If I remember correctly all of Apple computers for past few years were max possible in particular form factor.
MP 5.1 had 6 dimm slots, or 9 for triple channel.
I actually cannot remember a Apple computer that did not maxed out RAM.
Eh? The single processor MP 5,1 has 4 DIMM slots. The Dual processor model has 8 DIMM slots. 6 and 9 ... some alternative universe perhaps.
it is not "maxed out RAM". What Apple has not done is fail to match the number of memory controllers ( the native rank) The Xeon processor in the MP 5,1 has 3 memory controllers. Two of the DIMMs share a dual rank relationship with the 3rd controller. What Apple has generally done is avoid dual (or triple ) rank memory solutions. They did expanded a bit for the MP 4,1 and 5,1, but that is highly untypical of Apple system designs.
If ( and that is a somewhat of a leap) Apple selected EPYC then they probably would match the 8 memory controllers. The problem with EPYC is that it isn't going to scale down. EPYC may scale back 8-10 cores but I'd be surprised if it was more than that. I have doubts that Apple is going to want to start the Mac Pro price above that of the iMac Pro. The next iteration of Intel's or AMD's primarily workstation targeted CPU processor is going to be a better fit.
The iMac Pro and Mac Pro would be far more segmented on form factor and internal user modifiable differences. The iMac Pro doesn't even have sanctioned user modifiable memory. There are certain users who just want buy that. if the Mac Pro had user memory updates, a single x16 slot for the card some folks "have to have" (perhaps also a filled x4 slot that users could change) , and started at a price range of $2,500 - 3,300 they would probably do very well and not overlap with the iMac sales all that much. Similar with external factors. Some folks what to put the Mac Pro inside something else. Those folks aren't going to "bleed' into the iMac space.
While Threadripper has 64 PCI-e lanes, it only have 4 memory controllers. So does the Intel "Xeon E5 v5" ( Skylake-W and follow on). If Apple kept track record it would be 4 DIMMs. They might feel some heat from folks who pragmatically want "RAM SSD" to go to 8 DIMMs (and dual rank) with 'cheap' DIMMs. They may 'bend' on that (the mainstream iMac does) , but I wouldn't bet the farm on it.
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So not only it didn't support the full 9 DIMMs (single CPU) or 18 DIMMs (dual CPU) - the 4th DIMM on each CPU could only run in single-channel mode.
More of the usual nonsense on the "Apple can't' front. None of the major Windows PC workstation vendors implement triple ranked solutions for their workstation models either. The only narrow corner case systems that implement triple rank are the top end ( enterprise database, "mainframe killer' ) type servers. Dell 7000 series? Nope. HP Z80 series? Nope. Reverence to the core of the workstation market? None.
As for the MP 4,1 , 5.1 , Apple used a processor tray to implement the design. So there was no room for dual rank (let alone triple rank) in the two processor configuration. It doesn't make much sense to have one tray full dual rank and another primarily single rank. It just expands complexity. Tackling "two birds with one stone" makes lots of sense for a relatively low volume model. Too low a volume and the product would get axed by Apple's "minimum threshold" criteria. Think Apple's minimum volume threshold criteria is dumb? Do any of their competitors operate at Apple margins? Nope.
There is likely going to be logic board space limitations on the next Mac Pro also. The CPU packages are getting bigger which leads to even less space for triple rank on normal sized workstation logic boards. Part of the reason Apple went to using a processor tray was to support both single and dual socket with a single design. If Apple just goes single processor then they don't need the tray. They could take the space that a 2nd CPU package would soak up and throw that at a 2nd rank of DIMMs.