grpc - gRPC 中的 call_cq 和 notification_cq 有什么区别？

Question

https://github.com/grpc/grpc/blob/master/examples/cpp/helloworld/greeter_async_server.cc#L91

        service_->RequestSayHello(&ctx_, &request_, &responder_, cq_, cq_,
                                  this);

cq_ 的两次出现对我来说看起来很奇怪所以我深入研究了引导我的源代码

https://github.com/grpc/grpc/blob/master/include/grpcpp/impl/codegen/service_type.h#L92

  void RequestAsyncUnary(int index, ServerContext* context, Message* request,
                         internal::ServerAsyncStreamingInterface* stream,
                         CompletionQueue* call_cq,
                         ServerCompletionQueue* notification_cq, void* tag) {
    server_->RequestAsyncCall(methods_[index].get(), context, stream, call_cq,
                              notification_cq, tag, request);
  }

那么 call_cq有什么区别？和 notification_cq ?使用差异完成队列的潜在用途/好处是什么？

Answer 1

当提出同样的问题时，这是来自 grpc 的 google-groups 论坛的引述。
https://groups.google.com/forum/#!topic/grpc-io/V4NAQ77PMEo

Notification_cq gets the tag back indicating a call has started. All subsequent operations (reads, writes, etc) on that call report back to call_cq. For most async servers my recommendation is to use the same cq. Places where you might not:

1. Our sync API creates a cq per call under the covers... So it posts a general event >queue for notification_cq, and it's specific queue as call_cq.
2. If you want to be able to control when you accept incoming calls vs when you don't (by suspending polling on a notification_cq)
3. I'm sure folks can think of others.

这允许细粒度控制哪些线程处理哪些类型的事件(基于它们正在轮询的队列)。就像您可能有一个主线程轮询 notification_cq 和所有工作线程都轮询他们自己的 call_cqs 或类似的东西。