update tree maker to make it more robust

python/README.md

@@ -3,3 +3,4 @@ python wrapper for xgboost using ctypes
 see example for usage
 
 to make the python module, type make in the root directory of project
+

src/tree/updater_colmaker-inl.hpp

@@ -271,7 +271,8 @@ class ColMaker: public IUpdater<FMatrix> {
       }
       // start enumeration
       const unsigned nsize = static_cast<unsigned>(feat_set.size());
-      #pragma omp parallel for schedule(dynamic, 1)
+      const int batch_size = std::max(static_cast<int>(nsize / this->nthread / 32), 1);
+      #pragma omp parallel for schedule(dynamic, batch_size)
       for (unsigned i = 0; i < nsize; ++i) {
         const unsigned fid = feat_set[i];
         const int tid = omp_get_thread_num();