Only achieving 2.5Gbps over a 10Gbe direct connection between 2 machines

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I have 2 machines directly connected to each other with a 7 foot Cat6a ethernet cable (included in the box of the NIC cards). The PCIe x4 NIC I bought and in both machines is this: https://www.amazon.com/gp/product/B07CW2C2J1

I'm trying to debug why I'm getting almost exactly 2500Mbps transfer between these 2 machines. Any tips or obvious errors I'm overlooking to acheive closer to 10Gbps?

Here is what I've tested:

Configuration (Machine A)

Machine A ifconfig:

enp7s0    Link encap:Ethernet  HWaddr 24:5e:be:2c:c1:53  
      inet addr:2.0.0.20  Bcast:2.0.0.255  Mask:255.255.255.0
      inet6 addr: fe80::265e:beff:fe2c:c153/64 Scope:Link
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
      RX packets:17225416 errors:0 dropped:0 overruns:0 frame:0
      TX packets:7021731 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:1000 
      RX bytes:25712055299 (25.7 GB)  TX bytes:9701557546 (9.7 GB)

Machine A ip link:

3: enp7s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
link/ether 24:5e:be:2c:c1:53 brd ff:ff:ff:ff:ff:ff

Machine A ethtool enp7so:

Settings for enp7s0:
    Supported ports: [ TP ]
    Supported link modes:   100baseT/Full 
                            1000baseT/Full 
                            10000baseT/Full 
    Supported pause frame use: Symmetric
    Supports auto-negotiation: Yes
    Advertised link modes:  100baseT/Full 
                            1000baseT/Full 
                            10000baseT/Full 
    Advertised pause frame use: Symmetric
    Advertised auto-negotiation: Yes
    Speed: 10000Mb/s
    Duplex: Full
    Port: Twisted Pair
    PHYAD: 0
    Transceiver: external
    Auto-negotiation: on
    MDI-X: Unknown
    Supports Wake-on: g
    Wake-on: g
    Link detected: yes

Configuration (Machine B)

Machine B ifconfig:

enp101s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
    inet 2.0.0.10  netmask 255.255.255.0  broadcast 2.0.0.255
    inet6 fe80::265e:beff:fe2c:c0dc  prefixlen 64  scopeid 0x20<link>
    ether 24:5e:be:2c:c0:dc  txqueuelen 1000  (Ethernet)
    RX packets 2332894765  bytes 3532248694886 (3.5 TB)
    RX errors 0  dropped 0  overruns 0  frame 0
    TX packets 107128853  bytes 32005739542 (32.0 GB)
    TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

Machine B ip link:

3: enp101s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
link/ether 24:5e:be:2c:c0:dc brd ff:ff:ff:ff:ff:ff

Machine B ethtool enp101s0:

Settings for enp101s0:
Supported ports: [ TP ]
Supported link modes:   100baseT/Full 
                        1000baseT/Full 
                        10000baseT/Full 
                        2500baseT/Full 
                        5000baseT/Full 
Supported pause frame use: Symmetric
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes:  100baseT/Full 
                        1000baseT/Full 
                        10000baseT/Full 
                        2500baseT/Full 
                        5000baseT/Full 
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: 10000Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 0
Transceiver: internal
Auto-negotiation: on
MDI-X: Unknown
Link detected: yes

Debug Steps So Far

I did a netcat on /dev/zero on one machine to /dev/null on the other (B -> A):

3.15GiB 0:00:09 [ 353MiB/s]

I also ran ifperf with 2 window sizes (the default 64k and the below 256k) and saw identical results:

iperf -s -w 256k
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size:  416 KByte (WARNING: requested  250 KByte)
------------------------------------------------------------
[  4] local 2.0.0.10 port 5001 connected with 2.0.0.20 port 55364
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-10.0 sec  2.85 GBytes  2.45 Gbits/sec

Just to test to make sure and remove the network variable in the transfer: 

cat /dev/zero | pv > /dev/null
21.0GiB 0:00:04 [5.18GiB/s]

Best Answer

I remember I faced the same issue once. It was all related to LRO and GRO. You may want to disable large-receive-offload and generic-receive-offload on both ends and see if it makes the changes.

Run the following to temporarily disable those:

large-receive-offload

ethtool -K enp101s0 lro off

ethtool -K enp7s0 lro off

generic-receive-offload

ethtool -K enp101s0 gro off

ethtool -K enp7s0 gro off

By running the following you will actually examine the changes: 

ethtool -k enp7s0

please note that -k -K, uppercase makes changes and lowercase simply outputs the values

I always disable GRO and LRO on 10 NICs.

You can read more here: https://lwn.net/Articles/358910/

But try to disable it and see if it changes the speed, if it will, then I will give you the steps to make a permanent change in your /etc/network/interfaces.

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