broadcast subsampled feature correctly

src/common/random.h

@@ -17,6 +17,8 @@
 #include <numeric>
 #include <random>
 
+#include "io.h"
+
 namespace xgboost {
 namespace common {
 /*!
@@ -93,15 +95,24 @@ class ColumnSampler {
     if (colsample == 1.0f) return p_features;
     const auto& features = *p_features;
     CHECK_GT(features.size(), 0);
-    int n = std::max(1, static_cast<int>(colsample * features.size()));
+    unsigned long n = std::max(1, static_cast<int>(colsample * features.size()));
     auto p_new_features = std::make_shared<std::vector<int>>();
     auto& new_features = *p_new_features;
     new_features.resize(features.size());
     std::copy(features.begin(), features.end(), new_features.begin());
     std::shuffle(new_features.begin(), new_features.end(), common::GlobalRandom());
-    new_features.resize(n);
-    std::sort(new_features.begin(), new_features.end());
-
+    new_features.resize(static_cast<unsigned long>(n));
+    // std::sort(new_features.begin(), new_features.end());
+
+    // sync the subsampled columns
+    std::string s_cache;
+    common::MemoryBufferStream fc(&s_cache);
+    dmlc::Stream& fs = fc;
+    if (rabit::GetRank() == 0) {
+      fs.Write(new_features);
+    }
+    rabit::Broadcast(&s_cache, 0);
+    fs.Read(&new_features);
     // ensure that new_features are the same across ranks
     rabit::Broadcast(&new_features, 0);