From cb538cb832dad2be7574efd53b303948432233ed Mon Sep 17 00:00:00 2001
From: fis <jm.yuan@outlook.com>
Date: Mon, 10 Aug 2020 18:44:57 +0800
Subject: [PATCH] Slow it down.

---
 src/common/quantile.cc | 36 +++++++++---------------------------
 1 file changed, 9 insertions(+), 27 deletions(-)

diff --git a/src/common/quantile.cc b/src/common/quantile.cc
index 889be6e9104f..fe55b7128eb8 100644
--- a/src/common/quantile.cc
+++ b/src/common/quantile.cc
@@ -62,13 +62,8 @@ void HostSketchContainer::PushRowPage(SparsePage const &page,
 
   // Data groups, used in ranking.
   std::vector<bst_uint> const &group_ptr = info.group_ptr_;
-  size_t const num_groups = group_ptr.size() == 0 ? 0 : group_ptr.size() - 1;
   // Use group index for weights?
-  size_t group_ind = 0;
   auto batch = page.GetView();
-  if (use_group_ind_) {
-    group_ind = this->SearchGroupIndFromRow(group_ptr, page.base_rowid);
-  }
   dmlc::OMPException exec;
   // Parallel over columns.  Asumming the data is dense, each thread owns a set of
   // consecutive columns.
@@ -76,23 +71,21 @@ void HostSketchContainer::PushRowPage(SparsePage const &page,
   auto const is_dense = info.num_nonzero_ == info.num_col_ * info.num_row_;
   auto thread_columns_ptr = LoadBalance(page, columns_size_, nthread);
 
-#pragma omp parallel num_threads(nthread) firstprivate(group_ind)
+#pragma omp parallel num_threads(nthread)
   {
     exec.Run([&]() {
       auto tid = static_cast<uint32_t>(omp_get_thread_num());
       auto const begin = thread_columns_ptr[tid];
       auto const end = thread_columns_ptr[tid + 1];
+      size_t group_ind = 0;
 
       // do not iterate if no columns are assigned to the thread
       if (begin < end && end <= ncol) {
         for (size_t i = 0; i < batch.Size(); ++i) { // NOLINT(*)
           size_t const ridx = page.base_rowid + i;
           SparsePage::Inst const inst = batch[i];
-          if (use_group_ind_ && group_ptr[group_ind] == ridx &&
-              // maximum equals to weights.size() - 1
-              group_ind < num_groups - 1) {
-            // move to next group
-            group_ind++;
+          if (use_group_ind_) {
+            group_ind = this->SearchGroupIndFromRow(group_ptr, i + page.base_rowid);
           }
           size_t w_idx = use_group_ind_ ? group_ind : ridx;
           auto w = info.GetWeight(w_idx);
@@ -102,22 +95,11 @@ void HostSketchContainer::PushRowPage(SparsePage const &page,
               sketches_[ii].Push(p_data[ii].fvalue, w);
             }
           } else {
-            auto p_end = inst.data() + inst.size();
-            auto mid = static_cast<size_t>(
-                (static_cast<double>(begin) / static_cast<double>(ncol)) *
-                static_cast<double>(inst.size()));
-            if (mid < inst.size() / 2) {
-              for (auto it = p_data; it != p_end && it->index < end; ++it) {
-                if (it->index >= begin) {
-                  sketches_[it->index].Push(it->fvalue, w);
-                }
-              }
-            } else {
-              for (auto it = p_end - 1; it >= p_data && it->index >= begin;
-                   --it) {
-                if (it->index < end) {
-                  sketches_[it->index].Push(it->fvalue, w);
-                }
+            auto p_inst = inst.data();
+            for (size_t i = 0; i < inst.size(); ++i) {
+              auto const& entry = p_inst[i];
+              if (entry.index >= begin && entry.index < end) {
+                sketches_[entry.index].Push(entry.fvalue, w);
               }
             }
           }