Merge commit 'd87691ec603db325d5b1c5db1186295a748df7cc' as 'subtree/rabit'

subtree/rabit/guide/basic.cc

@@ -0,0 +1,29 @@
+/*!
+ *  Copyright (c) 2014 by Contributors
+ * \file basic.cc
+ * \brief This is an example demonstrating what is Allreduce
+ *
+ * \author Tianqi Chen
+ */
+#include <rabit.h>
+using namespace rabit;
+const int N = 3;
+int main(int argc, char *argv[]) {
+  int a[N];
+  rabit::Init(argc, argv);
+  for (int i = 0; i < N; ++i) {
+    a[i] = rabit::GetRank() + i;
+  } 
+  printf("@node[%d] before-allreduce: a={%d, %d, %d}\n",
+         rabit::GetRank(), a[0], a[1], a[2]);
+  // allreduce take max of each elements in all processes
+  Allreduce<op::Max>(&a[0], N);
+  printf("@node[%d] after-allreduce-max: a={%d, %d, %d}\n",
+         rabit::GetRank(), a[0], a[1], a[2]);
+  // second allreduce that sums everything up
+  Allreduce<op::Sum>(&a[0], N);
+  printf("@node[%d] after-allreduce-sum: a={%d, %d, %d}\n",
+         rabit::GetRank(), a[0], a[1], a[2]);  
+  rabit::Finalize();
+  return 0;
+}