Add environment variable to control max win sent length

bluefog/common/mpi_controller.cc

@@ -34,7 +34,11 @@ namespace common {
 // Due to unclear reason that mpi_put/get/accumlate under the
 // mpi_lock epoch cannot send too long vector in one time, we
 // define this number as the maximum size of win_ops can send.
-constexpr int MAX_WIN_SENT = 2000;
+static const char* BLUEFOG_MAX_WIN_SENT = std::getenv("BLUEFOG_MAX_WIN_SENT_LENGTH");
+static const int MAX_WIN_SENT =
+    BLUEFOG_MAX_WIN_SENT == nullptr
+        ? 2000
+        : std::strtol(BLUEFOG_MAX_WIN_SENT, nullptr, 10);
 
 // MPIController
 void MPIController::Initialize() {