You can find tons of used 10-40Gbps NICs on ebay for $50 or less. For a simple point-to-point connection with both sides having PCI-E slots that's going to be easier & cheaper to set up than thunderbolt will be. It'll also be more reliable & more practical since getting 40Gbps over Thunderbolt requires extremely short cables, whereas you can get DAC cables for 40Gbps QSFP or Infiniband up to 7M or so. And there's no confusion around what the cable can actually do, unlike the nightmare that is finding the right USB-C cable with the right signal integrity requirements at the right distance for what you actually need/want.
This has not been my experience -- I use both thunderbolt 3 and Mellanox ConnectX-3 40gbps cards, and have been looking for an excuse to add a Mini M1 to the homelab.
The cables and cards for PC Thunderbolt retrofit are far (like 3-5X!) more expensive than the MLNX cards and optical cables.
> The cables and cards for PC Thunderbolt retrofit are far (like 3-5X!) more expensive than the MLNX cards and optical cables.
I added new Thunderbolt3 PCIe cards (Gigabyte Titan Ridge) to two of my 2012 Mac Pros for around $100 each a bit over a year ago and the prices have dropped since, so I don't get how that is 3-5x more expensive than MLNX cards. Expense was way way lower a factor than the finesse it took to get MacOS to support them for me, but it was the only path that worked for my needs (different need than OP article) and so I endured the pain of getting it working.
