For desktop VMs using VMXnet3 NICs, you can significantly improve the peak video playback performance of your View desktop by simply setting the following registry setting to the value recommended by Microsoft:
HKLM\System\CurrentControlSet\Services\Afd\Parameters\FastSendDatagramThreshold to 1500
[As discussed in a Microsoft KB article here]
[N.B. A reboot of the desktop VM is required after changing this registry setting]
When running full-screen videos at 1080p resolution on a 2vCPU desktop, we see this deliver frame-rate improvements of up to 1.4X.
So, what does this do and why does it deliver these benefits?
The VMXNET3 adapter is a paravirtualized NIC designed for performance that, as of vSphere 5, supports interrupt coalescing. Virtual interrupt coalescing is similar to a physical NICs interrupt moderation and is useful in improving CPU efficiency for high throughput workloads. Unfortunately, out-of-the-box, Windows does not benefit from interrupt coalescing in many scenarios (those sending packets larger than 1024-bytes), because after sending a packet, Windows waits for a completion interrupt to be delivered before sending the next packet. By setting ParametersFastSendDatagramThreshold to the Microsoft recommended value of 1500 bytes you instruct Windows not to wait for the completion interrupt even when sending larger packets. Accordingly, you are allowing View and PCoIP (as well as other applications that send larger packets) to benefit from interrupt coalescing – reducing CPU load and improving network throughput for PCoIP — which translates into significantly improved video playback performance.
How do things work when using E1000 virtual adapters for example? Would adjusting this registry key help at all?
As it is a change to the way Winsock handles UDP packets I would have thought so – but curious to know as the above mentions specifically with regard VMXNET3 adapters.
E1000 does not support interrupt coalescing – FastSendDatagram relies on interrupt coalescing, so have to have VMXNET3
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Do you know if this will be reflected in the Windows 7/8 Optimization Guide and the associated script?
for what its worth Having deployed a win7 vm from MDT using the TS.xml from the Win 7 optimisation guide as of Jan 2013 the necessary registry entry wasn’t included for this.
The KB article covering this is #2040065.
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