On 08/29/2012 11:34 AM, Pozsár Balázs wrote: > > Hi all, > > I have been testing network throughput and latency and I was wondering > if my measurements are as expected. > For the test, I used Fedora 17 for both host and guest, using kernel > 3.5.2-3.fc17.86_64. > > Pinging an external server on the LAN from the host, using a gigabit > interface, the results are: > # ping -c 10 172.16.1.1 > PING 172.16.1.1 (172.16.1.1) 56(84) bytes of data. > 64 bytes from 172.16.1.1: icmp_req=1 ttl=64 time=0.109 ms > 64 bytes from 172.16.1.1: icmp_req=2 ttl=64 time=0.131 ms > 64 bytes from 172.16.1.1: icmp_req=3 ttl=64 time=0.145 ms > 64 bytes from 172.16.1.1: icmp_req=4 ttl=64 time=0.116 ms > 64 bytes from 172.16.1.1: icmp_req=5 ttl=64 time=0.110 ms > 64 bytes from 172.16.1.1: icmp_req=6 ttl=64 time=0.114 ms > 64 bytes from 172.16.1.1: icmp_req=7 ttl=64 time=0.112 ms > 64 bytes from 172.16.1.1: icmp_req=8 ttl=64 time=0.117 ms > 64 bytes from 172.16.1.1: icmp_req=9 ttl=64 time=0.119 ms > 64 bytes from 172.16.1.1: icmp_req=10 ttl=64 time=0.128 ms > > --- 172.16.1.1 ping statistics --- > 10 packets transmitted, 10 received, 0% packet loss, time 8999ms > rtt min/avg/max/mdev = 0.109/0.120/0.145/0.011 ms > > > Pinging the same external host on the LAN from the guest, the latency > seems to be much higher: > # ping -c 10 172.16.1.1 > PING 172.16.1.1 (172.16.1.1) 56(84) bytes of data. > 64 bytes from 172.16.1.1: icmp_req=1 ttl=64 time=0.206 ms > 64 bytes from 172.16.1.1: icmp_req=2 ttl=64 time=0.352 ms > 64 bytes from 172.16.1.1: icmp_req=3 ttl=64 time=0.518 ms > 64 bytes from 172.16.1.1: icmp_req=4 ttl=64 time=0.351 ms > 64 bytes from 172.16.1.1: icmp_req=5 ttl=64 time=0.543 ms > 64 bytes from 172.16.1.1: icmp_req=6 ttl=64 time=0.387 ms > 64 bytes from 172.16.1.1: icmp_req=7 ttl=64 time=0.348 ms > 64 bytes from 172.16.1.1: icmp_req=8 ttl=64 time=0.364 ms > 64 bytes from 172.16.1.1: icmp_req=9 ttl=64 time=0.345 ms > 64 bytes from 172.16.1.1: icmp_req=10 ttl=64 time=0.334 ms > > --- 172.16.1.1 ping statistics --- > 10 packets transmitted, 10 received, 0% packet loss, time 8999ms > rtt min/avg/max/mdev = 0.206/0.374/0.543/0.093 ms > > > The LAN, the host and guest are idle otherwise during the tests. > There are no iptables rules active. > The vhost_net and macvtap modules are loaded on the host, and qemu was > started (by libvirtd) with the -netdev vhost=on option. > The guest is using the virtio_net driver. > > Is this expected and normal, or do others see better latencies? Can I > try anything to make it better?
We've seen this, at least in once case the problem is due to the extra threads needed for virtualization; each one of them sits on a core, and if that core is in deep C state it will take quite a while to wake up. You can verify this by booting the host with idle=poll on the kernel command line, or simply running some load in the background. -- error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
