获取时间的开销

• time (s) => 4ns
• ftime (ms) => 39ns
• gettimeofday (us) => 30ns
• clock_gettime (ns) => 26ns (CLOCK_REALTIME)
• clock_gettime (ns) => 8ns (CLOCK_REALTIME_COARSE)
• clock_gettime (ns) => 26ns (CLOCK_MONOTONIC)
• clock_gettime (ns) => 9ns (CLOCK_MONOTONIC_COARSE)  （相对CLOCK_MONOTONIC不精确）
  
• clock_gettime (ns) => 170ns (CLOCK_PROCESS_CPUTIME_ID)
• clock_gettime (ns) => 154ns (CLOCK_THREAD_CPUTIME_ID)
• cached_clock (s) => 0ns