[j-nsp] high RTT on vMX(virtio mode) interfaces

[Martin T]

Hi!

I have a simple network topology where tap interface named
ge-0.0.1-vmx1(generated by vMX orchestration scripts) is connected to
virbr1 bridge port 1. This ge-0.0.1-vmx1 is mapped to ge-0/0/1
interface in Junos and has IPv4 address 10.210.0.1/24 configured.
virbr1 has 10.210.0.2/24 configured:

$ ip a sh dev virbr1
24: virbr1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue
state UP group default
link/ether fe:d6:c7:04:03:1b brd ff:ff:ff:ff:ff:ff
inet 10.210.0.2/24 scope global virbr1
valid_lft forever preferred_lft forever
inet6 fe80::fc06:aff:fe0e:fff1/64 scope link
valid_lft forever preferred_lft forever
$

When I ping the 10.210.0.2 from vMX, then the RTT is at least 20ms or
higher. Example:

***@vmx1> ping 10.210.0.2 source 10.210.0.1 count 100 rapid
PING 10.210.0.2 (10.210.0.2): 56 data bytes
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
--- 10.210.0.2 ping statistics ---
100 packets transmitted, 100 packets received, 0% packet loss
round-trip min/avg/max/stddev = 22.045/33.365/73.901/6.391 ms

***@vmx1>

I don't observe such behavior when I use a physical NIC. fxp0 of vmx1
is mapped to a physical NIC and 192.168.0.180 is a physical machine in
my network:

***@vmx1> show route 192.168.0.180 terse

inet.0: 8 destinations, 8 routes (8 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

A V Destination P Prf Metric 1 Metric 2 Next hop AS path
* ? 192.168.0.0/24 D 0 >fxp0.0

***@vmx1> ping 192.168.0.180 source 192.168.0.178 count 100 rapid
PING 192.168.0.180 (192.168.0.180): 56 data bytes
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
--- 192.168.0.180 ping statistics ---
100 packets transmitted, 100 packets received, 0% packet loss
round-trip min/avg/max/stddev = 0.103/0.137/0.295/0.023 ms

***@vmx1>

Both VCP and VFP have the lowest niceness value, load of the host
server is low, there is no netem configuration, which might affect the
latency, etc.

Is such latency expected when using virtio mode?


thanks,
Martin
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