Hello,
I currently occur a problem with latency by using KVM with virtio.
I totally ignore if the problem come from KVM, kernel or QEMU. I send
you first my question, maybe you will give me some answer.
On a debian stretch host, I've install all KVM (kernel 4.7), libvirt
(2.2) and QEMU (2.6) tools. On this host, vhost_net module is loaded and
used.
On it, 4 debian stretch guest were deployed with virtio-scsi and
virtio-net.
Between guest, no problem of bandwidth at all (an impressive 25Gb/s !).
Just the latency appears a little bit too high for what KVM will be
used.
The bridge for guest has been created by libvirt.
Ping with 64 bytes
Host -> Guest : 0.102/0.134/0.161/0.023 ms
Guest -> Host : 0.095/0.122/0.152/0.016 ms
Guest -> Guest : 0.118/0.184/0.218/0.025 ms
Now, I've found strange "solution". If I generate some traffic (100MB
with iperf), the ping could be really better ! :
# iperf -s & ping -c 20 162.168.0.23 :
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 85.3 KByte (default)
------------------------------------------------------------
PING 192.168.0.23 (192.168.0.23) 56(84) bytes of data.
64 bytes from 192.168.0.23: icmp_seq=1 ttl=64 time=0.130 ms
64 bytes from 192.168.0.23: icmp_seq=2 ttl=64 time=0.225 ms
64 bytes from 192.168.0.23: icmp_seq=3 ttl=64 time=0.212 ms
[ 4] local 192.168.0.22 port 5001 connected with 192.168.0.23 port 53350
64 bytes from 192.168.0.23: icmp_seq=4 ttl=64 time=0.063 ms
64 bytes from 192.168.0.23: icmp_seq=5 ttl=64 time=0.065 ms
64 bytes from 192.168.0.23: icmp_seq=6 ttl=64 time=0.063 ms
64 bytes from 192.168.0.23: icmp_seq=7 ttl=64 time=0.058 ms
64 bytes from 192.168.0.23: icmp_seq=8 ttl=64 time=0.067 ms
64 bytes from 192.168.0.23: icmp_seq=9 ttl=64 time=0.056 ms
64 bytes from 192.168.0.23: icmp_seq=10 ttl=64 time=0.053 ms
64 bytes from 192.168.0.23: icmp_seq=11 ttl=64 time=0.066 ms
64 bytes from 192.168.0.23: icmp_seq=12 ttl=64 time=0.058 ms
64 bytes from 192.168.0.23: icmp_seq=13 ttl=64 time=0.071 ms
[ ID] Interval Transfer Bandwidth
[ 4] 0.0-10.0 sec 119 MBytes 100 Mbits/sec
64 bytes from 192.168.0.23: icmp_seq=14 ttl=64 time=0.197 ms
64 bytes from 192.168.0.23: icmp_seq=15 ttl=64 time=0.211 ms
64 bytes from 192.168.0.23: icmp_seq=16 ttl=64 time=0.219 ms
64 bytes from 192.168.0.23: icmp_seq=17 ttl=64 time=0.199 ms
64 bytes from 192.168.0.23: icmp_seq=18 ttl=64 time=0.217 ms
64 bytes from 192.168.0.23: icmp_seq=19 ttl=64 time=0.197 ms
64 bytes from 192.168.0.23: icmp_seq=20 ttl=64 time=0.215 ms
--- 192.168.0.23 ping statistics ---
20 packets transmitted, 20 received, 0% packet loss, time 19004ms
rtt min/avg/max/mdev = 0.053/0.132/0.225/0.072 ms
I was thinking the issue came from large send offload.
I disabled all sg tso ufo gso gro with ethtool without any improvement.
(But bandwidth drop dramatically 25Gbps -> 4Gbps)
If I ping from a 3rd guest, one of guest implied on iperf I see a small
improvement (not as much as I can see on both node concerned by iperf).
And ping the 4th guest from the 3rd one, no impact at all, I still get
~190ms.
Do you have an idea how can I improve the latency without generate dummy
traffic ?
Thank you all for the good work with KVM, keep up with it ! :)
Best regards,
Greg
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