当计算图在设备之间划分之后，跨设备的 PartitionGraph 之间可能存在着数据依赖关系，因此 TF 在它们之间插入 Send/Recv 节点，这样就完成数据交互。而在分布式模式之中，Send/Recv 通过 RpcRemoteRendezvous 完成数据交换，所以我们需要先看看 TF 之中的数据交换机制 Rendezvous。
迄今为止，在分布式机器学习之中，我们看到了太多的 Rendezvous，其大多出现在弹性和通信相关部分，虽然具体意义各有细微不同，但是基本意义都差不多，就是来自其法语单词的原意：会合，聚会，集会，约会等。TensorFlow的Rendezvous是消息传输的通信组件和交换机制。
本文依旧深度借鉴了两位大神：
[TensorFlow Internals] (https://github.com/horance-liu/tensorflow-internals)，虽然其分析的不是最新代码，但是建议对 TF 内部实现机制有兴趣的朋友都去阅读一下，绝对大有收获。
https://home.cnblogs.com/u/deep-learning-stacks/ 西门宇少，不仅仅是 TensorFlow，其公共号还有更多其他领域，业界前沿。
本系列其他文章是：
[翻译] TensorFlow 分布式之论文篇 "TensorFlow : Large-Scale Machine Learning on Heterogeneous Distributed Systems"
[翻译] TensorFlow 分布式之论文篇 "Implementation of Control Flow in TensorFlow"
[源码解析] TensorFlow 分布式环境(1) --- 总体架构
[源码解析] TensorFlow 分布式环境(2)---Master 静态逻辑
[源码解析] TensorFlow 分布式环境(3)--- Worker 静态逻辑
[源码解析] TensorFlow 分布式环境(4) --- WorkerCache
[源码解析] TensorFlow 分布式环境(5) --- Session
[源码解析] TensorFlow 分布式环境(7) --- Worker 动态逻辑
在分布式模式之中，对跨设备的边会进行分裂，在边的发送端和接收端会分别插入 Send 节点和 Recv 节点。
我们假设 Worker 0 有两个 GPU，当插入Send 节点和 Recv 节点，效果如下，其中 Worker 1 发送给 Worker 之间的代表进程间通过 GrpcRemoteRendezvous 实现数据交换，Worker 0 内部两个 GPU 之间的虚线箭头代表进程内部通过 IntraProcessRendezvous 实现数据交换，Worker 之间的实线箭头表示使用 RPC 进行数据交换。
当执行某次 step，如果两个 Worker 需要交互数据，则：
其中send/recv 的数据传输是通过 WorkerInterface 的派生类作为接口完成的，WorkerInterface 则基于底层的 gRPC 通信库。
图 1 发送/接受
我们在学习 PyTorch 分布式时候，就知道每次分布式通信都需要有一个全局唯一的标识符，比如：
类似的，TF 也需要为每一个Send/Recv Pair 确定一个唯一的标识符，这样在多组消息并行发送时候，才不会发生消息错位。这个标识符就是 ParsedKey。
其定义如下：
// Parses the key constructed by CreateKey and parse src/dst device// names into structures respectively.struct ParsedKey { StringPiece src_device; DeviceNameUtils::ParsedName src; uint64 src_incarnation = 0; StringPiece dst_device; DeviceNameUtils::ParsedName dst; StringPiece edge_name; ParsedKey() {} ParsedKey(const ParsedKey& b) { *this = b; } ParsedKey& operator=(const ParsedKey& b); StringPiece FullKey() const { return buf_; } private: friend class Rendezvous; friend class SendOp; friend class RecvOp; std::string buf_;};具体生成字符串 key 结果如下：
src_device ; HexString(src_incarnation) ; dst_device ; name ; frame_iter.frame_id : frame_iter.iter_id具体代码如下：
/* static */string Rendezvous::CreateKey(const string& src_device, uint64 src_incarnation, const string& dst_device, const string& name, const FrameAndIter& frame_iter) { // NOTE: ';' is not used in the device name's job name. // // We include both sender and receiver in the key to facilitate // debugging. For correctness, we only need to encode the receiver. // // "src_incarnation" is used to distinguish a worker when it // restarts. char buf[strings::kFastToBufferSize]; return strings::StrCat( src_device, ";", strings::Uint64ToHexString(src_incarnation, buf), ";", dst_device, ";", name, ";", frame_iter.frame_id, ":", frame_iter.iter_id);}然后系统会使用 ParseKey 方法来解析key，生成 ParsedKey。ParseKey 对输入 key 的前四个域做了映射，抛弃第五个域 frame_iter.frame_id : frame_iter.iter_id。其他都直接对应字面意思，只是 edge_name 对应了 name。
/* static */Status Rendezvous::ParseKey(StringPiece key, ParsedKey* out) { if (key.data() == out->buf_.data()) { // Caller used our buf_ string directly, so we don't need to copy. (The // SendOp and RecvOp implementations do this, for example). DCHECK_EQ(key.size(), out->buf_.size()); } else { // Make a copy that our StringPieces can point at a copy that will persist // for the lifetime of the ParsedKey object. out->buf_.assign(key.data(), key.size()); } StringPiece s(out->buf_); StringPiece parts[5]; for (int i = 0; i < 5; i++) { parts[i] = ConsumeNextPart(&s, ';'); } if (s.empty() && // Consumed the whole string !parts[4].empty() && // Exactly five parts DeviceNameUtils::ParseFullName(parts[0], &out->src) && strings::HexStringToUint64(parts[1], &out->src_incarnation) && DeviceNameUtils::ParseFullName(parts[2], &out->dst) && !parts[3].empty()) { out->src_device = StringPiece(parts[0].data(), parts[0].size()); out->dst_device = StringPiece(parts[2].data(), parts[2].size()); out->edge_name = StringPiece(parts[3].data(), parts[3].size()); return Status::OK(); } return errors::InvalidArgument("Invalid rendezvous key: ", key);}Rendezvous 是一个抽象，用于从生产者向消费者传递张量。一个 rendezvous 是一个通道（channels）的表（table）。每个通道都由一个 rendezvous 键来标记。该键编码为<生产者，消费者>对，其中生产者和消费者是 tensorflow 设备。
生产者调用 Send() 方法在一个命名的通道上发送一个张量。消费者调用 Recv() 方法从一个指定的通道接收一个张量。一个张量的序列可以从生产者传递给消费者。 消费者按照生产者发送的顺序接收它们。
消费者可以在张量产生之前或之后安全地请求张量。 消费者可以选择进行阻塞式调用或提供回调：无论哪种情况，消费者都会在张量可用时收到它。 生产者永远不会阻塞。
RendezvousInterface 是接口类，定义了虚函数。ParsedKey 也是定义在这里（我们省略了这部分代码）。
class RendezvousInterface { public: struct Args { DeviceContext* device_context = nullptr; AllocatorAttributes alloc_attrs; CancellationManager* cancellation_manager = nullptr; // not owned. }; // The caller is a tensor producer and it sends a message (a tensor // "val" and a bool "is_dead") under the given "key". // // {val, is_dead} is bundled as a message sent and received. // Typically, is_dead is set by some control flow nodes // (e.g., a not-taken branch). args is passed by Send to the // Recv function to communicate any information that the Recv // function might need. This is typically only necessary for // Send/Recv on the same worker. // // Send() never blocks. virtual Status Send(const ParsedKey& key, const Args& args, const Tensor& val, const bool is_dead) = 0; // Callback provided by a tensor consumer waiting on the rendezvous. // It will be invoked when the tensor is available, or when a non-OK // status arises in the production of that tensor. It also gets // two Rendezvous::Args, one provided by the sender, the other by the // receiver, which may be needed when a non-CPU device is in use // by either side. typedef std::function<void(const Status&, const Args&, const Args&, const Tensor&, const bool)> DoneCallback; virtual void RecvAsync(const ParsedKey& key, const Args& args, DoneCallback done) = 0; // Synchronous wrapper for RecvAsync. Status Recv(const ParsedKey& key, const Args& args, Tensor* val, bool* is_dead, int64_t timeout_ms); Status Recv(const ParsedKey& key, const Args& args, Tensor* val, bool* is_dead); // Aborts all pending and future Send/Recv with the given "status". // StartAbort() does not wait for ongoing calls to finish. // REQUIRES: !status.ok() virtual void StartAbort(const Status& status) = 0; protected: virtual ~RendezvousInterface(); virtual bool is_cross_process() { return false; } friend class ProcessFunctionLibraryRuntime;};Rendezvous 类提供了最基本的 Send、Recv 和 RecvAsync 的实现，也提供了 ParseKey 功能。
// A reference-counted implementation of RendezvousInterface.//// This class is used in cases where a rendezvous may be shared between multiple// threads with no clear owner.class Rendezvous : public RendezvousInterface, public core::RefCounted { public: class Factory { public: // Default to a factory that evaluates to false. Factory() : valid_(false) {} Factory(std::function<Status(const int64_t, const DeviceMgr*, Rendezvous**)> create_fn, std::function<Status(const int64_t)> cleanup_fn) : valid_(true), create_fn_(std::move(create_fn)), cleanup_fn_(std::move(cleanup_fn)) {} // If no clean up fn is provided, just put in a dummy. // For backwards compatibility. explicit Factory( std::function<Status(const int64_t, const DeviceMgr*, Rendezvous**)> create_fn) : valid_(true), create_fn_(std::move(create_fn)), cleanup_fn_([](const int64_t step_id) { return Status::OK(); }) {} explicit operator bool() const { return valid_; } Status operator()(const int64_t step_id, const DeviceMgr* device_mgr, Rendezvous** rendez) const { return create_fn_(step_id, device_mgr, rendez); } Status CleanUp(const int64_t step_id) const { return cleanup_fn_(step_id); } private: bool valid_; std::function<Status(const int64_t, const DeviceMgr*, Rendezvous**)> create_fn_; std::function<Status(const int64_t)> cleanup_fn_; }; // Constructs a rendezvous key for the tensor of "name" sent from // "src_device" to "dst_device". The tensor is generated in the frame // and iteration specified by "frame_iter". static std::string CreateKey(const std::string& src_device, uint64 src_incarnation, const std::string& dst_device, const std::string& name, const FrameAndIter& frame_iter); static Status ParseKey(StringPiece key, ParsedKey* out);};RemoteRendezvous 继承了 Rendezvous，其只增加了一个纯虚函数 Initialize 方法。所有跨进程通信的派生类都需要重写此函数，因为需要借助 Session 成初始化工作。
RemoteRendezvous 可以处理两个远端进程之中生产者或消费者的情况，增加了与远程工作者协调的功能。RemoteRendezvous 遵循两阶段初始化策略：首先，对象被构建。最终，它们将被初始化。RendezvousMgrInterface 的客户端必须保证最终对返回的 RemoteRendezvous 调用了 nitialize 方法。
// RemoteRendezvous follow a 2-part initialization. First the objects are// constructed. Eventually, they will be initialized. Clients of the// RendezvousMgrInterface must guarantee to call Initialize on the returned// RemoteRendezvous eventually.//// Partially initialized RemoteRendezvous must respect the Rendezvous interface// (i.e. Send() must never block), however implementations are not expected to// actually perform the underlying operations until after the RemoteRendezvous// has been Initialize'd.class RemoteRendezvous : public Rendezvous { public: // Fully construct the RemoteRendezvous. virtual Status Initialize(WorkerSession* session) = 0; protected: bool is_cross_process() override { return true; }};因为跨进程通信存在不同协议，所以跨进程通信的各种 Rendezvous 都需要依据自己不同的协议来实现。所以 TF 在 RemoteRendezvous 和真正特化的各种 Rendezvous 中间加入了一个中间层 BaseRemoteRendezvous，这个类起到了承上启下的作用，提供了公共的 Send 和 Recv 方法，可以做到尽可能代码复用。
BaseRemoteRendezvous 主要成员变量是 Rendezvous* local_，代码之中大量使用了 BaseRecvTensorCall 作为参数，BaseRecvTensorCall 是通信的实体抽象。
// RemoteRendezvous is a Rendezvous which can handle either// the producer or consumer being in a remote process.//// Buffering of Tensor values is delegated to a "local" Rendezvous// obtained from NewLocalRendezvous(). This class just adds// functionality to coordinate with remote workers.class BaseRemoteRendezvous : public RemoteRendezvous { public: BaseRemoteRendezvous(const WorkerEnv* env, int64_t step_id); // Upgrades the BaseRemoteRendezvous to full initialization. Status Initialize(WorkerSession* session) override; // Forwards to local_, where the Tensor "val" will be buffered and // any waiting callback stored. Status Send(const ParsedKey& key, const Rendezvous::Args& args, const Tensor& val, const bool is_dead) override; // This method is called only by the RecvOp. It tests to see // whether the value will be produced by a local or remote device // and handles accordingly. In the local case it forwards to // local_, in the remote case it initiates an RPC request. void RecvAsync(const ParsedKey& key, const Rendezvous::Args& args, DoneCallback done) override; void StartAbort(const Status& status) override; // This method is called only by the local Worker, forwarded through // the same method on RendezvousMgr. This occurs when the Worker // has received a RecvTensor request, either locally or over the // network. In either case it needs to retrieve a locally buffered // value from local_, and give it to its caller. // // Runs "done" as soon as the tensor for "parsed" is available or an error // is detected. // // REQUIRES: "parsed" is one that will be Saved into the local rendezvous. void RecvLocalAsync(const ParsedKey& parsed, DoneCallback done); protected: virtual void RecvFromRemoteAsync(const Rendezvous::ParsedKey& parsed, const Rendezvous::Args& args, DoneCallback done) = 0; // Returns true if "src" and "dst" are located in the same worker, // and hence may use a local rendezvous. virtual bool IsSameWorker(DeviceNameUtils::ParsedName src, DeviceNameUtils::ParsedName dst); // If aborted, aborts "call". Otherwise, adds "call" into active_. void RegisterCall(BaseRecvTensorCall* call, const Rendezvous::Args& args); // Removes "call" from active_ if "call" is in active_. void DeregisterCall(BaseRecvTensorCall* call); WorkerSession* session(); bool is_initialized(); ~BaseRemoteRendezvous() override; const WorkerEnv* const env_; // Not owned. const int64_t step_id_; private: Rendezvous* local_; // Owns a Ref on this object. mutable mutex mu_; // Status given by StartAbort() if any. Status status_ TF_GUARDED_BY(mu_); WorkerSession* session_ TF_GUARDED_BY(mu_); // Not owned. // Data structures to handle calls when partially initialized. struct DeferredCall { const ParsedKey parsed; DoneCallback done; DeferredCall(const ParsedKey& parsed, DoneCallback done); }; std::vector<DeferredCall> deferred_calls_ TF_GUARDED_BY(mu_); typedef std::function<void()> InactiveCallback; std::unordered_map<BaseRecvTensorCall*, InactiveCallback> active_ TF_GUARDED_BY(mu_); bool is_initialized_locked() TF_SHARED_LOCKS_REQUIRED(mu_) { return session_ != nullptr; } // If "is_src" is true, checks that the rendezvous key "parsed"'s // source is in this process. If "is_src" is false, checks that the // rendezvous key "parsed"'s destination is in this process. Status ValidateDevices(const Rendezvous::ParsedKey& parsed, bool is_src); // Callback handling the case when a rendezvous has been // accomplished in local_ and the consumer is local to this process. // Tensor "in" will be copied into "out". The key "parsed" encodes // the src and dst devices. void SameWorkerRecvDone(const Rendezvous::ParsedKey& parsed, const Rendezvous::Args& in_args, const Rendezvous::Args& out_args, const Tensor& in, Tensor* out, StatusCallback done); // Must be called only if fully initialized. void RecvLocalAsyncInternal(const ParsedKey& parsed, DoneCallback done); TF_DISALLOW_COPY_AND_ASSIGN(BaseRemoteRendezvous);};class BaseRecvTensorCall { public: BaseRecvTensorCall() {} virtual ~BaseRecvTensorCall() {} virtual void Start(std::function<void()> recv_done) = 0; virtual void StartAbort(const Status& s) = 0; virtual Status status() const = 0; private: TF_DISALLOW_COPY_AND_ASSIGN(BaseRecvTensorCall);};在创建时候构建了一个 local Rendezvous，这个 local Rendezvous用来完成基本业务。
BaseRemoteRendezvous::BaseRemoteRendezvous(const WorkerEnv* env, int64_t step_id) : env_(env), step_id_(step_id), local_(NewLocalRendezvous()), session_(nullptr) {}Rendezvous* NewLocalRendezvous() { return new LocalRendezvousWrapper; }LocalRendezvousWrapper 定义如下：
class LocalRendezvousWrapper : public Rendezvous { public: LocalRendezvousWrapper() : impl_(this) {} Status Send(const ParsedKey& key, const Args& send_args, const Tensor& val, const bool is_dead) override { return impl_.Send(key, send_args, val, is_dead); } void RecvAsync(const ParsedKey& key, const Args& recv_args, DoneCallback done) override { impl_.RecvAsync(key, recv_args, std::move(done)); } void StartAbort(const Status& status) override { impl_.StartAbort(status); } private: LocalRendezvous impl_; TF_DISALLOW_COPY_AND_ASSIGN(LocalRendezvousWrapper);};我们接下来看看 BaseRemoteRendezvous 初始化方法，其中做了基础配置，比如设置session。
Status BaseRemoteRendezvous::Initialize(WorkerSession* session) { std::vector<DeferredCall> deferred_calls; { mutex_lock l(mu_); if (session_ != nullptr) { if (session_->worker_name() == session->worker_name()) { return Status::OK(); } Status s = errors::Internal( "Double init! Worker names would have changed from: ", session_->worker_name(), " -> ", session->worker_name()); return s; } session_ = session; std::swap(deferred_calls, deferred_calls_); } for (auto& call : deferred_calls) { RecvLocalAsyncInternal(call.parsed, std::move(call.done)); } return Status::OK();}RpcRemoteRendezvous 是 RemoteRendezvous 的 gRPC 协议实现。
class RpcRemoteRendezvous : public BaseRemoteRendezvous { public: RpcRemoteRendezvous(const WorkerEnv* env, int64_t step_id) : BaseRemoteRendezvous(env, step_id) {} protected: void RecvFromRemoteAsync(const Rendezvous::ParsedKey& parsed, const Rendezvous::Args& args, DoneCallback done) override; private: ~RpcRemoteRendezvous() override {} TF_DISALLOW_COPY_AND_ASSIGN(RpcRemoteRendezvous);};BaseRecvTensorCall 对应的派生类是 RpcRecvTensorCall。
// Used only to retrieve tensors from remote processes.class RpcRecvTensorCall : public BaseRecvTensorCall { public: RpcRecvTensorCall() : wi_(nullptr), dst_device_(nullptr) {} void Init(WorkerInterface* wi, int64_t step_id, StringPiece key, AllocatorAttributes alloc_attrs, Device* dst_device, const Rendezvous::Args& recv_args, Rendezvous::DoneCallback done) { wi_ = wi; alloc_attrs_ = alloc_attrs; dst_device_ = dst_device; recv_args_ = recv_args; done_ = std::move(done); req_.set_step_id(step_id); req_.set_rendezvous_key(key.data(), key.size()); req_.set_request_id(GetUniqueRequestId()); } void Reset() { // The RpcRemoteRendezvous using this object is responsible for calling // ReleaseWorker() before Reset(). alloc_attrs_ = AllocatorAttributes(); dst_device_ = nullptr; // We don't clear opts_ and assume that Init will set up the state for // opts_ appropriately. req_.Clear(); resp_.Clear(); { mutex_lock l(mu_); status_ = Status::OK(); } done_ = nullptr; } ~RpcRecvTensorCall() override { // Since only the RpcRecvTensorFreeList will delete an // RpcRecvTensorCall, we require that ReleaseWorker() has been called before // the user releases a Call object to the free list. CHECK_EQ(static_cast<WorkerInterface*>(nullptr), wi_) << "Leaking WorkerInterface in RpcRecvTensorCall destructor."; } void Start(std::function<void()> recv_done) override { StartRTCall(std::move(recv_done)); } void StartAbort(const Status& s) override { { mutex_lock l(mu_); status_.Update(s); } opts_.StartCancel(); } Status status() const override { mutex_lock l(mu_); return status_; } void ReleaseWorker(WorkerCacheInterface* worker_cache) { DCHECK_NE(static_cast<WorkerInterface*>(nullptr), wi_) << "RpcRecvTensorCall::ReleaseWorker() called twice."; worker_cache->ReleaseWorker(src_worker_, wi_); wi_ = nullptr; } const Tensor& tensor() const { return resp_.tensor(); } bool is_dead() const { return resp_.metadata().is_dead(); } Device* dst_device() const { return dst_device_; } const Rendezvous::Args& recv_args() const { return recv_args_; } const Rendezvous::DoneCallback& done() const { return done_; } private: friend class RpcRemoteRendezvous; // Start the main RecvTensor call, checking for an async abort. void StartRTCall(std::function<void()> recv_done) { resp_.InitAlloc(dst_device_, alloc_attrs_); auto abort_checked = std::make_shared<Notification>(); auto cb = [this, abort_checked, recv_done = std::move(recv_done)](const Status& s) { // Make sure the Rendezvous abort checking is finished before running the // callback, which might destroy the current call object. abort_checked->WaitForNotification(); if (!s.ok()) { mutex_lock l(mu_); status_.Update(s); } recv_done(); }; wi_->RecvTensorAsync(&opts_, &req_, &resp_, std::move(cb)); // NOTE: Check if the rendezvous was aborted after sending out the RPC. The // ordering is important because StartAbort could be called right before // the RecvTensorAsync request registers its RPC cancellation to opts_. // In that case, the previous StartAbort would not trigger the // cancellation of this call. Status s; { mutex_lock l(mu_); s = status_; } if (!s.ok()) { opts_.StartCancel(); } // Notify that the abort check has finished. abort_checked->Notify(); } string src_worker_; string src_rel_device_; WorkerInterface* wi_; // Not owned. AllocatorAttributes alloc_attrs_; Device* dst_device_; CallOptions opts_; RecvTensorRequest req_; TensorResponse resp_; Rendezvous::Args recv_args_; Rendezvous::DoneCallback done_; mutable mutex mu_; Status status_ TF_GUARDED_BY(mu_); TF_DISALLOW_COPY_AND_ASSIGN(RpcRecvTensorCall);};目前的逻辑关系具体如下：
图 2 Rendezvous 逻辑关系
RendezvousMgr 主要负责创建和销毁 RemoteRendezvous，其会跟踪一组本地的 rendezvous 实例，本工作者发送的所有张量都在 RendezvousMgr 中缓冲，直到张量被接收。 每个全局唯一的 "step_id" 对应于一个由 RendezvousMgr 管理的本地 rendezvous实例。
RendezvousMgrInterface 是接口类。
// RendezvousMgr keeps track of a set of local rendezvous instances.// All tensors sent by this worker are buffered in a RendezvousMgr// until the tensor is received. Each global unique "step_id"// corresponds to one local rendezvous instance managed by a// RendezvousMgr.//// E.g.,// Rendezvous* rendez = worker_env->rendezvous_mgr->Find(0x8935);// fork execution of an graph executor using "rendez" on thread 1;// fork execution of another graph executor using "rendez" on thread 2;// ...// join threads 1 and 2;//// In the example above, execution in thread 1 and 2 communicates with// each other by send/recv operations through the "rend".//// Tensors sent and recved through rendezvous managed by this// RendezvousMgr must have keys generated by Rendezvous::CreateKey.class RendezvousMgrInterface { public: RendezvousMgrInterface() {} virtual ~RendezvousMgrInterface() {} // Returns Rendezvous supporting send and recv among workers in the // "step_id". The caller takes ownership of one reference on the // returned Rendezvous instance. // // Note: the caller must guarantee to eventually call Initialize on the // returned RemoteRendezvous virtual RemoteRendezvous* Find(int64_t step_id) = 0; // Finds the local rendezvous instance for the "step_id". Runs // "done" when the tensor for "key" is produced or an error occurs. // // This method is used by the rpc handler of RecvTensor. virtual void RecvLocalAsync(int64_t step_id, const Rendezvous::ParsedKey& parsed, Rendezvous::DoneCallback done) = 0; // Synchronous wrapper for RecvLocalAsync. virtual Status RecvLocal(int64_t step_id, const Rendezvous::ParsedKey& parsed, Tensor* val, bool* is_dead) = 0; // Removes rendezvous for "step_id". // // TODO(zhifengc): Have a background thread in worker that // periodically calls CleanupAll(). virtual void Cleanup(int64_t step_id) = 0;};BaseRendezvousMgr 实现了基本功能，比如依据step_id查找Rendezvous。
class BaseRendezvousMgr : public RendezvousMgrInterface { public: explicit BaseRendezvousMgr(const WorkerEnv* worker_env); ~BaseRendezvousMgr() override; // Returns Rendezvous supporting send and recv among workers in the // "step_id". The caller takes ownership of one reference on the // returned Rendezvous instance. // // Note: the caller must guarantee to eventually call Initialize on the // returned RemoteRendezvous RemoteRendezvous* Find(int64_t step_id) override; // Finds the local rendezvous instance for the "step_id". Runs // "done" when the tensor for "key" is produced or an error occurs. // // This method is used by the rpc handler of RecvTensor. void RecvLocalAsync(int64_t step_id, const Rendezvous::ParsedKey& parsed, Rendezvous::DoneCallback done) override; // Synchronous wrapper for RecvLocalAsync. Status RecvLocal(int64_t step_id, const Rendezvous::ParsedKey& parsed, Tensor* val, bool* is_dead) override; // Removes rendezvous for "step_id". void Cleanup(int64_t step_id) override; protected: virtual BaseRemoteRendezvous* Create(int64_t step_id, const WorkerEnv* worker_env) = 0; private: // Maps step_id to rendezvous. typedef absl::flat_hash_map<int64_t, BaseRemoteRendezvous*> Table; // Not owned. const WorkerEnv* const worker_env_; mutex mu_; Table table_ TF_GUARDED_BY(mu_); BaseRemoteRendezvous* FindOrCreate(int64_t step_id); TF_DISALLOW_COPY_AND_ASSIGN(BaseRendezvousMgr);};在前面执行计算时候，我们看到了一些关于 Rendezvous 的使用，接下来我们就找几个情景来分析一下。
我们首先看看接受方的 worker。
Worker 在 DoRunGraph 方法之中会接受张量。
void Worker::DoRunGraph(CallOptions* opts, RunGraphRequestWrapper* request, MutableRunGraphResponseWrapper* response, StatusCallback done) { session->graph_mgr()->ExecuteAsync( request->graph_handle(), step_id, session.get(), request->exec_opts(), collector, response, cm, in, [this, step_id, response, session, cm, out, token, collector, device_profiler_session, opts, done](const Status& status) { Status s = status; if (s.ok()) { // 接受张量 s = session->graph_mgr()->RecvOutputs(step_id, out); } });}RecvOutputs 方法如下，就是依据step_id获取一个Rendezvous，然后接受消息。
Status GraphMgr::RecvOutputs(const int64_t step_id, NamedTensors* out) { Rendezvous* rendezvous = worker_env_->rendezvous_mgr->Find(step_id); Status s = RecvOutputsFromRendezvous(rendezvous, out, Rendezvous::Args()); rendezvous->Unref(); size_t output_size = 0; for (auto& p : *out) { output_size += p.second.AllocatedBytes(); } return s;}具体如下图所示，流程顺序如图上数字，其中第3步返回了一个Rendezvous，RecvOutputsFromRendezvous 是一个全局方法。
DoPartialRunGraph 会调用 RecvOutputsAsync 完成接受任务。
void Worker::DoPartialRunGraph(CallOptions* opts, RunGraphRequestWrapper* request, MutableRunGraphResponseWrapper* response, StatusCallback done) { const int64_t step_id = request->step_id(); const string& graph_handle = request->graph_handle(); Status s = recent_request_ids_.TrackUnique( request->request_id(), "PartialRunGraph (Worker)", request); std::shared_ptr<WorkerSession> session; if (request->create_worker_session_called()) { s = env_->session_mgr->WorkerSessionForSession(request->session_handle(), &session); } else { session = env_->session_mgr->LegacySession(); } GraphMgr::NamedTensors in; GraphMgr::NamedTensors* out = new GraphMgr::NamedTensors; s = PrepareRunGraph(request, &in, out); auto finish = [done, out, opts](const Status& s) { opts->ClearCancelCallback(); delete out; done(s); }; CancellationManager* cm = nullptr; bool is_new_partial_run = partial_run_mgr_.FindOrCreate(step_id, &cm); // Before we start doing anything, we set the RPC cancellation. opts->SetCancelCallback([this, cm, step_id]() { cm->StartCancel(); AbortStep(step_id); }); // If this is a new partial run request, the request will need to start the // executors. if (is_new_partial_run) { CancellationToken token; token = cancellation_manager_.get_cancellation_token(); cancellation_manager_.RegisterCallback(token, [cm]() { cm->StartCancel(); }); session->graph_mgr()->ExecuteAsync( graph_handle, step_id, session.get(), request->exec_opts(), nullptr /* collector */, nullptr /* response */, cm, in, [this, token, step_id, session](Status s) { cancellation_manager_.DeregisterCallback(token); partial_run_mgr_.ExecutorDone(step_id, s); }); } else { // Send the partial run's new inputs. s = session->graph_mgr()->SendInputs(step_id, in); } // 这里会调用到 RecvOutputsAsync 来接受张量 session->graph_mgr()->RecvOutputsAsync( step_id, out, [this, out, request, response, step_id, finish](Status s) { if (s.ok()) { // Construct and return the resp. for (const auto& p : *out) { const string& key = p.first; const Tensor& val = p.second; response->AddRecv(key, val); } } if (request->is_last_partial_run()) { partial_run_mgr_.PartialRunDone(step_id, finish, s); } else { finish(s); } });}RecvOutputsAsync 这里调用了 RecvOutputsFromRendezvousAsync。
void GraphMgr::RecvOutputsAsync(const int64_t step_id, NamedTensors* out, StatusCallback done) { Rendezvous* rendezvous = worker_env_->rendezvous_mgr->Find(step_id); std::vector<string> keys; std::vector<Tensor>* received_keys = new std::vector<Tensor>; keys.reserve(out->size()); received_keys->reserve(out->size()); for (const auto& p : *out) { keys.push_back(p.first); received_keys->push_back(p.second); } RecvOutputsFromRendezvousAsync( rendezvous, nullptr, {}, keys, received_keys, [done, rendezvous, received_keys, out, keys](const Status s) { rendezvous->Unref(); size_t output_size = 0; for (int i = 0, end = keys.size(); i < end; ++i) { (*out)[keys[i]] = (*received_keys)[i]; output_size += (*out)[keys[i]].AllocatedBytes(); } metrics::RecordGraphOutputTensors(output_size); delete received_keys; done(s); });}具体如下图，流程顺序如图上数字，其中第3步返回了一个Rendezvous，RecvOutputsFromRendezvousAsync是一个全局方法。
在 ExecuteAsync 之中会发送张量。
void GraphMgr::ExecuteAsync(const string& handle, const int64_t step_id, WorkerSession* session, const ExecutorOpts& opts, StepStatsCollector* collector, MutableRunGraphResponseWrapper* response, CancellationManager* cancellation_manager, const NamedTensors& in, StatusCallback done) { if (s.ok()) { // 发送张量 s = SendTensorsToRendezvous(rendezvous, nullptr, {}, keys, tensors_to_send); } // 执行子计算图 StartParallelExecutors( handle, step_id, item, rendezvous, ce_handle, collector, cost_graph, cancellation_manager, session, start_time_usecs, [item, rendezvous, ce_handle, done, start_time_usecs, input_size, step_id](const Status& s) { });}SendTensorsToRendezvous 如下：
Status SendTensorsToRendezvous( RendezvousInterface* rendezvous, DeviceContext* device_context, const std::vector<AllocatorAttributes>& alloc_attrs, const std::vector<string>& keys, gtl::ArraySlice<Tensor> tensors_to_send) { Rendezvous::ParsedKey parsed; for (int i = 0; i < keys.size(); ++i) { Rendezvous::Args rendez_args; rendez_args.device_context = device_context; if (!alloc_attrs.empty()) { rendez_args.alloc_attrs = alloc_attrs[i]; } TF_RETURN_IF_ERROR(Rendezvous::ParseKey(keys[i], &parsed)); TF_RETURN_IF_ERROR( rendezvous->Send(parsed, rendez_args, tensors_to_send[i], false)); } return Status::OK();}我们接下来就仔细分析一下如何接受和发送。
我们首先看看发送流程。Send 过程并不涉及跨进程传输，所以和本地场景下的 Send 传输过程相同，这里只是把张量放到 Worker 的本地 Table 之中，完全不涉及跨网络传输，是非阻塞的。
Send 方法调用了 local_->Send 完成功能。
Status BaseRemoteRendezvous::Send(const Rendezvous::ParsedKey& parsed, const Rendezvous::Args& args, const Tensor& val, const bool is_dead) { WorkerSession* sess = nullptr; { tf_shared_lock l(mu_); if (!status_.ok()) return status_; sess = session_; } if (!IsLocalDevice(sess->worker_name(), parsed.src_device)) { return errors::InvalidArgument( "Invalid rendezvous key (src): ", parsed.FullKey(), " @ ", sess->worker_name()); } // Buffers "val" and "device_context" in local_. return local_->Send(parsed, args, val, is_dead);}LocalRendezvous::Send 会把张量插入到本地表。
Status LocalRendezvous::Send(const Rendezvous::ParsedKey& key, const Rendezvous::Args& send_args, const Tensor& val, const bool is_dead) { uint64 key_hash = KeyHash(key.FullKey()); if (is_dead) { static auto* rendezvous_dead_values_sent = monitoring::Counter<2>::New( "/tensorflow/core/rendezvous_dead_values_sent", "The number of dead values sent between a pair of devices.", "send_device", "recv_device"); rendezvous_dead_values_sent ->GetCell(string(key.src_device), string(key.dst_device)) ->IncrementBy(1); } mu_.lock(); if (!status_.ok()) { // Rendezvous has been aborted. Status s = status_; mu_.unlock(); return s; } ItemQueue* queue = &table_[key_hash]; if (queue->head == nullptr || queue->head->type == Item::kSend) { // There is no waiter for this message. Append the message // into the queue. The waiter will pick it up when arrives. // Only send-related fields need to be filled. queue->push_back(new Item(send_args, val, is_dead)); mu_.unlock(); return Status::OK(); } // There is an earliest waiter to consume this message. Item* item = queue->head; // Delete the queue when the last element has been consumed. if (item->next == nullptr) { table_.erase(key_hash); } else { queue->head = item->next; } mu_.unlock(); // Notify the waiter by invoking its done closure, outside the // lock. DCHECK_EQ(item->type, Item::kRecv); (*item->recv_state.waiter)(Status::OK(), send_args, item->args, val, is_dead); delete item; return Status::OK();}此时逻辑如下，这里 Worker 0 指代的是一个工作者角色，并非是 Worker 类。
图 3 发送逻辑
发送端现在已经把准备好的张量放入本地 table。接收端需要从发送端的 table 取出张量，这里就涉及了跨进程传输。接受的处理过程是：
这里重点是：数据传输由 recv 部分发起，向 Send 方主动发出请求来触发通信过程。这与我们常见的模式不同。我们知道，Worker 之中既有同步调用，也有异步调用，我们选择异步调用来看看。先提前给出一个发送接受流程让大家有个整体认识。下图之中虚线表示返回张量。
图 4 发送接受整体逻辑
客户端逻辑如下：
全局函数 RecvOutputsFromRendezvousAsync 调用到了 rendezvous->RecvAsync。
void RecvOutputsFromRendezvousAsync( RendezvousInterface* rendezvous, DeviceContext* device_context, const std::vector<AllocatorAttributes>& alloc_attrs, const std::vector<string>& keys, std::vector<Tensor>* received_tensors, StatusCallback done) { if (keys.empty()) { done(Status::OK()); return; } received_tensors->reserve(keys.size()); std::vector< std::tuple<string, Tensor*, Rendezvous::ParsedKey, AllocatorAttributes>> arguments; for (int i = 0; i < keys.size(); ++i) { Rendezvous::ParsedKey parsed; Status s = Rendezvous::ParseKey(keys[i], &parsed); received_tensors->push_back(Tensor()); if (!s.ok()) { done(s); return; } AllocatorAttributes alloc_attr; if (!alloc_attrs.empty()) { alloc_attr = alloc_attrs[i]; } arguments.emplace_back(keys[i], &((*received_tensors)[i]), parsed, alloc_attr); } auto status_cb = new ReffedStatusCallback(std::move(done)); for (auto& p : arguments) { const string& key = std::get<0>(p); Tensor* val = std::get<1>(p); Rendezvous::ParsedKey parsed = std::get<2>(p); Rendezvous::Args rendez_args; rendez_args.device_context = device_context; rendez_args.alloc_attrs = std::get<3>(p); status_cb->Ref(); rendezvous->RecvAsync( parsed, rendez_args, [val, key, status_cb](const Status& s, const Rendezvous::Args& send_args, const Rendezvous::Args& recv_args, const Tensor& v, const bool is_dead) { Status status = s; if (status.ok()) { *val = v; if (is_dead) { status = errors::InvalidArgument("The tensor returned for ", key, " was not valid."); } } status_cb->UpdateStatus(status); status_cb->Unref(); }); } status_cb->Unref();}因为不在一个进程之内，所以调用到了 RecvFromRemoteAsync。
void BaseRemoteRendezvous::RecvAsync(const ParsedKey& parsed, const Rendezvous::Args& recv_args, DoneCallback done) { Status s = ValidateDevices(parsed, false /*!is_src*/); profiler::ScopedMemoryDebugAnnotation op_annotation("RecvAsync", step_id_); // Are src and dst in the same worker? if (IsSameWorker(parsed.src, parsed.dst)) { // 在同一个worker里面 // Recv the tensor from local_. local_->RecvAsync( parsed, recv_args, [this, parsed, done]( const Status& status, const Rendezvous::Args& send_args, const Rendezvous::Args& recv_args, const Tensor& in, bool is_dead) { Tensor* out = new Tensor; StatusCallback final_callback = [done, send_args, recv_args, out, is_dead](const Status& s) { done(s, send_args, recv_args, *out, is_dead); delete out; }; if (status.ok()) { SameWorkerRecvDone(parsed, send_args, recv_args, in, out, std::move(final_callback)); } else { final_callback(status); } }); return; } else { // 不在同一个worker里面 RecvFromRemoteAsync(parsed, recv_args, std::move(done)); }}RpcRemoteRendezvous 检查各项参数，准备 RpcRecvTensorCall，之后启动 call->Start()，Start() 里面调的是 StartRTCall()。RpcRecvTensorCall 继承了 BaseRecvTensorCall 这个抽象基类，是一次 gRPC 调用的抽象，其封装了复杂的后续调用链。这里关键点是如下两句，就是如何使用对应的 Worker 设置 RpcRecvTensorCall：
WorkerInterface* rwi = worker_cache->GetOrCreateWorker(call->src_worker_);call->Init(rwi, step_id_, parsed.FullKey(), recv_args.alloc_attrs, dst_device, recv_args, std::move(done));完整代码如下：
void RpcRemoteRendezvous::RecvFromRemoteAsync( const Rendezvous::ParsedKey& parsed, const Rendezvous::Args& recv_args, DoneCallback done) { CHECK(is_initialized()); Status s; // Prepare a RecvTensor call that can handle being aborted. // 生成一个 Call RpcRecvTensorCall* call = get_call_freelist()->New(); // key.src_device identifies a remote device. if (!DeviceNameUtils::SplitDeviceName(parsed.src_device, &call->src_worker_, &call->src_rel_device_)) { s = errors::Internal(parsed.src_device, " is invalid remote source device."); } WorkerSession* sess = session(); std::shared_ptr<WorkerCacheInterface> worker_cache = sess->GetSharedWorkerCache(); // The worker will be released in a subsequent call to // sess->worker_cache()->ReleaseWorker() (if the call has not yet been // initialized) or call->ReleaseWorker() (if it has been initialized). // 拿到对应的 Worker WorkerInterface* rwi = worker_cache->GetOrCreateWorker(call->src_worker_); Device* dst_device; if (s.ok()) { s = sess->device_mgr()->LookupDevice(parsed.dst_device, &dst_device); } if (!s.ok()) { if (rwi != nullptr) { sess->worker_cache()->ReleaseWorker(call->src_worker_, rwi); } get_call_freelist()->Release(call); done(s, Args(), recv_args, Tensor{}, false); return; } // 用 Worker 来初始化 call->Init(rwi, step_id_, parsed.FullKey(), recv_args.alloc_attrs, dst_device, recv_args, std::move(done)); // Record "call" in active_ so that it can be aborted cleanly. RegisterCall(call, recv_args); // Start "call". Ref(); call->Start([this, call, worker_cache]() { // Removes "call" from active_. Prevent StartAbort(). DeregisterCall(call); // If StartAbort was called prior to DeregisterCall, then the // current status should be bad. Status s = call->status(); // NOTE: *session() can potentially be deleted before we return from // call->done()(...), so we must release the worker before calling the // callback. call->ReleaseWorker(session()->worker_cache()); call->done()(s, Args(), call->recv_args(), call->tensor(), call->is_dead()); get_call_freelist()->Release(call); Unref(); });}RpcRecvTensorCall 的 Start 方法如下，结果又来到了 StartRTCall。
void RpcRecvTensorCall::Start(std::function<void()> recv_done) override { StartRTCall(std::move(recv_done));}RpcRecvTensorCall::StartRTCall 之中，会调用 Worker 的 RecvTensorAsync 来完成传输，其实就是 GrpcRemoteWorker 的 RecvTensorAsync。
// Start the main RecvTensor call, checking for an async abort.void RpcRecvTensorCall::StartRTCall(std::function<void()> recv_done) { resp_.InitAlloc(dst_device_, alloc_attrs_); auto abort_checked = std::make_shared<Notification>(); auto cb = [this, abort_checked, recv_done = std::move(recv_done)](const Status& s) { // Make sure the Rendezvous abort checking is finished before running the // callback, which might destroy the current call object. abort_checked->WaitForNotification(); if (!s.ok()) { mutex_lock l(mu_); status_.Update(s); } recv_done(); }; wi_->RecvTensorAsync(&opts_, &req_, &resp_, std::move(cb)); // NOTE: Check if the rendezvous was aborted after sending out the RPC. The // ordering is important because StartAbort could be called right before // the RecvTensorAsync request registers its RPC cancellation to opts_. // In that case, the previous StartAbort would not trigger the // cancellation of this call. Status s; { mutex_lock l(mu_); s = status_; } if (!s.ok()) { opts_.StartCancel(); } // Notify that the abort check has finished. abort_checked->Notify();}RecvTensorAsync 方法的缩减版本如下，于是我们回到了熟悉的 Worker 流程。
void GrpcRemoteWorker::RecvTensorAsync(CallOptions* call_opts, const RecvTensorRequest* request, TensorResponse* response, StatusCallback done) override { IssueRequest(request, response, recvtensor_, callback, call_opts);}目前我们完成了下图的右半部分，如图上圆圈所示。
现在我们来到了 Server 端，其实就是张量发送方。接收到 RecvTensorRequest 之后的逻辑如下：
GrpcWorkerServiceThread::HandleRPCsLoop 之中有一个 for 循环，插入了 1000 个处理机制，设定了 GrpcWorkerMethod::kRecvTensor 由 EnqueueRecvTensorRequestRaw() 处理。这是事先缓存，为了加速处理，而且 EnqueueRecvTensorRequestRaw 之中在处理一个消息之后，会调用 EnqueueRequestForMethod 再次插入一个处理机制。
void GrpcWorkerServiceThread::HandleRPCsLoop() { // TODO(ncteisen): This may require performance engineering. We can // change the number of threads, the number of handlers per thread, // or even decide to specialize certain threads to certain methods. SETUP_FOR_REQUEST(GetStatus, 1, false); SETUP_FOR_REQUEST(CreateWorkerSession, 1, false); SETUP_FOR_REQUEST(DeleteWorkerSession, 1, false); SETUP_FOR_REQUEST(CleanupAll, 1, false); SETUP_FOR_REQUEST(RegisterGraph, 1, false); SETUP_FOR_REQUEST(DeregisterGraph, 1, false); SETUP_FOR_REQUEST(Logging, 1, false); SETUP_FOR_REQUEST(Tracing, 1, false); SETUP_FOR_REQUEST(CompleteGroup, 10, true); SETUP_FOR_REQUEST(CompleteInstance, 10, true); SETUP_FOR_REQUEST(GetStepSequence, 10, true); SETUP_FOR_REQUEST(RecvBuf, 500, true); SETUP_FOR_REQUEST(RunGraph, 100, true); SETUP_FOR_REQUEST(CleanupGraph, 100, false); SETUP_FOR_REQUEST(MarkRecvFinished, 10, false); // TODO(ncteisen): Determine a better policy for enqueuing the // appropriate number of each request type. for (int i = 0; i < gtl::FindWithDefault( queue_depth_, static_cast<int>(GrpcWorkerMethod::kRecvTensor), 1000); ++i) { EnqueueRecvTensorRequestRaw(); // 设置 } void* tag; bool ok; while (cq_->Next(&tag, &ok)) { UntypedCall<GrpcWorkerServiceThread>::Tag* callback_tag = static_cast<UntypedCall<GrpcWorkerServiceThread>::Tag*>(tag); CHECK(callback_tag); callback_tag->OnCompleted(this, ok); }}这里会再次插入，会设定由 GrpcWorkerServiceThread::RecvTensorHandlerRaw 继续处理 GrpcWorkerMethod::kRecvTensor。
void EnqueueRecvTensorRequestRaw() { mutex_lock l(shutdown_mu_); if (!is_shutdown_) { Call<GrpcWorkerServiceThread, grpc::WorkerService::AsyncService, RecvTensorRequest, ::grpc::ByteBuffer>:: EnqueueRequestForMethod( worker_service_, cq_.get(), static_cast<int>(GrpcWorkerMethod::kRecvTensor), &GrpcWorkerServiceThread::RecvTensorHandlerRaw, true /* supports cancel*/); }}GrpcWorkerServiceThread 是服务端处理请求的线程类。这里就是调用 GrpcWorker 来继续处理。这里使用了 WorkerCall 来作为参数。WorkerCall 是服务端处理一次 gRPC 请求和响应的类，是个别名。
using WorkerCall = Call<GrpcWorkerServiceThread, grpc::WorkerService::AsyncService, RequestMessage, ResponseMessage>;代码具体如下：
void GrpcWorkerServiceThread::RecvTensorHandlerRaw( WorkerCall<RecvTensorRequest, ::grpc::ByteBuffer>* call) { Schedule([this, call]() { CallOptions* call_opts = new CallOptions; call->SetCancelCallback([call_opts]() { call_opts->StartCancel(); }); worker_->GrpcRecvTensorAsync( call_opts, &call->request, &call->response, [call, call_opts](const Status& s) { call->ClearCancelCallback(); delete call_opts; if (!s.ok()) { VLOG(3) << "Bad response from RecvTensor:" << s; } call->SendResponse(ToGrpcStatus(s)); }); }); EnqueueRecvTensorRequestRaw();}GrpcWorker 是真正负责处理请求逻辑的 Worker，是 GrpcRemoteWorker 的服务端版本。GrpcWorker::GrpcRecvTensorAsync 逻辑是：
// GrpcRecvTensorAsync: unlike the other Worker methods, which use protocol// buffers for a response object, to avoid extra protocol buffer serialization// overhead we generate our response directly into a ::grpc::ByteBuffer objectvoid GrpcWorker::GrpcRecvTensorAsync(CallOptions* opts, const RecvTensorRequest* request, ::grpc::ByteBuffer* response, StatusCallback done) { const int64_t request_id = request->request_id(); const int64_t step_id = request->step_id(); bool cache_enabled = (response_cache_ != nullptr && request_id != 0); auto do_response = [response, done, cache_enabled](const Tensor& tensor, bool is_dead, const Status& status) { if (status.ok()) { grpc::EncodeTensorToByteBuffer(is_dead, tensor, cache_enabled, response); } done(status); }; // If response cache is enabled and the response cache already contains the // request, we delegate this retry request to the response cache. Otherwise, // we add the request to the response cache and start the computation to // retrieve the requested data. if (cache_enabled && response_cache_->QueueRequest(request_id, step_id, do_response)) { return; } auto rendezvous_done = [this, request_id, do_response, cache_enabled]( const Tensor& tensor, bool is_dead, const Status& status) { if (cache_enabled) { // Data is ready. Process all pending requests in the response cache. response_cache_->OnRequestFinished(request_id, tensor, is_dead, status); } else { do_response(tensor, is_dead, status); } }; auto fail = [&rendezvous_done](const Status& status) { rendezvous_done(Tensor(), false, status); }; Status s = recent_request_ids_.TrackUnique( request_id, "RecvTensor (GrpcWorker)", *request); const string& key = request->rendezvous_key(); Rendezvous::ParsedKey parsed; s = Rendezvous::ParseKey(key, &parsed); Device* src_dev = nullptr; if (s.ok()) { s = PrepareRecvTensor(parsed, &src_dev); } // Request the tensor associated with the rendezvous key. // Note that we log the cancellation here but do not abort the current step. // gRPC can generate cancellations in response to transient network failures, // and aborting the step eliminates the opportunity for client side retries. // Repeated client failures will eventually cause the step to be aborted by // the client. opts->SetCancelCallback( [step_id]() { LOG(WARNING) << "RecvTensor cancelled for " << step_id; }); env_->rendezvous_mgr->RecvLocalAsync( step_id, parsed, [opts, rendezvous_done, src_dev, request]( const Status& status, const Rendezvous::Args& send_args, const Rendezvous::Args& recv_args, const Tensor& val, const bool is_dead) { opts->ClearCancelCallback(); if (status.ok()) { // DMA can only be used for Tensors that do not fall into // the following three odd edge cases: 1) a zero-size // buffer, 2) a dead tensor which has an uninit value, and // 3) the tensor has the on_host allocation attribute, // i.e. it's in CPU RAM *independent of its assigned // device type*. const bool on_host = send_args.alloc_attrs.on_host(); { // Non-DMA cases. if (src_dev->tensorflow_gpu_device_info() && (!on_host)) { DeviceContext* send_dev_context = send_args.device_context; AllocatorAttributes alloc_attrs; alloc_attrs.set_gpu_compatible(true); alloc_attrs.set_on_host(true); Allocator* alloc = src_dev->GetAllocator(alloc_attrs); Tensor* copy = new Tensor(alloc, val.dtype(), val.shape()); // "val" is on an accelerator device. Uses the device_context to // fill the copy on host. StatusCallback copy_ready = [rendezvous_done, copy, is_dead](const Status& s) { // The value is now ready to be returned on the wire. rendezvous_done(*copy, is_dead, s); delete copy; }; CopyDeviceToHost(&val, alloc, alloc, request->rendezvous_key(), src_dev, copy, send_dev_context, copy_ready); return; } } } rendezvous_done(val, is_dead, status); });}BaseRendezvousMgr::RecvLocalAsync 会从本地 Table 查找张量。
void BaseRendezvousMgr::RecvLocalAsync(int64_t step_id, const Rendezvous::ParsedKey& parsed, Rendezvous::DoneCallback done) { auto rendez = FindOrCreate(step_id); auto done_cb = [rendez, done = std::move(done)]( const Status& s, const Rendezvous::Args& send_args, const Rendezvous::Args& recv_args, const Tensor& v, bool dead) { rendez->Unref(); done(s, send_args, recv_args, v, dead); }; rendez->RecvLocalAsync(parsed, std::move(done_cb));}其实，最终调用到了 RecvLocalAsyncInternal，其关键代码是 local_->RecvAsync。
void BaseRemoteRendezvous::RecvLocalAsync(const ParsedKey& parsed, DoneCallback done) { // Test whether the rendezvous is initialized using a shared lock, to avoid // the need for exclusive access in the common case. if (TF_PREDICT_FALSE(!is_initialized())) { mutex_lock l(mu_); if (!is_initialized_locked()) { // RecvLocalAsync can be called (due to an incoming RecvTensor RPC from a // remote worker) before the RunStep (or PartialRunStep) RPC from the // master arrives. RecvLocalAsync thus buffers the arguments until after // the RemoteRendezvous is Initialize()'d, when it completes the // rendezvous logic. At some point after Initialize() is called, a Tensor // is produced locally that will then be sent in response to the incoming // RPC. DeferredCall call(parsed, std::move(done)); deferred_calls_.push_back(call); return; } } RecvLocalAsyncInternal(parsed, std::move(done));}void BaseRemoteRendezvous::RecvLocalAsyncInternal(const ParsedKey& parsed, DoneCallback done) { Status s = ValidateDevices(parsed, true /* is_src */); if (!s.ok()) { done(s, Args(), Args(), Tensor(), false); return; } local_->RecvAsync(parsed, Args(), std::move(done));}LocalRendezvous::RecvAsync 完成了从本地 table 读取张量的操作。
void LocalRendezvous::RecvAsync(const Rendezvous::ParsedKey& key, const Rendezvous::Args& recv_args, Rendezvous::DoneCallback done) { uint64 key_hash = KeyHash(key.FullKey()); mu_.lock(); if (!status_.ok()) { // Rendezvous has been aborted. Status s = status_; mu_.unlock(); done(s, Rendezvous::Args(), recv_args, Tensor(), false); return; } ItemQueue* queue = &table_[key_hash]; if (queue->head == nullptr || queue->head->type == Item::kRecv) { // There is no message to pick up. // Only recv-related fields need to be filled. CancellationManager* cm = recv_args.cancellation_manager; CancellationToken token = CancellationManager::kInvalidToken; bool already_cancelled = false; if (cm != nullptr) { // Increment the refcount when cancellation manager is present, to make // sure the rendezvous outlives the recv and its cancel callbacks. // This refcount is dropped in exactly one of the following cases: // (1) Recv registers cancellation callback to cm, and then cm is // cancelled, unref in the cancellation callback; // (2) Recv registers cancellation callback to cm, but cm is already // cancelled, unref in the already_cancelled check; // (3) Recv is successful, and item done callback finishes deregistering // the cancellation callback, unref in the item done callback; // (4) Recv is successful, but the item done callback fails to deregister // the cancellation callback because cm already StartCancel, in this // case the cancellation callback will be invoked by the cm anyway, // unref in the cancellation callback. if (rc_owner_) rc_owner_->Ref(); token = cm->get_cancellation_token(); already_cancelled = !cm->RegisterCallback(token, [this, token, key_hash] { Item* item = nullptr; { mutex_lock l(mu_); ItemQueue* queue = &table_[key_hash]; // Find an item in the queue with a cancellation token that matches // token, and remove it. if (queue->head != nullptr && queue->head->type == Item::kRecv) { for (Item *prev = nullptr, *curr = queue->head; curr != nullptr; prev = curr, curr = curr->next) { if (curr->recv_state.cancellation_token == token) { item = curr; if (queue->head->next == nullptr) { // We have a single-element queue, so we can erase it from // the table. table_.erase(key_hash); } else { // Remove the current item from the queue. if (curr == queue->head) { DCHECK_EQ(prev, nullptr); queue->head = curr->next; } else { DCHECK_NE(prev, nullptr); prev->next = curr->next; } if (queue->tail == curr) { queue->tail = prev; } } break; } } } } if (item != nullptr) { (*item->recv_state.waiter)( StatusGroup::MakeDerived( errors::Cancelled("RecvAsync is cancelled.")), Rendezvous::Args(), item->args, Tensor(), /*is_dead=*/false); delete item; } // Unref case (1) and (4) if (rc_owner_) rc_owner_->Unref(); }); } if (already_cancelled) { mu_.unlock(); // Unref case (2) if (rc_owner_) rc_owner_->Unref(); done(StatusGroup::MakeDerived( errors::Cancelled("RecvAsync is cancelled.")), Rendezvous::Args(), recv_args, Tensor(), /*is_dead=*/false); return; } // TODO(b/143786186): Investigate moving the allocation of Item outside // the lock. if (cm != nullptr) { // NOTE(mrry): We must wrap done with code that deregisters the // cancellation callback before calling the done callback, because the // cancellation manager may no longer be live after done is called. queue->push_back(new Item( recv_args, [this, cm, token, done = std::move(done)]( const Status& s, const Rendezvous::Args& send_args, const Rendezvous::Args& recv_args, const Tensor& v, bool dead) { // TryDeregisterCallback returns true when the cancellation callback // is successfully deregistered. If it fails because the CM already // StartAbort, Unref will happen inside the cancellation callback // when called by the CM. if (cm->TryDeregisterCallback(token)) { // Unref case (3) if (this->rc_owner_) this->rc_owner_->Unref(); } done(s, send_args, recv_args, v, dead); }, token)); } else { queue->push_back(new Item(recv_args, std::move(done), token)); } mu_.unlock(); return; } // A message has already arrived and is queued in the table under // this key. Consumes the message and invokes the done closure. Item* item = queue->head; // Delete the queue when the last element has been consumed. if (item->next == nullptr) { table_.erase(key_hash); } else { queue->head = item->next; } mu_.unlock(); // Invoke done() without holding the table lock. DCHECK_EQ(item->type, Item::kSend); done(Status::OK(), item->args, recv_args, *item->send_state.value, item->send_state.is_dead); delete item;}最终补齐了之前图的所有逻辑。或者我们也可以从另一种角度来看，如下图所示：
TensorFlow架构与设计：概述
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https://jcf94.com/2018/02/28/2018-02-28-tfunpacking3/
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