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MacOS - Max number of GPUs
 

MacOS - Max number of GPUs  

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highpass
(@highpass)
Eminent Member
Joined: 3 years ago
 

This isn't specifically eGPU related but I thought someone here might be able to answer.

Apparently recent versions of MacOS can only address 3 Nvidia GPUs + 1 AMD GPU. Is this true and can anyone elaborate on why?

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


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Flint Ironstag
(@flint-ironstag)
Estimable Member
Joined: 3 years ago
 

This is probably due to the fact that no currently shipping Mac has traditional PCIe slots.  The limitation seems to be in the method (enumeration I understand it's called) of allocating resources to GPUs at boot time.  On TB equipped Macs, that limit seems to be 2 eGPUs.  1 per bus, 2 on the same bus in the same enclosure, doesn't matter.

Occasionally a thread pops up somewhere about using a cMP to drive massive video walls.  You might also check the Octane forums - a few years ago there were some cMP users stacking Nvidia cards in.  There's not a ton of info out there on the theoretical max.  At one point in the Mac Pro's lifespan, you could configure it with 4 GPUs from Apple.  Then Magma, CUBIX, etc. had the aftermarket PCIe expansion chassis.

If 4 GPUs is indeed the limit again, this will need to be addressed on mMP at least.

MP 6,1 | 4c | d700
MP 6,1 | 6c | d500


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highpass
(@highpass)
Eminent Member
Joined: 3 years ago
 

Hi Flint

As far as I can tell the history is thus:

Prior to 10.8/10.9, a cMP could run 5 total Nvidia GPUs (1 internal for boot, 4 in a Cubix).
Since then the limit changed to 3 total, however addressed

...indeed you are right about Octane users. There is a thread somewhere (on InsanelyMac, which is down right now) wherein a hackintosh user did some dabbling around and got 4x Nvidia working. I wonder if the same process can be applied to a cMP. I realise this is typical cMP talk of smacking square pegs into round holes, but so what.

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


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Flint Ironstag
(@flint-ironstag)
Estimable Member
Joined: 3 years ago
 

I think it's an interesting topic.  It's also telling that I can't email Apple, or any PCIe expansion chassis manufacturer, and get an answer to this question.  Pretty sure HP would tell me:

"we support x number of y cards in z box".  

Or support at Magma or CUBIX could tell me the same thing.  Found one of Tutor's threads.  It's not Mac specific, but there's a ton of good information, and I think there's discussion about cMPs.  Also try searching for cMP PCIe splitters / risers.  Some folks were looking into these for the mining craze.  Not sure how far they got.

https://render.otoy.com/forum/viewtopic.php?f=40&t=43597

[edit] old bare feats article: 5GPUs in cMP + CUBIX.

http://barefeats.com/gpu680v7.html

Just checked eBay - they sure want a lot of money for them!  But if you bought the whole shebang new in 2010 (!!!), it's presumably paid for itself many times over, and is still faster than TB3, so not too bad.  A while back @mcarver316 found them on sale at Amazon for $400, but I haven't seen them since.

[edit] also forgot about Cyclone - there's a seller on eBay that appears to be stripping their boxes down to the backplanes and selling the components.  If you have a cMP, this might be a cheapish way to load up.

https://www.ebay.com/itm/Cyclone-Microsystems-5-Slot-PCI-Express-Gen-2-x16-Expansion-System-Kit-Mac-PC/142698306871?hash=item21397b5d37:g:IGQAAOSweExaF1TA

MP 6,1 | 4c | d700
MP 6,1 | 6c | d500


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Aled Brown
(@gelert42)
Eminent Member
Joined: 2 years ago
 

It's like groundhog day this thread! I keep reading the same post over and over and over  🙄

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


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highpass
(@highpass)
Eminent Member
Joined: 3 years ago
 

......Insanelymac is finally back up. Here is what one user altered to enable more than 3 GPUs. 

The user's post:  http://www.insanelymac.com/forum/topic/329008-3-gpus-work-4-gpus-fail-with-34couldn39t-allocate-runtime-area34/#entry2533997

The solution:  https://nickwoodhams.com/x99-hackintosh-osxaptiofixdrv-allocaterelocblock-error-update/

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


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Chippy McChipset
(@chippy-mcchipset)
Reputable Member
Joined: 2 years ago
 

Would be curious if you could hook up 4 eGPU to the new iMac Pro. That is 4 ports on two controllers (each with a full set of PCIe lanes per TB3 spec) AFAIK, so in theory should be possible (once Apple offers official support).

Thunderbolt 3 Macs, Sonnet and OWC eGPUs, 4K Displays, etc


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Tutor
(@tutor)
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Joined: 1 year ago
 

Whereas for a real MacPro 2009 to pre-cylinder/trashcan series (running pre-OS 10.8) max no. of Nvidia GPUs workable was 4 , now its 3.  Obviously, Apple OS has reduced GPU Input/Output (io) space.  I can, however, run additionally as a fourth GPU card an AMD 480.

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


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Chippy McChipset
(@chippy-mcchipset)
Reputable Member
Joined: 2 years ago
 

Upon further review I do think the practical answer to this question is "it depends on how many Thunderbolt 3 controllers your computer has." I don't think there's anything about 10.13.4 or later that prevents more than (for example) two eGPUs being used, but at the same time I don't know any Mac with more than two TB3 controllers. As soon as you connect two eGPU to ports on the same controller, each one is going to start cutting into the other one's performance most likely.

As a curiosity though I would like to see render benchmarks comparing two eGPU to four on Macs that don't have any inherent bandwidth limitations (a couple models have 20Gbps limitations on one of their TB3 ports).

Thunderbolt 3 Macs, Sonnet and OWC eGPUs, 4K Displays, etc


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Flint Ironstag
(@flint-ironstag)
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Joined: 3 years ago
 

Rominator / MVC has posted a MBP with 6 GPUs running (integrated, discrete, + 4 Titan Xp eGPU).

http://forum.netkas.org/index.php/topic,13703.0.html

MP 6,1 | 4c | d700
MP 6,1 | 6c | d500


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Chippy McChipset
(@chippy-mcchipset)
Reputable Member
Joined: 2 years ago
 

Wonder what the difference in performance for a given app function would be 2 eGPU vs. 3 vs. 4. Got to be a law of diminishing returns there based on total available bandwidth being shared.

Thunderbolt 3 Macs, Sonnet and OWC eGPUs, 4K Displays, etc


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Chippy McChipset
(@chippy-mcchipset)
Reputable Member
Joined: 2 years ago
 

Well, for Metal applications we now have an answer: 10.13.4-10.13.x the answer is currently "less than 4".

For 10.14 the answer will be "up to four". 

For non-metal stuff I think you can already do up to four, just a question of whether the cost justifies what will amount to less than a 1:1 increase in performance. Like if max performance is two eGPU (one on each TB3 controller), adding two more will not double the output most likely. Still hard to find conclusive data on this.

Thunderbolt 3 Macs, Sonnet and OWC eGPUs, 4K Displays, etc


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Flint Ironstag
(@flint-ironstag)
Estimable Member
Joined: 3 years ago
 

@Tutor - welcome Sir!  Didn't notice you'd popped in here a while back.

@Chippy-McChipset - very difficult to get definitive answers.  As always, performance depends on the app.  Password cracking is 1:1 as long as you have the CPU, RAM, and right rules to feed the GPUs.

MP 6,1 | 4c | d700
MP 6,1 | 6c | d500


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Chippy McChipset
(@chippy-mcchipset)
Reputable Member
Joined: 2 years ago
 

Alas I suspect password cracking is a very niche need in this community. 🙂 I think most people are worried about games and creative apps in that order.

Thunderbolt 3 Macs, Sonnet and OWC eGPUs, 4K Displays, etc


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(@guido)
New Member
Joined: 9 months ago
 

Hi there,
wanted to share my experience on this issue.
I'm using a classic Mac Pro 5.1, dual X5690, 96GB RAM, 512GB AHCI SSD, with 3 monitors running El Capitan.
External GPUs are attached to a Cyclone PCIe 427 expansion backplane.

GTX 780 + 2 x GTX 980ti + GT120 = Boot
GTX 780 + 2 x GTX 980ti + 2 x GT120 = No Boot
3 x GTX 980ti = Boot
3 x GTX 980ti + GT120 = No Boot
3 x GTX 980ti + ATI HD5870 = No Boot
2 x GTX 780 + 2 x GTX 980ti = Boot

From these experiments I deduce:
1. It does not seem to be limited to 3 Nvidia GPUs, 4 Nvidia GPUs are running well under El Capitan.
2. When running 3 x 980ti under El Capitan, no fourth GPU can be added, no matter if it's Nvidia oder ATI.

Kind regards
Guido

Mac Pro 5.1 Mac OS 10.11


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