diff --git a/PreLoad.cmake b/PreLoad.cmake
index a9d0d6512f9..dadaeb60a86 100644
--- a/PreLoad.cmake
+++ b/PreLoad.cmake
@@ -1,4 +1,11 @@
 # Use Ninja instead of Unix Makefiles by default.
+# https://stackoverflow.com/questions/11269833/cmake-selecting-a-generator-within-cmakelists-txt
+#
+# Reason: it have better startup time than make and it parallelize jobs more uniformly.
+# (when comparing to make with Makefiles that was generated by CMake)
+#
+# How to install Ninja on Ubuntu:
+#  sudo apt-get install ninja-build
 
 find_program(NINJA_PATH ninja)
 if (NINJA_PATH)