Re: ping latency using vhost_net, macvtap and virtio

[Avi Kivity <avi@redhat.com>]

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.


-- 
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