[MXNET-910] Multithreading inference. (apache#12456)

* add multi-threading inference.

* demo multi-threading inference.

* add new capi.

* make naive engine thread local.

* create an executor inside each thread.

* fix format.

* fix format.

* fix format.

* Revert "make naive engine thread local."

This reverts commit b9d844e.

* Update CAPI.

* add doc.

* fix lint.

* update example.

* update.

* fix.

* add check.

* fix.

* fix example.

* update name.

* update README.

example/image-classification/predict-cpp/Makefile

@@ -15,7 +15,7 @@ LDFLAGS+=`pkg-config --libs opencv`
 export MXNET_ROOT=`pwd`/../../..
 
 CFLAGS+=-Wall -I$(MXNET_ROOT)/include
-LDFLAGS+=$(MXNET_ROOT)/lib/libmxnet.so
+LDFLAGS+=$(MXNET_ROOT)/lib/libmxnet.so -lpthread
 
 image-classification-predict: image-classification-predict.o
 	g++ -O3 -o image-classification-predict image-classification-predict.o $(LDFLAGS)

example/image-classification/predict-cpp/README.md

@@ -1,5 +1,5 @@
 # Image Classification Example Using the C Predict API
-This is a simple predictor which shows how to use the MXNet C Predict API for image classification with a pre-trained ImageNet model.
+This is a simple predictor which shows how to use the MXNet C Predict API for image classification with a pre-trained ImageNet model in a single thread and multiple threads.
 
 ## Prerequisites
 
@@ -45,10 +45,10 @@ Run the example by passing it an image that you want to classify. If you don't h
   wget https://upload.wikimedia.org/wikipedia/commons/thumb/f/f4/Honeycrisp.jpg/1920px-Honeycrisp.jpg
   ```
 
-Then run the `image-classification-predict` program, passing the image as the argument.
+Then run the `image-classification-predict` program, passing the image as the first argument and the number of threads as the second parameter.
 
   ```bash
-  ./image-classification-predict 1920px-Honeycrisp.jpg
+  ./image-classification-predict 1920px-Honeycrisp.jpg 1
   ```
 
 ## Tips

example/image-classification/predict-cpp/image-classification-predict.cc

@@ -37,6 +37,7 @@
 #include <fstream>
 #include <vector>
 #include <memory>
+#include <thread>
 #include <iomanip>
 #include <opencv2/opencv.hpp>
 // Path for c_predict_api
@@ -179,14 +180,56 @@ void PrintOutputResult(const std::vector<float>& data, const std::vector<std::st
             "accuracy=" << std::setprecision(8) << best_accuracy << ")" << std::endl;
 }
 
+void predict(PredictorHandle pred_hnd, const std::vector<mx_float> &image_data,
+             NDListHandle nd_hnd, const std::string &synset_file, int i) {
+  auto image_size = image_data.size();
+  // Set Input Image
+  MXPredSetInput(pred_hnd, "data", image_data.data(), static_cast<mx_uint>(image_size));
+
+  // Do Predict Forward
+  MXPredForward(pred_hnd);
+
+  mx_uint output_index = 0;
+
+  mx_uint* shape = nullptr;
+  mx_uint shape_len;
+
+  // Get Output Result
+  MXPredGetOutputShape(pred_hnd, output_index, &shape, &shape_len);
+
+  std::size_t size = 1;
+  for (mx_uint i = 0; i < shape_len; ++i) { size *= shape[i]; }
+
+  std::vector<float> data(size);
+
+  MXPredGetOutput(pred_hnd, output_index, &(data[0]), static_cast<mx_uint>(size));
+
+  // Release NDList
+  if (nd_hnd) {
+    MXNDListFree(nd_hnd);
+  }
+
+  // Release Predictor
+  MXPredFree(pred_hnd);
+
+  // Synset path for your model, you have to modify it
+  auto synset = LoadSynset(synset_file);
+
+  // Print Output Data
+  PrintOutputResult(data, synset);
+}
+
 int main(int argc, char* argv[]) {
   if (argc < 2) {
     std::cout << "No test image here." << std::endl
-              << "Usage: ./image-classification-predict apple.jpg" << std::endl;
+              << "Usage: ./image-classification-predict apple.jpg [num_threads]" << std::endl;
     return EXIT_FAILURE;
   }
 
   std::string test_file(argv[1]);
+  int num_threads = 1;
+  if (argc == 3)
+    num_threads = std::atoi(argv[2]);
 
   // Models path for your model, you have to modify it
   std::string json_file = "model/Inception/Inception-BN-symbol.json";
@@ -214,25 +257,11 @@ int main(int argc, char* argv[]) {
                                         static_cast<mx_uint>(channels),
                                         static_cast<mx_uint>(height),
                                         static_cast<mx_uint>(width) };
-  PredictorHandle pred_hnd = nullptr;
 
   if (json_data.GetLength() == 0 || param_data.GetLength() == 0) {
     return EXIT_FAILURE;
   }
 
-  // Create Predictor
-  MXPredCreate(static_cast<const char*>(json_data.GetBuffer()),
-               static_cast<const char*>(param_data.GetBuffer()),
-               static_cast<int>(param_data.GetLength()),
-               dev_type,
-               dev_id,
-               num_input_nodes,
-               input_keys,
-               input_shape_indptr,
-               input_shape_data,
-               &pred_hnd);
-  assert(pred_hnd);
-
   auto image_size = static_cast<std::size_t>(width * height * channels);
 
   // Read Mean Data
@@ -259,40 +288,46 @@ int main(int argc, char* argv[]) {
 
   GetImageFile(test_file, image_data.data(), channels, cv::Size(width, height), nd_data);
 
-  // Set Input Image
-  MXPredSetInput(pred_hnd, "data", image_data.data(), static_cast<mx_uint>(image_size));
-
-  // Do Predict Forward
-  MXPredForward(pred_hnd);
-
-  mx_uint output_index = 0;
-
-  mx_uint* shape = nullptr;
-  mx_uint shape_len;
-
-  // Get Output Result
-  MXPredGetOutputShape(pred_hnd, output_index, &shape, &shape_len);
-
-  std::size_t size = 1;
-  for (mx_uint i = 0; i < shape_len; ++i) { size *= shape[i]; }
-
-  std::vector<float> data(size);
-
-  MXPredGetOutput(pred_hnd, output_index, &(data[0]), static_cast<mx_uint>(size));
-
-  // Release NDList
-  if (nd_hnd) {
-    MXNDListFree(nd_hnd);
+  if (num_threads == 1) {
+    // Create Predictor
+    PredictorHandle pred_hnd;
+    MXPredCreate(static_cast<const char*>(json_data.GetBuffer()),
+                 static_cast<const char*>(param_data.GetBuffer()),
+                 static_cast<int>(param_data.GetLength()),
+                 dev_type,
+                 dev_id,
+                 num_input_nodes,
+                 input_keys,
+                 input_shape_indptr,
+                 input_shape_data,
+                 &pred_hnd);
+    assert(pred_hnd);
+
+    predict(pred_hnd, image_data, nd_hnd, synset_file, 0);
+  } else {
+    // Create Predictor
+    std::vector<PredictorHandle> pred_hnds(num_threads, nullptr);
+    MXPredCreateMultiThread(static_cast<const char*>(json_data.GetBuffer()),
+                            static_cast<const char*>(param_data.GetBuffer()),
+                            static_cast<int>(param_data.GetLength()),
+                            dev_type,
+                            dev_id,
+                            num_input_nodes,
+                            input_keys,
+                            input_shape_indptr,
+                            input_shape_data,
+                            pred_hnds.size(),
+                            pred_hnds.data());
+    for (auto hnd : pred_hnds)
+      assert(hnd);
+
+    std::vector<std::thread> threads;
+    for (int i = 0; i < num_threads; i++)
+      threads.emplace_back(predict, pred_hnds[i], image_data, nd_hnd, synset_file, i);
+    for (int i = 0; i < num_threads; i++)
+      threads[i].join();
   }
-
-  // Release Predictor
-  MXPredFree(pred_hnd);
-
-  // Synset path for your model, you have to modify it
-  auto synset = LoadSynset(synset_file);
-
-  // Print Output Data
-  PrintOutputResult(data, synset);
+  printf("run successfully\n");
 
   return EXIT_SUCCESS;
 }

include/mxnet/c_predict_api.h

@@ -119,6 +119,39 @@ MXNET_DLL int MXPredCreatePartialOut(const char* symbol_json_str,
                                      mx_uint num_output_nodes,
                                      const char** output_keys,
                                      PredictorHandle* out);
+
+/*!
+ * \brief create predictors for multiple threads. One predictor for a thread.
+ * \param symbol_json_str The JSON string of the symbol.
+ * \param param_bytes The in-memory raw bytes of parameter ndarray file.
+ * \param param_size The size of parameter ndarray file.
+ * \param dev_type The device type, 1: cpu, 2:gpu
+ * \param dev_id The device id of the predictor.
+ * \param num_input_nodes Number of input nodes to the net,
+ *    For feedforward net, this is 1.
+ * \param input_keys The name of input argument.
+ *    For feedforward net, this is {"data"}
+ * \param input_shape_indptr Index pointer of shapes of each input node.
+ *    The length of this array = num_input_nodes + 1.
+ *    For feedforward net that takes 4 dimensional input, this is {0, 4}.
+ * \param input_shape_data A flatted data of shapes of each input node.
+ *    For feedforward net that takes 4 dimensional input, this is the shape data.
+ * \param num_threads The number of threads that we'll run the predictors.
+ * \param out An array of created predictor handles. The array has to be large
+ *   enough to keep `num_threads` predictors.
+ * \return 0 when success, -1 when failure.
+ */
+MXNET_DLL int MXPredCreateMultiThread(const char* symbol_json_str,
+                                      const void* param_bytes,
+                                      int param_size,
+                                      int dev_type, int dev_id,
+                                      mx_uint num_input_nodes,
+                                      const char** input_keys,
+                                      const mx_uint* input_shape_indptr,
+                                      const mx_uint* input_shape_data,
+                                      int num_threads,
+                                      PredictorHandle* out);
+
 /*!
  * \brief Change the input shape of an existing predictor.
  * \param num_input_nodes Number of input nodes to the net,