Mellanox ConnectX-3 slow transfer speed

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New Member
May 30, 2020

I have tested a couple of MCX-311A card (latest driver and firmware) under Mint(Ubuntu) Linux with iperf. They work fine (9.x Gbit/s).
I have tested these cards in a fresh-install Win10 21H2 (all default) on a Optiplex 9020. The iperf is about 8-9Gbit/s.

When I test these cards in a DELL Precision 3620 (i7-7700, C236 chipset, Win10-1909, Bitlocker On).
iperf is about 5Gbits/s transmit, 1.3Gbit/s receive.
However, SMB file transfer (on a unencrypted SSD) speed is about 800MByte/s transmit and 160MByte/s receive. The major problem is the receibve speed.

In the driver - advanced page in device manager, I have changed all TX and RX buffer size to 4096. Jumbo (MTU) left default 1514 (I think it is equivalent to layer 3's 1500, which is commonly seen, and plus 14 bytes of L2 header). I don't think 1500 MTU is the cause, because these cards can achieve 9.x Gbit/s even with MTU 1500 under Linux.

I have also tried an Intel X520 card in the Precision 3620, it's iperf performance is also unacceptable (3.xGbits transmit, 1.3Gbit/s receive).

I don't know what to do now.


Active Member
Jan 29, 2022

Look at the PCIe slot configuration where you installed the Mellanox card.

Verify if other cards doesn't share the PCIe lanes with the NIC.

Since you report that asymetry between the rates of up and down I'm guess of a software problem most likely antivirus throttling the connections.
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