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I wonder if Apple also intends to introduce ECC LPDDR5 on the Mac Pro. Other than additional expansion, I’m struggling to see what else they can add to distinguish it from the Mac Studio.
In order for an Apple Silicon Mac Pro to make any sense whatsoever, its SOC will need have to have support for off-package memory and substantially more PCI-E lanes than the M1 Ultra. Therefore it seems all but certain to me that it will debut the M2 chip family. Apple isn't going to give up the substantial performance benefits of on-package unified memory in order to support DIMMs. Therefore I predict that we'll see…
People also seem often to forget that interconnects are a significant amount of modern power budgets - look at the Epic IO die often using more than the cores in many workloads. It may be the the M1 family looks less attractive when you actually have to add these requirements.
Perhaps there's some possibility of having both a tightly-coupled RAM package and also have an extensible memory system - though that has significant management complexity if you try to treat it like a cache, or likely needs app support if it's some NUMA system where they're mapped separately. But possible, at "just" the cost of the extra memory controller.