaluate_layers_;
  auto& train_loss_tensor = network->train_loss_tensor_;
  auto& evaluate_loss_tensor = network->evaluate_loss_tensor_;
  auto& train_loss = network->train_loss_;
  auto& evaluate_loss = network->evaluate_loss_;
  auto& enable_cuda_graph = network->enable_cuda_graph_;
  auto& raw_metrics = network->raw_metrics_;

  // 会进行GPU内存分配，这里大量使用了 create_block，其中就是 BufferBlockImpl
  std::shared_ptr> blobs_buff =
      GeneralBuffer2::create();

  std::shared_ptr> train_weight_buff = blobs_buff->create_block();
  std::shared_ptr> train_weight_buff_half = blobs_buff->create_block<__half>();
  std::shared_ptr> wgrad_buff = nullptr;
  std::shared_ptr> wgrad_buff_half = nullptr;

  if (!inference_flag) {
    if (use_mixed_precision) {
      auto id = gpu_resource->get_local_id();
      wgrad_buff_half =
          (grouped_all_reduce)
              ? std::dynamic_pointer_cast>(exchange_wgrad)
                    ->get_network_wgrad_buffs()[id]
              : std::dynamic_pointer_cast>(exchange_wgrad)
                    ->get_network_wgrad_buffs()[id];
      wgrad_buff = blobs_buff->create_block();  // placeholder
    } else {
      auto id = gpu_resource->get_local_id();
      wgrad_buff = (grouped_all_reduce)
                       ? std::dynamic_pointer_cast>(exchange_wgrad)
                             ->get_network_wgrad_buffs()[id]
                       : std::dynamic_pointer_cast>(exchange_wgrad)
                             ->get_network_wgrad_buffs()[id];
      wgrad_buff_half = blobs_buff->create_block<__half>();  // placeholder
    }
  } else {
    wgrad_buff = blobs_buff->create_block();
    wgrad_buff_half = blobs_buff->create_block<__half>();
  }

  std::shared_ptr> evaluate_weight_buff = blobs_buff->create_block();
  std::shared_ptr> evaluate_weight_buff_half =
      blobs_buff->create_block<__half>();
  std::shared_ptr> wgrad_buff_placeholder = blobs_buff->create_block();
  std::shared_ptr> wgrad_buff_half_placeholder =
      blobs_buff->create_block<__half>();
  std::shared_ptr> opt_buff = blobs_buff->create_block();
  std::shared_ptr> opt_buff_half = blobs_buff->create_block<__half>();

  // 建立训练网络层
  if (!inference_flag) {
    // create train layers
    create_layers(j_array, train_tensor_entries, blobs_buff, train_weight_buff,
                  train_weight_buff_half, wgrad_buff, wgrad_buff_half, train_loss_tensor,
                  gpu_resource, use_mixed_precision, enable_tf32_compute, num_networks_in_global,
                  scaler, enable_cuda_graph, inference_flag, train_layers, train_loss, nullptr,
                  top_layers, bottom_layers);
  }

  // 建立评估网络层
  // create evaluate layers
  create_layers(j_array, evaluate_tensor_entries, blobs_buff, evaluate_weight_buff,
                evaluate_weight_buff_half, wgrad_buff_placeholder, wgrad_buff_half_placeholder,
                evaluate_loss_tensor, gpu_resource, use_mixed_precision, enable_tf32_compute,
                num_networks_in_global, scaler, enable_cuda_graph, inference_flag, evaluate_layers,
                evaluate_loss, &raw_metrics);

  // 建立优化器
  // create optimizer
  if (!inference_flag) {
    if (use_mixed_precision) {
      auto opt_param = get_optimizer_param(j_optimizer);

      network->optimizer_ = std::move(Optimizer::Create(opt_param, train_weight_buff->as_tensor(),
                                                        wgrad_buff_half->as_tensor(), scaler,
                                                        opt_buff_half, gpu_resource));
    } else {
      auto opt_param = get_optimizer_param(j_optimizer);

      network->optimizer_ =
          std::move(Optimizer::Create(opt_param, train_weight_buff->as_tensor(),
                                      wgrad_buff->as_tensor(), scaler, opt_buff, gpu_resource));
    }
  } else {
    try {
      TensorEntry pred_tensor_entry = evaluate_tensor_entries.back();
      if (use_mixed_precision) {
        network->pred_tensor_half_ = Tensor2<__half>::stretch_from(pred_tensor_entry.bag);
      } else {
        network->pred_tensor_ = Tensor2::stretch_from(pred_tensor_entry.bag);
      }
    } catch (const std::runtime_error& rt_err) {
      std::cerr << rt_err.what() << std::endl;
      throw;
    }
  }

  // 初始化网络其他信息
  network->train_weight_tensor_ = train_weight_buff->as_tensor();
  network->train_weight_tensor_half_ = train_weight_buff_half->as_tensor();
  network->wgrad_tensor_ = wgrad_buff->as_tensor();
  network->wgrad_tensor_half_ = wgrad_buff_half->as_tensor();
  network->evaluate_weight_tensor_ = evaluate_weight_buff->as_tensor();
  network->evaluate_weight_tensor_half_ = evaluate_weight_buff_half->as_tensor();
  network->opt_tensor_ = opt_buff->as_tensor();
  network->opt_tensor_half_ = opt_buff_half->as_tensor();

  CudaDeviceContext context(gpu_resource->get_device_id());
  blobs_buff->allocate();

  return network;
}

4.5.1 create_layers

create_layers 有两个版本，分别是HugeCTR/src/parsers/create_network.cpp 和 HugeCTR/src/cpu/create_network_cpu.cpp，我们使用 create_network.cpp 的代码来看看。

其实就是遍历从配置读取的json数组，然后建立每一层，因为层类型太多，所以我们只给出了两个例子。

void create_layers(const nlohmann::json& j_array, std::vector& tensor_entries,
                   const std::shared_ptr>& blobs_buff,
                   const std::shared_ptr>& weight_buff,
                   const std::shared_ptr>& weight_buff_half,
                   const std::shared_ptr>& wgrad_buff,
                   const std::shared_ptr>& wgrad_buff_half,
                   Tensor2& loss_tensor, const std::shared_ptr& gpu_resource,
                   bool use_mixed_precision, bool enable_tf32_compute, int num_networks_in_global,
                   float scaler, bool& enable_cuda_graph, bool inference_flag,
                   std::vector>& layers, std::unique_ptr& loss,
                   metrics::RawMetricMap* raw_metrics, std::vector* top_layers = nullptr,
                   std::vector* bottom_layers = nullptr) {
  
  for (unsigned int i = 1; i < j_array.size(); i++) { // 遍历json数组
    const nlohmann::json& j = j_array[i];
    const auto layer_type_name = get_value_from_json(j, "type");
    Layer_t layer_type;


    std::vector output_tensor_entries;
    // 这里获得本层的输入和输出
    auto input_output_info = get_input_tensor_and_output_name(j, tensor_entries);
    
    switch (layer_type) {
      // 建立对应的每一层
      case Layer_t::ReduceMean: {
        int axis = get_json(j, "axis").get();
        // 本层输入
        Tensor2 in_tensor = Tensor2::stretch_from(input_output_info.inputs[0]);
        Tensor2 out_tensor;
        emplaceback_layer(
            new ReduceMeanLayer(in_tensor, out_tensor, blobs_buff, axis, gpu_resource));
        // 本层输出
        output_tensor_entries.push_back({input_output_info.output_names[0], out_tensor.shrink()});
        break;
      }

      case Layer_t::Softmax: {
        // 本层输入
        Tensor2 in_tensor = Tensor2::stretch_from(input_output_info.inputs[0]);
        Tensor2 out_tensor;
        blobs_buff->reserve(in_tensor.get_dimensions(), &out_tensor);
        // 本层输出
        output_tensor_entries.push_back({input_output_info.output_names[0], out_tensor.shrink()});
        emplaceback_layer(new SoftmaxLayer(in_tensor, out_tensor, blobs_buff, gpu_resource));
        break;
      }
    }  // end of switch
    
  }  // for layers
}

4.5.2 层实现

HugeCTR 属于一个具体而微的深度学习系统，它实现的具体层类型如下：

enum class Layer_t {
  BatchNorm,
  BinaryCrossEntropyLoss,
  Reshape,
  Concat,
  CrossEntropyLoss,
  Dropout,
  ELU,
  InnerProduct,
  FusedInnerProduct,
  Interaction,
  MultiCrossEntropyLoss,
  ReLU,
  ReLUHalf,
  GRU,
  MatrixMultiply,
  Scale,
  FusedReshapeConcat,
  FusedReshapeConcatGeneral,
  Softmax,
  PReLU_Dice,
  ReduceMean,
  Sub,
  Gather,
  Sigmoid,
  Slice,
  WeightMultiply,
  FmOrder2,
  Add,
  ReduceSum,
  MultiCross,
  Cast,
  DotProduct,
  ElementwiseMultiply
};

我们使用 SigmoidLayer 作为例子，大家来看看。

/**
 * Sigmoid activation function as a derived class of Layer
 */
template 
class SigmoidLayer : public Layer {
  /*
   * stores the references to the input tensors of this layer.
   */
  Tensors2 in_tensors_;
  /*
   * stores the references to the output tensors of this layer.
   */
  Tensors2 out_tensors_;

 public:
  /**
   * Ctor of SigmoidLayer.
   * @param in_tensor the input tensor
   * @param out_tensor the output tensor which has the same dim with in_tensor
   * @param device_id the id of GPU where this layer belongs
   */
  SigmoidLayer(const Tensor2& in_tensor, const Tensor2& out_tensor,
               const std::shared_ptr& gpu_resource);

  /**
   * A method of implementing the forward pass of Sigmoid
   * @param stream CUDA stream where the foward propagation is executed
   */
  void fprop(bool is_train) override;
  /**
   * A method of implementing the backward pass of Sigmoid
   * @param stream CUDA stream where the backward propagation is executed
   */
  void bprop() override;
};

其前向传播如下：

template 
void SigmoidLayer::fprop(bool is_train) {
  CudaDeviceContext context(get_device_id());

  int len = in_tensors_[0].get_num_elements();

  auto fop = [] __device__(T in) { return T(1) / (T(1) + exponential(-in)); };

  MLCommon::LinAlg::unaryOp(out_tensors_[0].get_ptr(), in_tensors_[0].get_ptr(), len, fop,
                            get_gpu().get_stream());

#ifndef NDEBUG
  cudaDeviceSynchronize();
  CK_CUDA_THROW_(cudaGetLastError());
#endif
}

其后向传播如下：

template 
void SigmoidLayer::bprop() {
  CudaDeviceContext context(get_device_id());

  int len = in_tensors_[0].get_num_elements();

  auto bop = [] __device__(T d_out, T d_in) {
    T y = T(1) / (T(1) + exponential(-d_in));
    return d_out * y * (T(1) - y);
  };

  MLCommon::LinAlg::binaryOp(in_tensors_[0].get_ptr(), out_tensors_[0].get_ptr(),
                             in_tensors_[0].get_ptr(), len, bop, get_gpu().get_stream());

#ifndef NDEBUG
  cudaDeviceSynchronize();
  CK_CUDA_THROW_(cudaGetLastError());
#endif
}

至此，HugeCTR 已经初始化完毕，接下来可以开始训练了，我们摘录官方HugeCTR_Webinar 之中的图来给大家做一个梳理，这里CSR是嵌入层依赖的数据格式，我们下文会分析。

4.5.3 层与层之间如何串联

我们尚且有一个疑问，那就是层与层之间如何串联起来？

在create_layers之中有如下代码：

// 获取本层的输入和输出
auto input_output_info = get_input_tensor_and_output_name(j, tensor_entries);

get_input_tensor_and_output_name 的代码如下，可以看到，每一层都会记录自己的输入和输出，结合内部解析模块，这些层就建立其了逻辑关系。

static InputOutputInfo get_input_tensor_and_output_name(
    const nlohmann::json& json, const std::vector& tensor_entries) {
  auto bottom = get_json(json, "bottom");
  auto top = get_json(json, "top");

  // 从jason获取输入，输出名字
  std::vector bottom_names = get_layer_names(bottom);
  std::vector top_names = get_layer_names(top);

  std::vector bottom_bags;
  // 把输出组成一个向量列表
  for (auto& bottom_name : bottom_names) {
    for (auto& top_name : top_names) {
      if (bottom_name == top_name) {
        CK_THROW_(Error_t::WrongInput, "bottom and top include a same layer name");
      }
    }
    TensorBag2 bag;
    if (!get_tensor_from_entries(tensor_entries, bottom_name, &bag)) {
      CK_THROW_(Error_t::WrongInput, "No such bottom: " + bottom_name);
    }
    bottom_bags.push_back(bag);
  }
  return {bottom_bags, top_names}; // 返回
}

最终，建立流水线逻辑关系如下：

0x05 训练

具体训练代码逻辑如下：

• 需要 reader 先读取一个 batchsize 的数据。
• 开始解析数据。
• 嵌入层进行前向传播，即从参数服务器读取embedding，进行处理。
• 对于网络层进行前向传播和后向传播，具体区分是多卡，单卡，多机，单机等。
• 嵌入层反向操作。
• 多卡之间交换dense参数的梯度。
• 嵌入层更新sparse参数。
• 各个流进行同步。

bool Session::train() {
  try {
    // 确保 train_data_reader_ 已经启动
    if (train_data_reader_->is_started() == false) {
      CK_THROW_(Error_t::IllegalCall,
                "Start the data reader first before calling Session::train()");
    }

#ifndef DATA_READING_TEST
    // 需要 reader 先读取一个 batchsize 的数据。
    long long current_batchsize = train_data_reader_->read_a_batch_to_device_delay_release();
    if (!current_batchsize) {
      return false; // 读不到就退出，没有数据了
    }
    #pragma omp parallel num_threads(networks_.size()) //其后语句将被networks_.size()个线程并行执行
    { 
      
      size_t id = omp_get_thread_num();
      CudaCPUDeviceContext ctx(resource_manager_->get_local_gpu(id)->get_device_id());
      cudaStreamSynchronize(resource_manager_->get_local_gpu(id)->get_stream());
    }
    // reader 可以开始解析数据
    train_data_reader_->ready_to_collect();
#ifdef ENABLE_PROFILING
    global_profiler.iter_check();
#endif

    // If true we're gonna use overlaping, if false we use default
    if (solver_config_.use_overlapped_pipeline) {
      train_overlapped();
    } else {
      for (const auto& one_embedding : embeddings_) {
        one_embedding->forward(true); // 嵌入层进行前向传播，即从参数服务器读取embedding，进行处理
      }

      // Network forward / backward
      if (networks_.size() > 1) { // 因为之前是把模型分别拷贝到GPU之上，所以size大于1，就说明多卡
        // 单机多卡或多机多卡
        // execute dense forward and backward with multi-cpu threads
        #pragma omp parallel num_threads(networks_.size())
        {
          // dense网络的前向反向
          size_t id = omp_get_thread_num();
          long long current_batchsize_per_device =
              train_data_reader_->get_current_batchsize_per_device(id);
          networks_[id]->train(current_batchsize_per_device); // 前向操作
          const auto& local_gpu = resource_manager_->get_local_gpu(id);
          local_gpu->set_compute_event_sync(local_gpu->get_stream());
          local_gpu->wait_on_compute_event(local_gpu->get_comp_overlap_stream());
        }
      } else if (resource_manager_->get_global_gpu_count() > 1) {
        // 多机单卡
        long long current_batchsize_per_device =
            train_data_reader_->get_current_batchsize_per_device(0);
        networks_[0]->train(current_batchsize_per_device); // 前向操作
        const auto& local_gpu = resource_manager_->get_local_gpu(0);
        local_gpu->set_compute_event_sync(local_gpu->get_stream());
        local_gpu->wait_on_compute_event(local_gpu->get_comp_overlap_stream());
      } else {
        // 单机单卡
        long long current_batchsize_per_device =
            train_data_reader_->get_current_batchsize_per_device(0);
        networks_[0]->train(current_batchsize_per_device); // 前向操作
        const auto& local_gpu = resource_manager_->get_local_gpu(0);
        local_gpu->set_compute_event_sync(local_gpu->get_stream());
        local_gpu->wait_on_compute_event(local_gpu->get_comp_overlap_stream());
        networks_[0]->update_params();
      }

      // Embedding backward
      for (const auto& one_embedding : embeddings_) {
        one_embedding->backward(); // 嵌入层反向操作
      }

      // Exchange wgrad and update params
      if (networks_.size() > 1) {
        #pragma omp parallel num_threads(networks_.size())
        {
          size_t id = omp_get_thread_num();
          exchange_wgrad(id); // 多卡之间交换dense参数的梯度
          networks_[id]->update_params();
        }
      } else if (resource_manager_->get_global_gpu_count() > 1) {
        exchange_wgrad(0);
        networks_[0]->update_params(); 
      } 
      for (const auto& one_embedding : embeddings_) {
        one_embedding->update_params(); // 嵌入层更新sparse参数
      }

      // Join streams 各个流进行同步
      if (networks_.size() > 1) {
        #pragma omp parallel num_threads(networks_.size())
        {
          size_t id = omp_get_thread_num();
          const auto& local_gpu = resource_manager_->get_local_gpu(id);
          local_gpu->set_compute2_event_sync(local_gpu->get_comp_overlap_stream());
          local_gpu->wait_on_compute2_event(local_gpu->get_stream());
        }
      }
      else {
        const auto& local_gpu = resource_manager_->get_local_gpu(0);
        local_gpu->set_compute2_event_sync(local_gpu->get_comp_overlap_stream());
        local_gpu->wait_on_compute2_event(local_gpu->get_stream());
      }
      return true;
    }
#else
      data_reader_->read_a_batch_to_device();
#endif

  } catch (const internal_runtime_error& err) {
    std::cerr << err.what() << std::endl;
    throw err;
  } catch (const std::exception& err) {
    std::cerr << err.what() << std::endl;
    throw err;
  }
  return true;
}

训练流程如下：

至此，我们大体知道了 HugeCTR如何初始化和训练，下一篇我们介绍如何读取数据。

0xFF 参考

https://developer.nvidia.com/blog/introducing-merlin-hugectr-training-framework-dedicated-to-recommender-systems/

https://developer.nvidia.com/blog/announcing-nvidia-merlin-application-framework-for-deep-recommender-systems/

https://developer.nvidia.com/blog/accelerating-recommender-systems-training-with-nvidia-merlin-open-beta/

HugeCTR源码阅读

embedding层如何反向传播

https://web.eecs.umich.edu/~justincj/teaching/eecs442/notes/linear-backprop.html

HugeCTR_Webinar