Transitioning the Mac Pro to Apple Silicon

a selfie of me wearing the Mac Pro 'believe' shirt designed by John Siracusa, with my Mac Pro in the background
Everyone’s busy thinking about Apple’s upcoming VR headset, but I’m more interested in a product Apple has publicly stated is coming: the Apple Silicon Mac Pro. I would be surprised if it’s actually getting announced at this WWDC, but I hold out hope that Apple’s making something great.

The current Mac Pro was released in late 2019 and is vastly different from its contemporary Macs, in that the entire system is a modular tower-based desktop. The video card is removable. The memory is removable. You can add third party hardware cards to the computer at will.

The Mac Pro has this modularity for two reasons. First, it’s vastly more expandable than the other Macs. You can configure your Mac pro with up to an astounding 28 processor cores, and up to 1.5 terabytes of RAM. You can install incredibly powerful graphics cards in the machine. The second reason for this modularity is to accommodate pro workflows that involve installing custom hardware in the Mac.

These are use cases reserved for the most demanding of pro Mac users. Apple doesn’t break down Mac sales figures, but I would be surprised if the Mac Pro ever topped 1% of the Mac’s total unit sales.

John Siracusa makes the case more eloquently than I ever could in The Case for a True Mac Pro Successor. The Mac Pro isn’t a practical computer for Apple to make in terms of the money it earns them, but it’s important to the Mac overall that as a software platform it will scale from a being a simple personal computer to being used in advanced scientific research. It’s aspirational in many ways, but from a practical standpoint it’s enticing to know that you can use the same computer to do all of those things.

In the years between Apple’s roundtable discussion about pro Macs and June 2019 when they finally announced the machine, there was intense discussion among Apple bloggers and podcasters wondering whether Apple was going to bring a truly no-compromises machine to the market, or if the next pro Mac was going to be akin to the 2013 Mac Pro (lovingly and not-so-lovingly referred to as the “trash can”). These tone of these discussions tended toward pessimism after seeing years of Apple making missteps with the Mac lineup, compromising utility of pro Macs in favor of aesthetics that pros weren’t asking for.

Pundits were right to be extremely skeptical of Apple’s commitment to pro users at the time. Not only was the trash can Mac Pro poorly received for not suiting people’s needs, it was languishing because its design was suboptimal for more powerful GPUs. The MacBook Pro got revamped in 2016 after years of stagnation only to be worse for pro users in key ways (not least of which was the choice to use the butterfly switch keyboard in these laptops). Apple had been showing some promising signs that they were listening to concerns, but it remained to be seen whether they would make a pro level machine that would satisfy the most pro users.

That pessimism turned out to be unfounded because the 2019 Mac Pro was revealed to be a machine that compromised on absolutely nothing (with a price tag that also didn’t compromise). It checked off every box, with plenty of expansion slots, support for high-bandwidth MPX module cards to support hardware that had more data to move, a processor with up to 28 cores, and even a rack-mount option. It threw in a couple USB-A ports in addition to a handful of Thunderbolt ports (including two on the top of the computer) and it was highly user-serviceable. The entire machine was designed for thermal load with a beautiful lattice of round holes in front to maximize airflow and internals that were designed to ferry air through the machine.

The 2019 Mac Pro was a love letter to pro users, and it makes no sense to me for it to have been a one-off computer. After hearing John Ternus confirm after announcing the M1 Mac Studio that they were still making an Apple Silicon version of the Mac Pro, I became even more convinced that Apple’s next Mac Pro is going to be free of compromises in the same way.

Apple Silicon Architecture

All of Apple’s existing Apple Silicon Macs are a tightly integrated affair. The M1 chips are entire systems on a chip (SOCs), with CPU cores, GPU cores, neural engine cores, and memory all in one unit. That makes sense given that the roots of Apple Silicon are in iPads and iPhones.

This is an incredible design for all of the Macs Apple has transitioned so far to Apple Silicon. Laptops get incredible battery life due to the power efficiency. The computers themselves get to be incredibly sleek, like the new iMac. Having one big pool of RAM that is used by both the CPUs and the GPUs saves space and money, and it’s faster too because there’s no time wasted copying data from RAM to the GPU.

For Apple’s more powerful laptops and desktops, Apple scales up their chip design by fusing together multiple SOCs.

With this approach Apple can make machines with as much as 128 gigs of memory, 20 CPU cores, and 64 GPU cores. That’s a lot, but it doesn’t come close to how generously you can configure a Mac Pro.

So what’s Apple to do?

Option 1: Use the same architecture and take the L

The simplest thing Apple could do here (and very much a prevailing theory among those who speculate about Apple) is to release a Mac Pro that contains the M1/2 Ultra chip, perhaps with an optional Extreme chip that is 2 Ultra chips to double the number of cores and maximum memory once more.

The machine could still be offered in a Mac Pro looking case and maybe there would be PCI slots still for pros who need to add more cards for connectivity and such.

The problem, though, is that Apple would be replacing the Intel Mac Pro with a Mac Pro that is substantially less capable in key ways. Yes, the unified memory might be faster, but that’s not going to mean much if your workloads demand a terabyte or more of memory. There’s also a GPU issue here too; although Apple’s in-house GPUs are really power efficient, they aren’t as fast as the best graphics cards you can get from AMD or Nvidia.

This would be the most cost effective strategy for Apple, though, since it would mean they can leverage their existing chip designs without doing any radical new designs that will only be used in one machine. If Apple went this route again we might see them revisiting the 2013 Mac Pro philosophy to upgradeability and modularity, where customers were encouraged to augment functionality by attaching external devices via Thunderbolt.

Option 2: Use a traditional PC architecture

Another option Apple has here would be to make a Mac Pro that is essentially identical in architecture to the Intel Mac Pro, but it swaps out the Intel CPU with an Apple Silicon CPU.

In this case the chip could be pared down, maybe losing or deactivating the integrated GPUs and memory in favor of the RAM slots. Maybe it would be an option to have a mix of unified memory and extra memory in slots so users who don’t need a big GPU wouldn’t need to purchase one and could rely on the GPU built in. There would continue to be MPX modules for installing high end cards and several PCI slots for other hardware.

This makes an Apple Silicon Mac Pro that’s no-compromises on specs and upgradeability, but it would be missing out on the benefit of unified memory which Apple has been touting as a huge advantage of Apple Silicon. It would seem weird to me to build a machine without that.

Option 3: Deconstruct the SoC

Finally, there’s the more ambitious approach: instead of trying to cram everything into these tiny SoCs, Apple could instead build a custom Mac Pro logic board that applies everything Apple has learned about making fast, tightly integrated chips, but implemented on large footprint with modularity.

Instead of having memory soldered on, Apple could instead build a very fast interconnect to a bank of memory slots, saving us from needing to choose between having memory that’s fast and without latency, and being able to have an absolute crap ton of memory. In the same way that Apple’s MPX modules have high bandwidth connectivity to the rest of the system, Apple could make a successor to MPX that allows installing high-performance graphics cards with full-bandwidth access to all of the system memory. Having slots be this high-performance also gives brand new options for expandability. If you want to be running a lot of ML workloads, you could install a card with a massive array of Neural Engine cores, all of which have direct memory access.

We could take this a step further and make every major part of the SOC slotted, including the CPU cores, giving customers total ability to customize whatever part of their system is needed by their workloads. Need more CPU? There’s a card for that. More GPU? There are cards for that! Need more disk I/O? There’s a card for that too!

What will Apple choose?

I speculate that we’ll see Apple choose Option 2 or 3, or maybe some hybrid between the two.

Of course, Option 3 would be the biggest R&D undertaking and it would be the biggest stretch for Apple because it would mean developing a completely custom architecture just for the Mac Pro, and that architecture is not likely to be one that would scale down to their other machines very well. Also I’m not at all well versed in computer hardware design; it’s entirely possible that what I’m suggesting won’t work within the laws of physics because Apple’s current chips rely on physical proximity to be able to achieve the low latencies they enjoy (after all, Grace Hopper taught me that a nanosecond is about 11.8 inches).

I still look back to the 2019 Mac Pro announcement (it was the best WWDC), and although Apple didn’t say it in so many words, that announcement felt like a recommitment to the kinds of pro users who would be buying a Mac Pro. Apple thinks for the long term, and they weren’t going to make the investment they made into designing the 2019 Mac Pro if it was just going to become a one-off product. And it similarly doesn’t make sense to go all out with the 2019 Mac Pro if the intent was to make subsequent Mac Pro computers successors in name only.

It’s taken Apple many years but I think they have finally come around to the idea that they need to let pro Macs be pro Macs, and it’s safe for us to expect a Mac Pro that not only has specs that lap every other Mac (and almost every other computer on the market), but modularity and flexibility that the most demanding and tiny subset of Mac users need.

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