+/*
+ * There are three way to measure time in OS X:
+ * 1. nanoseconds
+ * 2. absolute time (abstime unit equal to the length of one bus cycle),
+ * schedule of thread/timer are counted by absolute time, but abstime
+ * in different mac can be different also, so we wouldn't use it.
+ * 3. clock interval (1sec = 100hz). But clock interval only taken by KPI
+ * like tsleep().
+ *
+ * We use nanoseconds (uptime, not calendar time)
+ *
+ * clock_get_uptime() :get absolute time since bootup.
+ * nanouptime() :get nanoseconds since bootup
+ * microuptime() :get microseonds since bootup
+ * nanotime() :get nanoseconds since epoch
+ * microtime() :get microseconds since epoch
+ */