Hyper-V Virtual Switch - Affect Host NIC Speed

[zkrr01]

I am seeing something I don't understand. I have a 40GbE NIC on my Windows machines that I was getting about 3000MB/s transfers from host a to host b. I then defined that NIC to Hyper-V on both hosts. Once I did that, the transfers on my machines went from 3000MB/s to 800MB/s.

Note that there were no activity on the Hyper-V machines. After many hours of trying to find out what was causing the slowdown, I decided to simply remove the 40GbE Virtual Switch definition on both hosts. I then ran my speed tests and I was back to my normal 3000MB/s.

What is going on?

 

[cesmith9999]

Which server OS? It makes a difference. 2012 r2? 2016? 2019?

Chris

 

[zkrr01]

It's on Windows 10 Pro for workstations. Based on your reply, it sounds like I have exceeded the number of NICS allowed for Windows 10 for
workstations. I have three. a 1GbE for management, a 10GbE for backup and a 40GbE for fast SSD transfers. I added the third recently since all my SSD's were faster than the 10GbE link could handle. However only the 1GbE and the 40 GbE are defined to Hyper-V.

 

[Connorise]

Try to disable VMQ on your NIC's. But I do not believe that this will work, standard external vNIC working with 10Gbe so I believe this is the reason.

 

[LaMerk]

I believe you are using iPerf for the tests... During the test, check the CPU core usage, it could be a bottleneck.

Here is a link to the similar discussion: Slow download speed after creating External Virtual Switch - Windows 10 Forums

 

[darklight]

Exactly, this "I believe you are using iPerf for the tests... During the test, check the CPU core usage, it could be a bottleneck."

From what I am aware you might be seeing a bottleneck from CPU side.

 

[zkrr01]

The problem was I was running on Windows 10 for workstations. It appears that Microsoft only allows RDMA performance with servers and not workstations. Its a pure money issue, not a technical issue! Once I started testing with Linux based machines my problems went away.

 

[Connorise]

>The problem was I was running on Windows 10 for workstations. It appears that Microsoft only allows RDMA performance with servers and not workstations. Its a pure money issue, not a technical issue!

Okay, cool! Thanks for sharing this!