Now that an eGPU can accelerate a Mac’s internal display, the question that naturally arises is how significantly the performance of the eGPU would be affected. I wanted to not only provide insight on this question but also show the performance of a graphics card inside a Mac desktop, the Mac Pro tower. This is the last modern Mac with full-length, internal PCIe slots.
The chosen graphics card for this comparison is an EVGA GTX 980 Ti Hybrid. It makes for a very nice pairing with the AKiTiO Node for an eGPU setup. The GTX 980 Ti Hybrid mounts inside the Mac Pro tower well with the only challenge coming from routing the liquid cooling lines.
The Mac Pro tower I tested is a 2010 model 5,1 with two 4x and two 16x PCIe 2.0 slots. In my opinion this is still the best Mac desktop available even though it’s six years old. The modular design makes upgrading components a joy. I’ve previously upgraded one of these Mac Pro towers to have 2x hex-core Xeon CPUs, GTX 980 Ti GPU, 128GB RAM, 512GB PCIe flash storage, and 4x 8TB Helium hard drives; all were mounted internally.
On the eGPU side, I’m testing the GTX 980 Ti Hybrid + AKiTiO Node with a Late 2016 13″ MacBook Pro through Thunderbolt 3 connection. For internal display acceleration, I’m using a headless ghost HDMI adapter. External display eGPU tests are through an HP 22″ monitor. The Unigine benchmarks ran at 1920 x 1080 fullscreen, max settings.
|GTX 980 Ti Hybrid||Max PCIe Speed||Unigine Valley||Unigine Heaven|
|Mac Pro Internal PCIe Slot||32Gbps-PCIe||2,026 (48.4 FPS)||1,658 (65.8 FPS)|
|MacBook Pro External Display||32Gbps-TB3||2,389 (57.1 FPS)||1,756 (69.7 FPS)|
|MacBook Pro Internal Display||32Gbps-TB3||1,814 (43.4 FPS)||1,084 (43.0 FPS)|
I’m sure you’re asking why the numbers from the Mac Pro internal 16x PCIe 2.0 slot are lower than External Display eGPU. Well, Apple crippled non-Mac GPUs to only be able to run at 2.5 Gbps lane speed. This gimpy connection effectively makes the graphics card run at 16x PCIe 1.1, which is close to 4x PCIe 3.0.
The important takeaway is the performance loss when you use an eGPU without an external display. Using an eGPU setup with an external display in macOS yields at least 30% higher performance than feeding that signal back into the internal display of your Mac.
It’s a considerable hit on performance. We all expected it though. Whether this trade-off is worth it depends on your situation and how you plan on using an eGPU. If you have questions about your external graphics card build, visit our forums.
[Nando] Q: How much bandwidth overhead does Optimus/X-connect accelerated internal LCD mode add?
As discussed on the eGPU.io forum, subtracting this value from our total link bandwidth, with a reference of how much bandwidth is needed for acceptable performance will then tell us if our eGPU is truly a portable desktop-replacement solution. One that doesn’t require an external LCD.
|Resolution||Internal LCD overhead* (Gbps)||Theoretic remaining bandwidth for eGPU (Gbps)
|3840×2160 – 4k/UHD||14.9||17.1||1.1||unfeasible||unfeasible|
|2880×1800 – 15″ Retina||9.3||22.7||6.7||0.7||unfeasible|
|2560×1600 – 13″ Retina||7.4||24.6||8.6||2.6||unfeasible|
|1920×1080 – FHD||3.7||28.3||12.3||6.3||0.3|
|1680×1050 – WSXGA+||3.2||28.8||12.8||6.8||0.8|
|1366×768 – WXGA||1.9||30.1||14.1||8.1||2.1|
Lime: ~ 71% desktop GPU performance (x4 1.1 8Gbps or more bandwidth)
From this table comparison we see:
- x4 3.0 32Gbps-TB3 NVidia/AMD accelerated internal LCD traffic traffic overhead still leaves us with at least our desired x4 2.0 16Gbps or more bandwidth for pure GPU traffic.
- 16Gbps-TB2 with an external LCD to maximise it’s narrower PCIe link also gives ~85% desktop GPU performance levels. Internal LCD mode up to FHD can still deliver more than 71% desktop GPU performance.
- Slower 10Gbps-TB1, 4Gbps-EC2, 4Gbps-mPCIe2 links will see compromised performance, particularly at higher resolutions. Those require an external LCD attached, application of Frame Limiting tweaks and disabling of eGPU audio devices to maximise their narrow bandwidth for GPU traffic.