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Date: Fri, 2 Oct 2020 16:51:07 +0800 From: 叶小龙 <muryo.ye@...il.com> To: linux-kernel@...r.kernel.org, netdev@...r.kernel.org Subject: Why ping latency is smaller with shorter send interval? Hi, net experts, Hope this is the right place to ask the question :) Recently I've tried to measure the network latency between two machines by using ping, one interesting observation I found is that ping latency will be smaller if I use a shorter interval with -i option. For example, when I use default ping (interval is 1s), then the ping result is as below with avg latency 0.062ms # ping 9.9.9.2 -c 10 PING 9.9.9.2 (9.9.9.2) 56(84) bytes of data. 64 bytes from 9.9.9.2: icmp_seq=1 ttl=64 time=0.059 ms 64 bytes from 9.9.9.2: icmp_seq=2 ttl=64 time=0.079 ms 64 bytes from 9.9.9.2: icmp_seq=3 ttl=64 time=0.060 ms 64 bytes from 9.9.9.2: icmp_seq=4 ttl=64 time=0.072 ms 64 bytes from 9.9.9.2: icmp_seq=5 ttl=64 time=0.048 ms 64 bytes from 9.9.9.2: icmp_seq=6 ttl=64 time=0.069 ms 64 bytes from 9.9.9.2: icmp_seq=7 ttl=64 time=0.067 ms 64 bytes from 9.9.9.2: icmp_seq=8 ttl=64 time=0.055 ms 64 bytes from 9.9.9.2: icmp_seq=9 ttl=64 time=0.058 ms 64 bytes from 9.9.9.2: icmp_seq=10 ttl=64 time=0.055 ms --- 9.9.9.2 ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9001ms rtt min/avg/max/mdev = 0.048/0.062/0.079/0.010 ms Then I use "-i 0.001", the lateny (0.038) is way better than defaut ping # ping 9.9.9.2 -i 0.001 -c 10 PING 9.9.9.2 (9.9.9.2) 56(84) bytes of data. 64 bytes from 9.9.9.2: icmp_seq=1 ttl=64 time=0.069 ms 64 bytes from 9.9.9.2: icmp_seq=2 ttl=64 time=0.039 ms 64 bytes from 9.9.9.2: icmp_seq=3 ttl=64 time=0.034 ms 64 bytes from 9.9.9.2: icmp_seq=4 ttl=64 time=0.033 ms 64 bytes from 9.9.9.2: icmp_seq=5 ttl=64 time=0.033 ms 64 bytes from 9.9.9.2: icmp_seq=6 ttl=64 time=0.033 ms 64 bytes from 9.9.9.2: icmp_seq=7 ttl=64 time=0.034 ms 64 bytes from 9.9.9.2: icmp_seq=8 ttl=64 time=0.036 ms 64 bytes from 9.9.9.2: icmp_seq=9 ttl=64 time=0.037 ms 64 bytes from 9.9.9.2: icmp_seq=10 ttl=64 time=0.038 ms --- 9.9.9.2 ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9ms rtt min/avg/max/mdev = 0.033/0.038/0.069/0.012 ms ping loopback shows the similar result. Default ping avg latency is 0.049ms # ping 127.0.0.1 -c 10 PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data. 64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.032 ms 64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.049 ms 64 bytes from 127.0.0.1: icmp_seq=3 ttl=64 time=0.054 ms 64 bytes from 127.0.0.1: icmp_seq=4 ttl=64 time=0.058 ms 64 bytes from 127.0.0.1: icmp_seq=5 ttl=64 time=0.049 ms 64 bytes from 127.0.0.1: icmp_seq=6 ttl=64 time=0.042 ms 64 bytes from 127.0.0.1: icmp_seq=7 ttl=64 time=0.052 ms 64 bytes from 127.0.0.1: icmp_seq=8 ttl=64 time=0.052 ms 64 bytes from 127.0.0.1: icmp_seq=9 ttl=64 time=0.053 ms 64 bytes from 127.0.0.1: icmp_seq=10 ttl=64 time=0.055 ms --- 127.0.0.1 ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9001ms rtt min/avg/max/mdev = 0.032/0.049/0.058/0.010 ms ping with "-i 0.001" shows 0.014ms avg latency. # ping 127.0.0.1 -i 0.001 -c 10 PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data. 64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.040 ms 64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.014 ms 64 bytes from 127.0.0.1: icmp_seq=3 ttl=64 time=0.012 ms 64 bytes from 127.0.0.1: icmp_seq=4 ttl=64 time=0.011 ms 64 bytes from 127.0.0.1: icmp_seq=5 ttl=64 time=0.011 ms 64 bytes from 127.0.0.1: icmp_seq=6 ttl=64 time=0.011 ms 64 bytes from 127.0.0.1: icmp_seq=7 ttl=64 time=0.011 ms 64 bytes from 127.0.0.1: icmp_seq=8 ttl=64 time=0.010 ms 64 bytes from 127.0.0.1: icmp_seq=9 ttl=64 time=0.010 ms 64 bytes from 127.0.0.1: icmp_seq=10 ttl=64 time=0.011 ms --- 127.0.0.1 ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9ms rtt min/avg/max/mdev = 0.010/0.014/0.040/0.008 ms I'm using centos 7.2 with kernel 3.10. I am very confused about the result. As I understand it, it doesn't matter how frequently I send packets, each packet's latency should be the same. So How can I understand it from network stack point of view? Any thoughts or suggestions would be highly appreciated. Thanks, Xiaolong
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