The tale of the search for hidden 40Gbps

Not everyone knows that Thunderbolt 3/4+ is not only those incomprehensible usb-c ports in computers of a certain fruit company, and just starting to appear by default in the latest Intel/AMD computer lines, but also a surprisingly fast network interface. Moreover, as far as I understand, besides Macs, it is also supported by Linux and Windows - this is the Thunderbolt Networking standard. Also, USB4 seems to have some capabilities in this area, but USB4 is a stripped-down Thunderbolt, and who knows how much each manufacturer has stripped it down, USB is just a swamp.

But we only have a few MacBooks with Thunderbolt ports, so the experiments will be with them.

And yes, for those who don't know, a superficial explanation for further understanding - iperf is a console software for checking network speed, on one of the computers it runs as a server, it will receive traffic, on the other - as a client, it will send traffic. It has a lot of options and settings, but in this case, they are not critical.

We take two MacBook Pros (with M2 Pro and M3 Pro, Apple with these "pros" is a bit tiring), connect them with a kind of usb-c, but thicker and more expensive cable with a lightning bolt, run iperf, and see:
0.00-10.01 sec 44.0 GBytes 37.8 Gbits/sec
Beautiful.

We replace one of the Pros with a MacBook Air (M2, without "pro"). Run iperf, observe.
The Pro is the client:
0.00-10.01 sec 24.6 GBytes 21.1 Gbits/sec
The Air is the client:
0.00-10.01 sec 41.4 GBytes 35.6 Gbits/sec
Something is lacking for the Air on reception.

Moreover, the CPU load was less than 150% everywhere, which is not too scary for 8-12 core machines. Most likely, the Air has some bottleneck with memory bandwidth.

Now we make the scheme a bit more complicated, there is a built-in bridge in macOS, let there be three devices, the scheme: Pro <-> Pro <-> Air, and we start driving traffic between the two endpoints, and the middle one - let it suffer from bridging.

If the "left" Pro is the client, we get:
0.00-10.01 sec 19.4 GBytes 16.7 Gbits/sec
Again, the Air has a bottleneck on reception.

If the Air is the client - we see:
0.00-10.01 sec 36.8 GBytes 31.6 Gbits/sec
Not bad either, who at home can boast of such a network for three computers?

Moreover, the "average" pro - still lost 350% cpu in the last case. That is, even for a rack-level router - it is completely unsuitable, which is not surprising at such speeds. But for some kind of microcluster, you can come up with a fun topology.

So, this is a mini-test. I hope someone found it interesting.

What could this mean in more or less ordinary life? Well, if you don't tune anything, then my files are copied via a regular smb between Macs at a speed of about a gigabyte per second. Where the bottleneck is, it is not very clear, it looks more like smb than a disk, but it is still much faster than wifi. And probably sometimes someone might find it useful.

PS: But with all this, there is an obvious problem - long cables do not exist, as far as I know