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I connected two MBPs via thunderbolt, via thunderbolt bridge which should provide 10Gbit/s bidirectional connection on one of its channels.

I have them connected over a thunderbolt 2 male male cable and a thunderbolt 2 to 3 adapter.

The real life single connection speed is 3.3 Gbit/s on one direction and 528 Mbit/s in the other direction.

The tests are done with iperf3 single TCP connections (multiple ones are equal or worse in aggregate I noticed).

Over LAN I have 900 to 940 Mbit/s in both directions.

Why could this 527 Mbit/s be (so low)?

The 3 Gbit is when the older mac is pushing data to the newer one. The 500 Mbit is when the older Mac is pulling (or the newer one is pushing).

All the tests are single connection, one client, one server, one direction of transfer (never bidirectional, never multiple ones at the same time).

The ping is 0.7ms over thunderbolt (both directions) vs 0.9ms over ethernet (median latencies).

I am using Apple adapter & cable.

From the newer MBP:

nicolae.marasoiu@newerMac ~ % iperf3 -c 169.254.139.97 -R
Connecting to host 169.254.139.97, port 5201
Reverse mode, remote host 169.254.139.97 is sending
[  5] local 169.254.33.131 port 55704 connected to 169.254.139.97 port 5201
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec   378 MBytes  3.17 Gbits/sec
^C[  5]   1.00-1.38   sec   145 MBytes  3.20 Gbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.38   sec  0.00 Bytes  0.00 bits/sec                  sender
[  5]   0.00-1.38   sec   523 MBytes  3.18 Gbits/sec                  receiver
iperf3: interrupt - the client has terminated
nicolae.marasoiu@OVO4939MB ~ % iperf3 -c 169.254.139.97
Connecting to host 169.254.139.97, port 5201
[  5] local 169.254.33.131 port 56691 connected to 169.254.139.97 port 5201
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec  67.4 MBytes   566 Mbits/sec
[  5]   1.00-2.00   sec  67.3 MBytes   565 Mbits/sec
[  5]   2.00-2.40   sec  26.6 MBytes   565 Mbits/sec

The ssh + pv show the same. On the other interface (ethernet), the rate is 900 Mbit/s in the same direction:

nicolae.marasoiu@OVO4939MB ~ % iperf3 -Z -c 192.168.0.12
Connecting to host 192.168.0.12, port 5201
[  5] local 192.168.0.13 port 61780 connected to 192.168.0.12 port 5201
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec  68.4 MBytes   573 Mbits/sec
[  5]   1.00-2.00   sec   106 MBytes   893 Mbits/sec
[  5]   2.00-3.00   sec   106 MBytes   889 Mbits/sec
[  5]   3.00-4.00   sec   106 MBytes   890 Mbits/sec
[  5]   4.00-5.00   sec   105 MBytes   885 Mbits/sec
[  5]   5.00-6.00   sec   106 MBytes   891 Mbits/sec
[  5]   6.00-7.00   sec   106 MBytes   889 Mbits/sec

So ethernet raises to 900 Mbit/s in any direction after one or two inferior speeds, while thunderbolt raises to 3 GBit/s in one direction and around 550 Mbit in the other.

The older Mac has an old OS (El Capitain) which could affect things like drivers - I can't upgrade it because its graphical card is broken and I am staying on the Intel on-board graphical unit, but from the service they told me that the graphical card bypass they made will no longer work with a newer MacOS.

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After I installed Ubuntu on the older Mac (on an external drive, Sandisk extreme), I obtained 8 Gbit in one direction and 3 Gbit in the other. This confirmed that the main problem was the old drivers & OS.

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