Sky/Kaby Lake "U" CPU OPI GT2 half-bandwidth issue
The following was uncovered in finding why the Alienware Graphics Amplifier CUDA-Z Speed was operating at x2 3.0 rather than x4 3.0 link speed.
Affected ULV “U” CPU specced systems, eg: i7-6500U, would require a BIOS fix to switch the OPI (DMI) from the low-power GT2 mode (~x2 3.0) to the high-performance GT4 mode (~x4 3.0). The vendor would be using the Intel FITC tool to modify that bootstrap in the ME firmware, a component of the BIOS.
A GT2 OPI may register the link as being x4 3.0 using hwinfo64, but when performance tested will show bandwidth equivalent to x4 2.0 as shown below:
|GT2 OPI (x4 2.0)||GT4 OPI (x4 3.0)|
I know I’m late to the party, but I’m wondering if it is indeed the CPU<->PCH link that’s the culprit. According to Anandtech, the OPI interface on ULV processors can be configured to either “GT2” or “GT4” rates, the former equivalent to PCIe 3.0 x2 and the latter equivalent to PCIe 3.0 x4 or DMI 3.0. If the Alienware is set to use the slower rate, that would limit eGPU performance.
OPI GT2 (x2 3.0) rather than GT4 (x4 3.0) is looking to be the performance handicap
Given Dell XPS 9350 runs it’s OPI at GT2 rates which limit SSD performance to < 1800MB/s it's likely the AW13 R2 does too. A AW13 R2 NVME / M.2 SSD seq read/write benchmark would tell us but as yet I haven't found one.
Based on the following, a ME firmware mod is requires to switch on OPI GT4 mode:
Further confirmed by the following link “Changed FITC setting, OPI Link Speed to GT4 – this is the performance fix for PCIe 3.0 x4 NVMe SSDs”
I’d suggest engage Dell support requesting (likely) a BIOS fix here.
This topic needs more attention. I got an XPS 13 9360 this week with a Kabylake “U” CPU. While I have not paired it up with an eGPU enclosure, I found it has this same low-power GT2 DMI mode. I swapped the 128GB PCIe drive for a Samsung 500GB 960 EVO. This connection is confirmed to run at x4 PCIe 3.0 but Read and Write speed are restricted at 1,800MB/s. I installed Samsung Magician as well as Samsung NVMe drivers to make sure I eliminated other possible causes.
I have an HP Spectre x360, model 13-w0xx, with the dual TB3 ports. I haven’t been able to test it directly with an eGPU box but I believe I’m also affected by the GT2 OPI issue. Based on the feedback I’ve seen from the HP reps on their support forums, I’m not going to hold my breath waiting for a firmware fix to flip things to GT4. But I still want the convenience of a single-plug docking+charging station, and I’d like to have as much eGPU power as I can pull off without bottlenecking elsewhere. Given that the machine has a 2-core i7-7500U and is hamstrung by the GT2 problem, what is the fastest GPU I should bother with? I’ve seen benchmarks that a GTX 1080 or 1080ti is overkill because the i7-7500U will bottleneck and that a GTX 1070 is the sweet spot, but I think those tests were run on a GT4 system. How much further do I lower the GPU bar in view of the OPI issue? I don’t want to run out and get a shiny new 1070 if that’s also overkill.
The interesting thing about this particular 2 in 1 is that it shares the same motherboard and bios as the 7275 model which Dell supposedly claims supports pcie x4. I’m going to check the motherboard to see if there are any DIP switches that are available, other than that there must be something specific in the bios checking for the 7275 model that allows it to run pcie x4 in GT4 OPI as there isn’t anything in the BIOS that allows the change.