[DIST] Enable multiple thread and tracker, make rabit and xgboost more thread-safe by using thread local variables.

src/common/common.cc

@@ -4,12 +4,20 @@
  * \brief Enable all kinds of global variables in common.
  */
 #include "./random.h"
+#include "./thread_local.h"
 
 namespace xgboost {
 namespace common {
+/*! \brief thread local entry for random. */
+struct RandomThreadLocalEntry {
+  /*! \brief the random engine instance. */
+  GlobalRandomEngine engine;
+};
+
+typedef ThreadLocalStore<RandomThreadLocalEntry> RandomThreadLocalStore;
+
 GlobalRandomEngine& GlobalRandom() {
-  static GlobalRandomEngine inst;
-  return inst;
+  return RandomThreadLocalStore::Get()->engine;
 }
 }
 }  // namespace xgboost

src/common/random.h

@@ -61,7 +61,8 @@ typedef RandomEngine GlobalRandomEngine;
 
 /*!
  * \brief global singleton of a random engine.
- *  Only use this engine when necessary, not thread-safe.
+ *  This random engine is thread-local and
+ *  only visible to current thread.
  */
 GlobalRandomEngine& GlobalRandom(); // NOLINT(*)
 

src/common/thread_local.h

@@ -6,6 +6,8 @@
 #ifndef XGBOOST_COMMON_THREAD_LOCAL_H_
 #define XGBOOST_COMMON_THREAD_LOCAL_H_
 
+#include <dmlc/base.h>
+
 #if DMLC_ENABLE_STD_THREAD
 #include <mutex>
 #endif