decouple bagging with num_threads

docs/FAQ.rst

@@ -1,3 +1,6 @@
+.. role:: raw-html(raw)
+    :format: html
+
 LightGBM FAQ
 ############
 
@@ -82,8 +85,13 @@ You may also use the CPU version.
 6. Bagging is not reproducible when changing the number of threads.
 -------------------------------------------------------------------
 
+:raw-html:`<strike>`
 LightGBM bagging is multithreaded, so its output depends on the number of threads used.
 There is `no workaround currently <https://github.com/microsoft/LightGBM/issues/632>`__.
+:raw-html:`</strike>`
+
+Starting from `#2804 <https://github.com/microsoft/LightGBM/pull/2804>`__ bagging result doesn't depend on the number of threads.
+So this issue should be solved in the latest version.
 
 7. I tried to use Random Forest mode, and LightGBM crashes!
 -----------------------------------------------------------

docs/Parameters.rst

@@ -559,7 +559,7 @@ IO Parameters
 
 -  ``data_random_seed`` :raw-html:`<a id="data_random_seed" title="Permalink to this parameter" href="#data_random_seed">&#x1F517;&#xFE0E;</a>`, default = ``1``, type = int, aliases: ``data_seed``
 
-   -  random seed for data partition in parallel learning (excluding the ``feature_parallel`` mode)
+   -  random seed for sampling data to construct histogram bins
 
 -  ``output_model`` :raw-html:`<a id="output_model" title="Permalink to this parameter" href="#output_model">&#x1F517;&#xFE0E;</a>`, default = ``LightGBM_model.txt``, type = string, aliases: ``model_output``, ``model_out``
 

include/LightGBM/config.h

@@ -523,7 +523,7 @@ struct Config {
   double histogram_pool_size = -1.0;
 
   // alias = data_seed
-  // desc = random seed for data partition in parallel learning (excluding the ``feature_parallel`` mode)
+  // desc = random seed for sampling data to construct histogram bins
   int data_random_seed = 1;
 
   // alias = model_output, model_out

include/LightGBM/utils/threading.h

@@ -1,12 +1,16 @@
 /*!
  * Copyright (c) 2016 Microsoft Corporation. All rights reserved.
- * Licensed under the MIT License. See LICENSE file in the project root for license information.
+ * Licensed under the MIT License. See LICENSE file in the project root for
+ * license information.
  */
 #ifndef LIGHTGBM_UTILS_THREADING_H_
 #define LIGHTGBM_UTILS_THREADING_H_
 
+#include <LightGBM/meta.h>
+#include <LightGBM/utils/common.h>
 #include <LightGBM/utils/openmp_wrapper.h>
 
+#include <algorithm>
 #include <functional>
 #include <vector>
 
@@ -37,6 +41,23 @@ class Threading {
       *block_size = cnt;
     }
   }
+  template <typename INDEX_T>
+  static inline void BlockInfoForceSize(int num_threads, INDEX_T cnt,
+                                        INDEX_T min_cnt_per_block,
+                                        int* out_nblock, INDEX_T* block_size) {
+    *out_nblock = std::min<int>(
+        num_threads,
+        static_cast<int>((cnt + min_cnt_per_block - 1) / min_cnt_per_block));
+    if (*out_nblock > 1) {
+      *block_size = (cnt + (*out_nblock) - 1) / (*out_nblock);
+      // force the block size to the times of min_cnt_per_block
+      *block_size = (*block_size + min_cnt_per_block - 1) / min_cnt_per_block *
+                    min_cnt_per_block;
+    } else {
+      *block_size = cnt;
+    }
+  }
+
   template <typename INDEX_T>
   static inline int For(
       INDEX_T start, INDEX_T end, INDEX_T min_block_size,
@@ -58,6 +79,116 @@ class Threading {
   }
 };
 
-}   // namespace LightGBM
+template <typename INDEX_T, bool TWO_BUFFER>
+class ParallelPartitionRunner {
+ public:
+  ParallelPartitionRunner(INDEX_T num_data, INDEX_T min_block_size)
+      : min_block_size_(min_block_size) {
+    num_threads_ = 1;
+#pragma omp parallel
+#pragma omp master
+    { num_threads_ = omp_get_num_threads(); }
+    left_.resize(num_data);
+    if (TWO_BUFFER) {
+      right_.resize(num_data);
+    }
+    offsets_.resize(num_threads_);
+    left_cnts_.resize(num_threads_);
+    right_cnts_.resize(num_threads_);
+    left_write_pos_.resize(num_threads_);
+    right_write_pos_.resize(num_threads_);
+  }
+
+  ~ParallelPartitionRunner() {}
+
+  void ReSize(INDEX_T num_data) {
+    left_.resize(num_data);
+    if (TWO_BUFFER) {
+      right_.resize(num_data);
+    }
+  }
+
+  template<bool FORCE_SIZE>
+  INDEX_T Run(
+      INDEX_T cnt,
+      const std::function<INDEX_T(int, INDEX_T, INDEX_T, INDEX_T*, INDEX_T*)>& func,
+      INDEX_T* out) {
+    int nblock = 1;
+    INDEX_T inner_size = cnt;
+    if (FORCE_SIZE) {
+      Threading::BlockInfoForceSize<INDEX_T>(num_threads_, cnt, min_block_size_,
+                                             &nblock, &inner_size);
+    } else {
+      Threading::BlockInfo<INDEX_T>(num_threads_, cnt, min_block_size_, &nblock,
+                                    &inner_size);
+    }
+
+    OMP_INIT_EX();
+#pragma omp parallel for schedule(static, 1)
+    for (int i = 0; i < nblock; ++i) {
+      OMP_LOOP_EX_BEGIN();
+      INDEX_T cur_start = i * inner_size;
+      INDEX_T cur_cnt = std::min(inner_size, cnt - cur_start);
+      offsets_[i] = cur_start;
+      if (cur_cnt <= 0) {
+        left_cnts_[i] = 0;
+        right_cnts_[i] = 0;
+        continue;
+      }
+      auto left_ptr = left_.data() + cur_start;
+      INDEX_T* right_ptr = nullptr;
+      if (TWO_BUFFER) {
+        right_ptr = right_.data() + cur_start;
+      }
+      // split data inner, reduce the times of function called
+      INDEX_T cur_left_count =
+          func(i, cur_start, cur_cnt, left_ptr, right_ptr);
+      if (!TWO_BUFFER) {
+        // reverse for one buffer
+        std::reverse(left_ptr + cur_left_count, left_ptr + cur_cnt);
+      }
+      left_cnts_[i] = cur_left_count;
+      right_cnts_[i] = cur_cnt - cur_left_count;
+      OMP_LOOP_EX_END();
+    }
+    OMP_THROW_EX();
+
+    left_write_pos_[0] = 0;
+    right_write_pos_[0] = 0;
+    for (int i = 1; i < nblock; ++i) {
+      left_write_pos_[i] = left_write_pos_[i - 1] + left_cnts_[i - 1];
+      right_write_pos_[i] = right_write_pos_[i - 1] + right_cnts_[i - 1];
+    }
+    data_size_t left_cnt = left_write_pos_[nblock - 1] + left_cnts_[nblock - 1];
+
+    auto right_start = out + left_cnt;
+#pragma omp parallel for schedule(static)
+    for (int i = 0; i < nblock; ++i) {
+      std::copy_n(left_.data() + offsets_[i], left_cnts_[i],
+                  out + left_write_pos_[i]);
+      if (TWO_BUFFER) {
+        std::copy_n(right_.data() + offsets_[i], right_cnts_[i],
+                    right_start + right_write_pos_[i]);
+      } else {
+        std::copy_n(left_.data() + offsets_[i] + left_cnts_[i], right_cnts_[i],
+                    right_start + right_write_pos_[i]);
+      }
+    }
+    return left_cnt;
+  }
+
+ private:
+  int num_threads_;
+  INDEX_T min_block_size_;
+  std::vector<INDEX_T> left_;
+  std::vector<INDEX_T> right_;
+  std::vector<INDEX_T> offsets_;
+  std::vector<INDEX_T> left_cnts_;
+  std::vector<INDEX_T> right_cnts_;
+  std::vector<INDEX_T> left_write_pos_;
+  std::vector<INDEX_T> right_write_pos_;
+};
+
+}  // namespace LightGBM
 
-#endif   // LightGBM_UTILS_THREADING_H_
+#endif  // LightGBM_UTILS_THREADING_H_