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An eGPU is Useless for Photoshop and Lightroom Unless...

The 2018 Mac mini has four Thunderbolt 3 ports with two busses. The 2019 iMac 5K has only a single Thunderbolt 3 bus, thus half the bandwidth—hobbled, more on that below.

David Cwrites:

You know about eGPUs (and similar) things, right?

It seems to me having only a single Thunderbolt 3 bus is a handicap if you plan to run external storage, e.g. SSDs with an eGPU. maybe the render is so fast it doesn’t matter or maybe it’s fine with one GPU because you need only one data channel?

The eGPU route does seem a way around a non-replaceable graphics card, but I don’t know if using one is generic or can only be whatever specific model(s) the OS supports, i.e. whatever Apple deigns to allow.

The 2018 Mac mini has four Thunderbolt 3 ports but I am unable to find out how many buses they run on. I guess two Thunderbolt 3 ports on the same bus would be slower than on two separate buses, but that also assumes a single bus isn’t fast enough two handle two thunderbolt 3 devices at full speed. Ignorance on display here…

DIGLLOYD: the 2018 Mac mini has four Thunderbolt 3 ports on two busses, so it has double the bandwidth of the 2019 iMac 5K.

It’s a serious handicap to have only one bus; see Understanding Thunderbolt 3 Bandwidth. The 'hit' is severe when an external 5K or 4K display is used because a big chunk of available bandwidth is sucked up for the display. That is why the 2019 iMac 5K is a flawed machine and should be avoided for anyone contemplating an external 5K or 4K display.

An eGPU is 100% useless for Photoshop and LR unless it is driving main screen — those programs use only one GPU, the one for the main display. Think iMac 5K, iMac Pro, all laptops — for all of these, unless the built-in display is configured as the secondary display, the eGPU does nothing for Photoshop and Lightroom — they won’t use it. Actually, I’m not sure an eGPU is used by Photoshop and/or Lightroom even if the built-in display is the secondary display. Maybe readers can confirm/refute that.

When testing the 2018 Mac mini, I discovered this issue. The reason I did not post any eGPU benchmarks for the 2018 Mac mini is that I had trouble getting the eGPU to drive the screen reliably. So if the screen is driven by the built-in graphics, the eGPU is 100% useless for Photoshop and Lightroom — it goes unused.

From Adobe: GPU troubleshooting and FAQ:

Lightroom CC/Lightroom 6 currently doesn't take advantage of more than one graphics processor. Using two video adapters does not enhance performance.

From Photoshop graphics processor (GPU) card FAQ:

Lightroom CC/Lightroom 6 currently doesn't take advantage of more than one graphics processor. Using two video adapters does not enhance performance.

The unstated huge downside left unsaid in both of above quotes is that using built-in graphics for the built-in display can mean the eGPU goes unused. As the built-in display cannot be driven by an eGPU, that in turn implies that even if Photoshop/LR will use the GPU for the main screen, the user must then force the built-in display to be the secondary display (if indeed that works).

See also: Apple: https://support.apple.com/en-us/HT208544 and

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