Very slow up/slow download speeds if Hyper-V external switch enabled

cereberus · Aug 8, 2021

A long standing issue for many Hyper-V users is when you set up an external wifi switch in Hyper-V, it kills the download and/or upload speeds.

I have found a simple solution for Intel NICs. I have no idea how general this solution is.

Very slow up/download speeds with HyperV external switch setup - Windows 10 Help Forums

A long standing issue for many Hyper-V users is when you set up an external wifi switch in Hyper-V, it kills the download and/or upload speeds. On my ASUS with an Intel NIC, my uploads and downloads are only 2% of normal speeds making internet virtually u

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rypmag · Jan 1, 2022

OMG - you saved me so much time with this fix tonight. Couldn't figure out why my network performance on download had halved and upload had gone to less than 1Mb from 500Mb. Thank you! I had been building some Linux hosts that I wanted to run as VM's, they all worked great but the alas the host did not :).

stefantemp · Aug 7, 2022

Worked for me on the Upload with an Intel Wi-Fi Adapter. Thank you.

For quick ref...

Slow download - disable packet coalescing on the wifi card settings in device manager.

Slow upload - disable large send offload (for IPV4 and IPV6) from the external vethernet switch that is bridged (not the default vethernet switch).

jimbo45 · Aug 9, 2022

stefantemp said:
For quick ref...

Slow download - disable packet coalescing on the wifi card settings in device manager.

Slow upload - disable large send offload (for IPV4 and IPV6) from the external vethernet switch that is bridged (not the default vethernet switch).

The main problem is in the "Network virtualisation" - another solution is to attach a separate nic to the VM say a USB device- those modern usb wifi ones are capable of very high speeds.

Host-guest communication is always a pain when NAT is involved -- . Pass thru the actual hardware to the VM usually results in better performance in these cases rather than having to share with the host as in that case tere's then "double" traffic -- first from the target / visited web site via the "virtual nic" on the host and then through the "internal Host / VM" network.

You might get better performance too if you access the VM from a remote machine but RDP is frustratingly slow and not very secure. If it's file transfer you want just use SSH or similar.

Cheers
jimbo

cereberus · Aug 9, 2022

jimbo45 said:
The main problem is in the "Network virtualisation" - another solution is to attach a separate nic to the VM say a USB device- those modern usb wifi ones are capable of very high speeds.

Host-guest communication is always a pain when NAT is involved -- . Pass thru the actual hardware to the VM usually results in better performance in these cases rather than having to share with the host as in that case tere's then "double" traffic -- first from the target / visited web site via the "virtual nic" on the host and then through the "internal Host / VM" network.

You might get better performance too if you access the VM from a remote machine but RDP is frustratingly slow and not very secure. If it's file transfer you want just use SSH or similar.

Cheers
jimbo

This is a different issue to overall performance.

If Hyper-V is set up to use default network i.e. behind a NAT, Windows works fine in sharing the NIC with Hyper-V.

The issue arises when you are using an external switch so VM and Host are on same subnet (no NAT), then Windows falls over with some NICs (Intel are prone to this), and download/upload speeds are severely reduced. The issue is tied up with large packet sizes which are supposed to improve performance, but does the complete opposite.

There is clearly a bug either in Windows or the drivers, but disabling coalescing usuall sorts it.

jimbo45 · Aug 9, 2022

cereberus said:
This is a different issue to overall performance.

If Hyper-V is set up to use default network i.e. behind a NAT, Windows works fine in sharing the NIC with Hyper-V.

The issue arises when you are using an external switch so VM and Host are on same subnet (no NAT), then Windows falls over with some NICs (Intel are prone to this), and download/upload speeds are severely reduced. The issue is tied up with large packet sizes which are supposed to improve performance, but does the complete opposite.

There is clearly a bug either in Windows or the drivers, but disabling coalescing usuall sorts it.

Hi there

I agree - but :

I'm not a Network Guru but it's not hard to tell when you have an internet connection available at nearly 10Gbps and connection between HOST and VM can be as slow as 200mbps/sec even with fast SSD's / Nvme's -- and with spinners it can be much slower - often less than 100mbps.

In my Biased Linux view a Host with KVM/QEMU to a Windows VM I can easily get almost the full USB-C spec between transferring files etc between HOST and VM on Host->VM or VM-> Host communication and on external Internet only limited by the download speed of the target website.

HYPER-V is potentially capable of giving fantastic throughput beteen VM and HOST but a lot of work still seems to be needed on "Virtual NIC vio drivers".

Cheers
jimbo

Very slow up/slow download speeds if Hyper-V external switch enabled

cereberus

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