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Solved!  8)

So it was the source host sending malformed packets. I'm not 100% certain as to why but I blame Windows. It is possible that Windows drivers were somehow affecting NV settings on the NICs so problem persisted across OSs.

Reflashing the NIC and resetting Windows drivers solved it.

I was only able to confirm that it was happening by doing a LOT more wiresharking on both ends of the network with  Promiscuous mode enabled across all interfaces.

[[Problem statement]]

Alright. I give up. I spent 4 nights on it and have no clue as to what is going on.

[Problem statement]
In a 10 gigabit 2-tier home network 1 host mystically causes 100% CPU load and has speeds waaaay below link speed. Looks like no other hosts are affected.

Let's dig in.

[Network info, diagnostic steps]
- full-sized image attached

Apologies for a very quick drawing. So this is a part of the network in question. All links are 10Gbps on different media (twinwax, MMF, CAT6A).

ICX6610 serves as the main inter-vlan router and right now has a permissive ACL. Clients use ICX6610 as a default gateway and it forwards internet-bound traffic to OPN and through it to the internet via VLAN128.

Vlans: VLAN20, VLAN33 are client VLANs. VLAN128 is for the icx <> OPN connection only and VLAN64 is for OPN <> modem.


Method of testing is iperf3 running directly on OPN.

• Client 1 on VLAN20 speeds to OPN are below 1Gbps and cause 100% cpu load on OPN.
• Client 2 is on the same host as OPN physically but uses dedicated hardware. Achieves link speed (9.90Gbps) at ~ 50% CPU load on OPN
• Client 3 is a separate non-virtualized Linux machine that also works at link speed (~9.1GBps) at about 50% CPU

• Client 1 <> Client 2 is link speed
  
• Client 1 <> Client 3 is also link speed

Attempted bringing client 1 into VLAN128 (so icx6610 doesn't do any routing) - same.

Notes:

• client 1 is windows but was booted into linux and had exactly the same speed.
• NIC was replaced with a different model to test on both - no notable difference.
• Wireguard would occasionally show TCP window full and ACKd unseen segment.
• PFsense presents the same behaviour
• Last know working configuration was on PFsense 2.5.2 so a while ago. Have no intention going back there
• Disabling firewall didn't help, while booting linux livecd on the same machine produced link speeds as expected

[Configuration]
OPNsense is qemu/libvirt virtualized with passthrough of an Intel IX (Intel Corporation 82599ES 10-Gigabit SFI/SFP+) NIC.
Client 2 on the same machine has a different NIC passed through it.

OPNsense has 12 cores allocated to it of Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz and 32GB of RAM

The issue presents itself on a clean installation as well as on a tuned OPNsense.

top -aSCHIP shows that the 100% CPU load is generated by interrupts.  intr{swi1: netisr XX}  and kernel{if_io_tqg_X}.

Any suggestions are welcome as I still can't diagnose the issue. Happy to provide any additional details.