Scale loss op use event

paddle/fluid/framework/parallel_executor.cc

@@ -124,12 +124,17 @@ struct ScaleLossGradOpHandle : public OpHandle {
   float coeff_;
   Scope *scope_;
   platform::Place place_;
+  cudaEvent_t ev_;
 
   explicit ScaleLossGradOpHandle(size_t num_dev, Scope *scope,
                                  platform::Place place)
       : coeff_(static_cast<float>(1.0 / num_dev)),
         scope_(scope),
-        place_(place) {}
+        place_(place) {
+    PADDLE_ENFORCE(cudaEventCreateWithFlags(&ev_, cudaEventDisableTiming));
+  }
+
+  ~ScaleLossGradOpHandle() { PADDLE_ENFORCE(cudaEventDestroy(ev_)); }
 
   void Run() override {
     std::string var_name = static_cast<VarHandle *>(this->outputs_[0])->name_;
@@ -141,16 +146,23 @@ struct ScaleLossGradOpHandle : public OpHandle {
     if (platform::is_cpu_place(place_)) {
       *tmp = coeff_;
     } else {
-      memory::Copy(
-          boost::get<platform::CUDAPlace>(place_), tmp, platform::CPUPlace(),
-          &coeff_, sizeof(float),
+      auto stream =
           static_cast<platform::CUDADeviceContext *>(this->dev_ctx_[place_])
-              ->stream());
+              ->stream();
+      memory::Copy(boost::get<platform::CUDAPlace>(place_), tmp,
+                   platform::CPUPlace(), &coeff_, sizeof(float), stream);
+      PADDLE_ENFORCE(cudaEventRecord(ev_, stream));
     }
   }
 
   void Wait(platform::DeviceContext *waited_dev) override {
+    if (platform::is_cpu_place(waited_dev->GetPlace())) {
       this->dev_ctx_.at(place_)->Wait();
+    } else {
+      auto stream =
+          static_cast<platform::CUDADeviceContext *>(waited_dev)->stream();
+      PADDLE_ENFORCE(cudaStreamWaitEvent(stream, ev_, 0));
+    }
   }
 };