After some recent updates to the 6.13 kernels in Fedora41 I have noticed very slow network speeds when communicating from a Windows Hyper-V virtual machine to a Fedora41 Hyper-V virtual machine. In this setup I use a private virtual network that allows communication between virtual machines and the host. I have first noticed the issue when copying files from Fedora to Windows over NFS. I have then also checked Samba and it was also very slow. In both cases Windows is the client.
When starting the same Fedora41 installation with an older kernel (6.11.7-300.fc41.x86_64) the issue is solved. I have run a iperf3 test which also shows the issue.
6.13.10-200.fc41.x86_64
Windows server
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 144 MBytes 121 Mbits/sec 4552 sender
[ 5] 0.00-10.00 sec 142 MBytes 119 Mbits/sec receiver
Windows client
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-10.00 sec 4.53 GBytes 3.89 Gbits/sec sender
[ 4] 0.00-10.00 sec 4.53 GBytes 3.89 Gbits/sec receiver
6.11.7-300.fc41.x86_64
Windows server
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 7.72 GBytes 6.63 Gbits/sec 0 sender
[ 5] 0.00-10.00 sec 7.72 GBytes 6.63 Gbits/sec receiver
Windows client
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-10.00 sec 5.20 GBytes 4.47 Gbits/sec sender
[ 4] 0.00-10.00 sec 5.20 GBytes 4.47 Gbits/sec receiver
When copying files over NFS
6.13.10-200.fc41.x86_64: about 15 MB/s
6.11.7-300.fc41.x86_64: about 700 MB/s
On another host I have a fedora41 vm that had only 6.13 kernels. After a manual installation of kernel 6.12.15 the issue was also solved. On this host the difference was even more pronounced.
6.13.10-200.fc41.x86_64: about 15 MB/s
6.12.15-200.fc41.x86_64: about 1.2 GB/s
Is this a known issue in the 6.13 kernels?