I did say this some time ago and even when the iMac Pro was first announced....this product is/was a stop-gap for the redesigned Mac Pro...to fill the "Pro"/"Enthusiast" niche...
My suspicion is that it was going to be
the next Mac Pro but was rejected by key customers/developers. Its quite well developed for a stop-gap - even though it is basically iMac shaped, non-trivial work went into the cooling re-design. It also carries many of the fingerprints of the trashcan: particularly, heavily dependent on a particular processor and GPU models with no real update roadmap.
At the time of the famous "future of the Mac Pro" press conference in April 2017, where the camera was carefully avoiding the blood on the carpet, the iMac Pro would already have been in quite an advanced stage of development - just a couple of months away from announcement at WWDC, while the then-mythical "modular" Mac Pro was pretty clearly nowhere. Methinks the new iMac Pro had grown up in an echo-chamber until they showed it to a few key partners in early 2017 and got laughed at - hence the U-turn. Still, just guessing...
When it was available the 18 core iMac Pro had equivalent cpu performance to the 16 core Mac Pro, but with the included monitor it cost thousands less.
That's the big problem with the Mac Pro - you pay a hefty premium for a chassis with the
potential to be upgraded to a 28 core, quad-GPU, 1.5TB monster. If you're happy with a
mere 16-core, a
piddling 256GB of RAM, a single high-end GPU that leaves change from 2 grand, and could slum it with a mere 2-3 full-width PCIe slots, then you're wasting money on a grossly over-specified (not to mention grossly over-engineered and not particularly well designed) chassis.
OTOH the "value for money" of the whole iMac range rather hinges on that display being "worth" about $1200. If you
didn't want a built-in, 27", 5k, P3 display, that display is worth $0 - or less, since you're left with a computer entirely built around a display you don't want.
If I am not wrong, it’s because of the intel processor in the newer Macs that have support for the 6k display. The iMac Pro, with its older processor, doesn’t get said benefit.
I believe it's not the CPU as such but the Thunderbolt controller chips - support for DisplayPort 1.4 over Thunderbolt (which the 6k display needs) was only added with the Titan Ridge chips in 2018.
1. How will they support up to 1.5 TB of memory?
2. How will they support the performance equivalent of 2 graphics cards, 64 GB each?
Those are two very interesting questions which won't be answered until we see the M2 (or whatever it will be called) chip. I find it hard to imagine that we'll see more than ~64GB of RAM mounted "on-package" M1 style - but there's no reason why an "M2" couldn't just use regular (non-low power) "external" RAM instead of/as well as on-package LPDDR.
We'll presumably get a new integrated - either on-chip or on-die - GPU to give any
single dGPU a run for its money, but I suspect the route for 2- or 4- GPU monsters is either (a) multiple Apple Silicon processors or (b) just support AMD etc. GPUs via PCIe...
Question for the experts: can two or more M1 chips be combined?
If so, it's not hard to imagine the iMac to get 2M1, and the Mac Pro to get like 10M1 or something like that.
Multiple CPUs are a thing (see the whole"scaleable" Xeon range - plenty of workstations out there with 2 x 28 core Xeons - heck, the original Mac Pro 1,1 had dual Xeon CPUs) and it would be up to Apple to build the necessary capability into future Apple Silicon processors and MacOS frameworks. I'm not saying that it isn't rocket surgery - but Apple are big enough and ugly enough to employ rocket surgeons.
M1 is primarily a chip for the MacBook Air and its successors, and isn't going to cut it in the higher-end MBPs, let alone an (i)Mac Pro replacement. So it is not really about whether the M1 has features to support muti-processors.
You can build a cluster computer out of almost anything (and Apple used to have a software product - XGrid - for exactly that). Its really a case of how much effort goes into providing high-speed CPU interconnects and smart software to make it look more like one big computer. The applications being run on current high-end x86 workstations are already pretty dependent on parallelism for performance, anyway.
I'd guess that it is quite likely that a future high-end Mac Pro will feature multiple Apple Silicon SoCs based on the chips in lower-end Macs - because the alternative would be for Apple to produce a dedicated high-end Xeon-killer chip
just for the relatively tiny top sliver of the Mac market, which would be hugely expensive.