You achieved a combined throughput of 17.4 Gbits/sec (~2.15 Gigabytes per second).
Your goal of exceeding 10 Gbps over Thunderbolt 4 is confirmed.
Performance Analysis
- Throughput: Moving 20.2 GBytes in 10 seconds confirms your Thunderbolt networking layer (
thunderbolt-net) is functioning at full speed. - Overhead Expectation: While Thunderbolt 4 supports up to 40 Gbps PHY link speed, Thunderbolt Networking encapsulates Ethernet packets inside PCIe/Thunderbolt frames. Reaching 17–22 Gbps point-to-point network throughput is typical for Linux peer-to-peer laptop connections due to kernel TCP stack and CPU interrupt handling.
- TCP Retransmissions (
Retr: 82,860): The high number of retransmissions indicates packet drops, likely caused by TCP buffer limits, CPU core saturation, or small MTU sizes under heavy multi-stream load (-P 8).
Steps to Maximize Throughput
To reduce packet drops and approach ~22–25 Gbps, apply the following optimizations:
1. Increase MTU to 9000 (Jumbo Frames)
Standard Ethernet uses an MTU of 1500 bytes. Enabling Jumbo Frames reduces CPU interrupt overhead significantly.
- On Proxmox:
ip link set dev thunderbolt0 mtu 9000
- On Linux Mint:
sudo ip link set dev thunderbolt0 mtu 9000
2. Increase TCP Socket Buffers
Run these commands on both Proxmox and Linux Mint to prevent packet drops at high bitrates:
sudo sysctl -w net.core.rmem_max=16777216
sudo sysctl -w net.core.wmem_max=16777216
sudo sysctl -w net.ipv4.tcp_rmem="4096 87380 16777216"
sudo sysctl -w net.ipv4.tcp_wmem="4096 65536 16777216"
3. Re-test Bandwidth
Run iperf3 again with 4 parallel streams instead of 8:
iperf3 -c 192.168.99.2 -P 4 -t 10