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20 gbps or 40 gbps cable and link width (x4 lane)
 

20 gbps or 40 gbps cable and link width (x4 lane)  

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marcotini
(@marcotini)
Active Member
Joined: 2 years ago
 

Currently I'm using a MacBook Pro and a Razer v2 and in the system preferences (on PCIE) I notice that if I use a 40 gbps (50 cm) or the 20 gbps (1 m) it's appear to be always the same result:

Link Width: x4 
Link Speed: 8.0 GT/s

I don't understand why the 20 gbps it's not cropped. Also in the fps when I play games, there is no difference between the two cables.
There is a way to test in a better way?

Pending: Add my system information and expected eGPU configuration to my signature to give context to my posts


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Arbystrider
(@arbystrider)
Eminent Member
Joined: 2 years ago
 

Use CUDA-Z to figure out the device-to-host and host-to-device bandwidth.
Use GPU-Z to figure out link width.

ThinkPad X230t, i7-3520M @ 2.9 GHz, Windows 7
EVGA GTX 1070 Hybrid, 2113 MHz @ 1043 mV
x1.2Opt, PE4C V3.0, 4Gbps-EC2


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joevt
(@joevt)
Reputable Member
Joined: 2 years ago
 

The Thunderbolt tunnel does not affect PCIe link speed and width at the ends of the tunnel. The device receives or sends PCIe data using a x4 link at 8.0 GT/s, but that doesn't mean it's going to saturate that link.

Think of a USB 2.0 controller in a 5 GT/s x1 slot. The slot sends/receives PCIe data at 5 GT/s (500 MB/s) but it's not going to let you send or receive more than 50 MB/s (or whatever USB 2.0 lets you transfer). In this example, if you are transferring USB data at 50 MB/s then nothing is being transferred on the PCIe link for 90% of the time.

In a non-Thunderbolt environment, PCIe devices have to contend with other PCIe devices. In a Thunderbolt environment, you also have to keep in mind the Thunderbolt bandwidth limits from the cable (40 or 20 Gbps), the version of Thunderbolt (40 or 20 or 10 Gbps), and the type of traffic (PCIe in Thunderbolt 3 is limited to 22 Gbps).

The game performance is not affected much by link speed when your display is connected directly to the graphics card in the eGPU.
what-is-the-bandwidth-with-4x-5-0gt-s

You can google for "PCI-Express Scaling" on techpowerup.com to find results ranging from 8 Gbps up to 126 Gbps:
https://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_1080_PCI_Express_Scaling/24.html
They did the RTX 2080 TI recently:
https://www.techpowerup.com/reviews/NVIDIA/GeForce_RTX_2080_Ti_PCI-Express_Scaling/3.html

Pending: Add my system information and expected eGPU configuration to my signature to give context to my posts


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