From 3fcd16ede3dd71b269fed8ae213d18491b65f186 Mon Sep 17 00:00:00 2001
From: fengjiayi <fengjiayi@baidu.com>
Date: Tue, 6 Mar 2018 19:08:23 +0800
Subject: [PATCH] init double buffer

---
 paddle/fluid/framework/reader.cc | 41 ++++++++++++++++++++++++++++++++
 paddle/fluid/framework/reader.h  | 25 +++++++++++++++++++
 2 files changed, 66 insertions(+)

diff --git a/paddle/fluid/framework/reader.cc b/paddle/fluid/framework/reader.cc
index dc1caa72a4c..9cdce11d375 100644
--- a/paddle/fluid/framework/reader.cc
+++ b/paddle/fluid/framework/reader.cc
@@ -112,5 +112,46 @@ void BatchReader::ReadNext(std::vector<LoDTensor>* out) {
     out->push_back(out_tensor);
   }
 }
+
+void DoubleBufferReader::ReadNext(std::vector<LoDTensor>* out) {
+  std::unique_lock<std::mutex> lck(mtx_);
+  while (write_pos_ == read_pos_) {
+    buffer_not_empty_.wait(lck);
+  }
+
+  out->clear();
+  out->resize(buffer_[read_pos_].size());
+  // TODO(fengjiayi): This copy shall be reduced.
+  for (size_t i = 0; i < buffer_[read_pos_].size(); ++i) {
+    TensorCopy(buffer_[read_pos_][i], platform::CPUPlace(), &out[i]);
+    out[i].set_lod(buffer_[read_pos_][i].lod());
+  }
+
+  ++read_pos_;
+  if (read_pos_ >= kDoubleBufferSize) {
+    read_pos_ = 0;
+  }
+  buffer_not_full_.notify_all();
+}
+
+bool DoubleBufferReader::HasNext() {
+  return reader_->HasNext() || !buffer_.empty();
+}
+
+void DoubleBufferReader::ProducerThreadFunc() {
+  while (reader_->HasNext()) {
+    std::unique_lock<std::mutex> lck(mtx);
+    while (((write_pos_ + 1) % kDoubleBufferSize) == read_pos_) {
+      buffer_not_full_.wait(lck);
+    }
+    reader_->ReadNext(&buffer_[write_pos_]);
+    ++write_pos_;
+    if (write_pos_ >= kDoubleBufferSize) {
+      write_pos_ = 0;
+    }
+    buffer_not_empty_.notify_all();
+  }
+}
+
 }  // namespace framework
 }  // namespace paddle
diff --git a/paddle/fluid/framework/reader.h b/paddle/fluid/framework/reader.h
index 4a5eba5fb73..917412ce9b2 100644
--- a/paddle/fluid/framework/reader.h
+++ b/paddle/fluid/framework/reader.h
@@ -16,10 +16,13 @@
 
 #include "paddle/fluid/framework/ddim.h"
 #include "paddle/fluid/framework/lod_tensor_array.h"
+#include "paddle/fluid/framework/threadpool.h"
 
 namespace paddle {
 namespace framework {
 
+static constexpr size_t kDoubleBufferSize = 3;
+
 class ReaderBase {
  public:
   explicit ReaderBase(const std::vector<DDim>& shapes) : shapes_(shapes) {
@@ -135,6 +138,28 @@ class BatchReader : public DecoratedReader {
   std::vector<std::vector<LoDTensor>> buffer_;
 };
 
+class DoubleBufferReader : public DecoratedReader {
+ public:
+  DoubleBufferReader(ReaderBase* reader)
+      : DecoratedReader(reader), buffer_(kDoubleBufferSize) {
+    framework::Async(std::bind(&DoubleBufferReader::ProducerThreadFunc, this));
+  }
+
+  void ReadNext(std::vector<LoDTensor>* out) override;
+  bool HasNext() const override;
+
+ private:
+  void ProducerThreadFunc();
+
+  std::vector<std::vector<LoDTensor>> buffer_;
+  size_t write_pos_;
+  size_t read_pos_;
+
+  std::mutex mtx_;
+  std::condition_variable buffer_not_full_;
+  std::condition_variable buffer_not_empty_;
+};
+
 // The ReaderHolder is used as readers' unified wrapper,
 // making it easier to access different type readers in Variables.
 class ReaderHolder {
-- 
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