How to debug gigabit Ethernet on linux?

During testing network, I'm facing a very low throughput with gigabit Ethernet, which was implemented as RGMII mode, as follows:

$iperf - s
Interval       Transfer     Bandwidth
0.0-10.0 sec   211 MBytes   176 Mbits/sec

$iperf - c <IP>
Interval       Transfer     Bandwidth
0.0-10.0 sec   101 MBytes   83.6 Mbits/sec

I'm just a beginner with network on Linux then could anyone guide me some direction to debug and fix this issue?

The only thing I tried so far is that reading ethtool. There is nothing wrong observe here.

Settings for eth1:
        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Half 1000baseT/Full
        Supported pause frame use: Symmetric
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Half 1000baseT/Full
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: Yes
        Speed: 1000Mb/s
        Duplex: Full
        Port: MII
        PHYAD: 2
        Transceiver: external
        Auto-negotiation: on
        Supports Wake-on: d
        Wake-on: d
        Current message level: 0x00000000 (0)

        Link detected: yes