Linux在運行 ping 命令的時候，里面有一項輸出叫 mdev，如下所示：
ping www.baidu.com
PING www.a.shifen.com (163.177.151.110) 56(84) bytes of data.
64 bytes from 163.177.151.110: icmp_seq=1 ttl=52 time=6.77 ms
64 bytes from 163.177.151.110: icmp_seq=2 ttl=52 time=6.51 ms
64 bytes from 163.177.151.110: icmp_seq=3 ttl=52 time=6.49 ms
64 bytes from 163.177.151.110: icmp_seq=4 ttl=52 time=6.49 ms
64 bytes from 163.177.151.110: icmp_seq=5 ttl=52 time=6.49 ms
64 bytes from 163.177.151.110: icmp_seq=6 ttl=52 time=6.47 ms
^C
--- www.a.shifen.com ping statistics ---
6 packets transmitted, 6 received, 0% packet loss, time 5488ms
rtt min/avg/max/mdev = 6.478/6.541/6.777/0.141 ms
mdev 就是 Mean Deviation 的縮寫，它表示這些 ICMP 包的 RTT 偏離平均值的程度，這個值越大說明你的網速越不穩定
rtt是傳輸的時間延遲Round-Trip Time