Network bandwidth is unstable between Host and VM Guest

[3motion]

Host uses Linux bridge, VM Guest uses Virtio NIC model. I enabled the iperf3 server on the Host and then tested on the VM Guest,Then I get the following result:

[ 5] 0.00-1.00 sec 2.08 GBytes 17.9 Gbits/sec 0 2.53 MBytes
[ 5] 1.00-2.00 sec 2.91 GBytes 25.0 Gbits/sec 0 2.85 MBytes
[ 5] 2.00-3.00 sec 3.06 GBytes 26.3 Gbits/sec 0 3.02 MBytes
[ 5] 3.00-4.00 sec 1.04 GBytes 8.89 Gbits/sec 0 3.02 MBytes
[ 5] 4.00-5.00 sec 1.02 GBytes 8.80 Gbits/sec 0 3.02 MBytes
[ 5] 5.00-6.00 sec 2.18 GBytes 18.7 Gbits/sec 0 3.02 MBytes
[ 5] 6.00-7.00 sec 2.26 GBytes 19.4 Gbits/sec 0 3.02 MBytes
[ 5] 7.00-8.00 sec 1.83 GBytes 15.7 Gbits/sec 0 3.02 MBytes
[ 5] 8.00-9.00 sec 1.24 GBytes 10.6 Gbits/sec 0 3.02 MBytes
[ 5] 9.00-10.00 sec 1.58 GBytes 13.6 Gbits/sec 0 3.02 MBytes

Normally, the test result will be a stable 20Gbps,I ran the same test on another Proxmox server and the results were normal.

My hardware and environment are as follows:

Motherboard: Supermicro H12SSL-i
CPU: AMD EPYC 7Y83
RAM: 256GB RECC 3200Mhz
Startup Disk: Samsung 980Pro 1Tb

Another normal server is Lenovo HR650X with Dual intel Xeon Gold 6138 and 192GB of 2133Mhz RAM.

 

[shanreich]

How does the VM Config look like?
Have you checked the load on the host / guest during the tests?
What exact iperf commands are you using to test?
Have you checked both directions? (-R inverts the direction)