Fix MultiGradientMachine error

paddle/gserver/gradientmachines/MultiGradientMachine.cpp

@@ -346,7 +346,9 @@ Evaluator* MultiGradientMachine::makeEvaluator() const {
 void MultiGradientMachine::eval(Evaluator* evaluator) const {
   for (auto& thread : threads_) {
     SetDevice device(thread->getDeviceId());
-    thread->getGradientMachine()->eval(evaluator);
+    if (thread->hasInputData()) {
+      thread->getGradientMachine()->eval(evaluator);
+    }
   }
 }
 
@@ -356,14 +358,20 @@ void MultiGradientMachine::getOutArgs(std::vector<Argument>* outArgs,
     REGISTER_TIMER("waitOutArgs");
     thread->waitOutArgsReady();
   }
-  outArgs_.resize(threads_[0]->getOutArgs().size());
+  // outArgs_.size() only need to be calculated once.
+  static int size = threads_[threads_.size() - 1]->getOutArgs().size();
+  outArgs_.resize(size);
 
   REGISTER_TIMER("copyOutArgs");
   for (size_t i = 0; i < outArgs_.size(); ++i) {
     std::vector<Argument> args;
     args.reserve(threads_.size());
     for (auto& thread : threads_) {
-      args.push_back(thread->getOutArgs()[i]);
+      // If the thread input is empty, then the output is empty.
+      auto tmp = thread->getOutArgs();
+      if (tmp.size() > 0) {
+        args.push_back(tmp[i]);
+      }
     }
     outArgs_[i].concat(args, useGpu_, outArgStream_, passType);
   }
@@ -534,7 +542,7 @@ void TrainerThread::prefetch() {
 void TrainerThread::forward() {
   if (!inArgsCopied_) {
     REGISTER_TIMER("copyInArgs");
-    copyInArgs();
+    batchSize_ = copyInArgs();
   } else {
     inArgsCopied_ = false;
   }
@@ -564,7 +572,12 @@ void TrainerThread::forward() {
 
   {
     REGISTER_TIMER("thread_forward");
-    gradientMachine_->forward(inArgs_, &outArgs_, multiMachine_->getPassType());
+    if (batchSize_ > 0) {
+      gradientMachine_->forward(
+        inArgs_, &outArgs_, multiMachine_->getPassType());
+    } else {
+       outArgs_.clear();
+    }
   }
   outArgsReadySem_.post();
 }
@@ -574,7 +587,13 @@ void TrainerThread::backward() {
   if (multiMachine_->isPassGrad()) {
     copyOutputGrad();
   }
-  gradientMachine_->backward(backwardCallback_);
+  if (batchSize_ > 0) {
+    gradientMachine_->backward(backwardCallback_);
+  } else {
+    for (size_t i = parameters_.size(); i > 0; i--) {
+      backwardCallback(parameters_[i - 1].get());
+    }
+  }
   if (multiMachine_->hasNonstaticCpuParamters()) {
     mergeCpuGradients();
   }
@@ -732,7 +751,7 @@ void TrainerThread::notifyValueReady(int paramId) {
   notifyValueDispatch(paramId);
 }
 
-void TrainerThread::copyInArgs() {
+int TrainerThread::copyInArgs() {
   const std::vector<Argument>& fullInArgs = multiMachine_->getInArgs();
   int numThreads = multiMachine_->getAllThreads().size();
   int32_t numSequences = fullInArgs[0].getNumSequences();
@@ -748,7 +767,7 @@ void TrainerThread::copyInArgs() {
   }
 
   if (copySize == 0) {
-    return;
+    return 0;
   }
 
   for (size_t i = 0; i < fullInArgs.size(); i++) {
@@ -758,6 +777,7 @@ void TrainerThread::copyInArgs() {
         copySize,
         FLAGS_parallel_nn ? false : multiMachine_->useGpu());
   }
+  return copySize;
 }
 
 void TrainerThread::mergeCpuGradients() {

paddle/gserver/gradientmachines/MultiGradientMachine.h

@@ -387,6 +387,9 @@ public:
   /// copy the output gradient from the main GradientMachine.
   void copyOutputGrad();
 
+  /// Whether the thread has input data.
+  bool hasInputData() { return batchSize_ != 0; }
+
 protected:
   void mergeCpuGradients();
 
@@ -407,7 +410,7 @@ protected:
   void copyGradToBufferThread();
   void gradCollectThread();
 
-  void copyInArgs();
+  int copyInArgs();
   void forward();
   void backward();
   void backwardCallback(Parameter* para);
@@ -467,6 +470,8 @@ protected:
 
   /// indicate whether inArgs is copied before forward()
   bool inArgsCopied_;
+
+  int batchSize_;
 };
 
 }  // namespace paddle