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My MacPro (Late 2013, OSX Yosemite) has two AMD FirePro D500 GPUs and three Cinema HD Displays. When I look at the System report all of the Displays show up under one GPU and the other GPU has nothing.

Is the other GPU (slot-1) being utilized at all? How do I tell?

Can I move a display to the unused GPU?
Will this help performance for rendering or calculations?

AMD FirePro D500:

  Chipset Model:    AMD FirePro D500
  Type: GPU
  Bus:  PCIe
  Slot: Slot-1
  PCIe Lane Width:  x16
  VRAM (Total): 3072 MB
  Vendor:   ATI (0x1002)
  Device ID:    0x679e
  Revision ID:  0x0000
  ROM Revision: 113-C3861J-687
  gMux Version: 4.0.11 [3.2.8]
  EFI Driver Version:   01.0B.687

AMD FirePro D500:

  Chipset Model:    AMD FirePro D500
  Type: GPU
  Bus:  PCIe
  Slot: Slot-2
  PCIe Lane Width:  x16
  VRAM (Total): 3072 MB
  Vendor:   ATI (0x1002)
  Device ID:    0x679e
  Revision ID:  0x0000
  ROM Revision: 113-C3861J-687
  gMux Version: 4.0.11 [3.2.8]
  EFI Driver Version:   01.0B.687
  Displays:
Cinema HD:
  Display Type: LCD
  Resolution:   2560 x 1600
  Pixel Depth:  32-Bit Color (ARGB8888)
  Display Serial Number:    [redacted]
  Main Display: Yes
  Mirror:   Off
  Online:   Yes
  Rotation: Supported
  Adapter Type: Apple Mini DisplayPort To Dual-Link DVI Adapter
  Adapter Firmware Version: 1.03
Cinema HD:
  Display Type: LCD
  Resolution:   2560 x 1600
  Pixel Depth:  32-Bit Color (ARGB8888)
  Display Serial Number:    [redacted]
  Mirror:   Off
  Online:   Yes
  Rotation: Supported
  Adapter Type: Apple Mini DisplayPort To Dual-Link DVI Adapter
  Adapter Firmware Version: 1.03
Cinema HD:
  Display Type: LCD
  Resolution:   2560 x 1600
  Pixel Depth:  32-Bit Color (ARGB8888)
  Display Serial Number:    [redacted]
  Mirror:   Off
  Online:   Yes
  Rotation: Supported
  Adapter Type: Apple Mini DisplayPort To Dual-Link DVI Adapter
  Adapter Firmware Version: 1.01
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    How are the Displays physically attached? To one or both GPU's and or daisy chained to one another, etc.? – user3439894 May 31 '18 at 20:26
  • They each plug into their own thunderbolt port. Not daisy chained. It doesn’t seem to matter which ports I use, the results are the same – MERM May 31 '18 at 20:29
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    I do not have a Mac Pro, so I can't really be of much help however, have a look at Use multiple displays with your Mac Pro (Late 2013) to see if any info there helps. – user3439894 May 31 '18 at 20:39
  • As per that document I put each display on its own thunderbolt bus. Didn't change the GPU assignments though. – MERM May 31 '18 at 20:45
  • FWIW (probably not much) I recall that windows used to keep one GPU basically powered down until the load couldn't be handled by a single GPU. Only then did it power up the other GPU and redistribute the workload. I expect (but don't know) MacOS is doing the same. There just isn't any benefit to having 2 GPUs wafting along at 25% capacity when you can have a single GPU trucking along at 50% – Terry May 31 '18 at 21:06
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Have you tried doing some heavy graphics load on two different monitors? I've read in the past that the 2013 Mac Pro attempts to use only one GPU for the most part and leave the second one available for OpenCL / GPU compute functions. However, if you increase the load on the GPU enough it will switch one of the displays over to the unused GPU (e.g. run a demo like Unigine Heaven on one and Valley on the other screen).

Note that macOS does NOT support two GPUs driving one display (a.k.a. CrossFire or SLI), so you would only use the second GPU for graphics if you have two displays connected and the load is sufficient to switch on the second GPU.

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