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Comment by alberth | Hacker News Reader

I'm cross posting a question I had from the Mac Studio thread (currently unanswered).

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Mac Pro scale up?

How is this going to scale up to a Mac Pro, especially related to RAM?

The Ultra caps at 128 GB of RAM (which isn't much for video editing, especially given that the GPU uses the system RAM). Today's Mac Pro goes up to 1.5TB (and has dedicated video RAM above this).

If the Mac Pro is say, 4 Ultra's stacked together - that means the new Mac Pro will be capped at 512GB of RAM. Would Apple stack 12 Ultra's together to get to 1.5TB of RAM? Seems unlikely.

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I think some of this can be guessed from the SoC codenames

https://en.wikipedia.org/wiki/List_of_Apple_codenames

M1 Max is Jade C-Die => 64GB

M1 Ultra is Jade 2C-Die => 128GB

There is a still unreleased SoC called Jade 4C-Die =>256GB

So I think that's the most we'll see this generation, unless they somehow add (much slower) slotted RAM

If they were to double the max RAM on M2 Pro/Max (Rhodes Chop / Rhodes 1C), which doesn't seem unreasonable, that would mean 512GB RAM on the 4C-Die version, which would be enough for _most_ Mac Pro users.

Perhaps Apple is thinking that anyone who needs more than half a Terabyte of RAM should just offload the work to some other computer somewhere else for the time being.

I do think it's a shame that in some ways the absolute high-end will be worse than before, but I also wonder how many 1.5TB Mac Pros they actually sold.

How is slotted RAM slower? 6400Mhz DIMM exists. This would match the specs of the RAM on the M1 Max. Even octa-channel has been done before so the memory bus would have the exact same width, latency and clock frequency.

The memory bandwidth of the M1 Max is 400 GB/s with 64GB of RAM, where as the memory bandwidth of Corsair's 6400MHz DDR5 32GB RAM module is 51GB/s per stick, or 102GB/s for the M1 Max equivalent.

51GB/s * 8 (octa-channel, not dual channel as you are calculating) is 408 GB/s. Basically the same as the M1 Max. It's not fair to use an off the shelf product since even if the RAM is slotted Apple wouldn't use an off the shelf product.

Whether they use slotted RAM or not has nothing to do with performance. It's a design choice. For the mobile processors it makes total sense to save space. But for the Mac pro they might as well use slotted RAM. Unless they go for HBM which does offer superior performance.

Is 8 channel RAM doable, are there downsides? If no to both, why don't high end x86 processors have it?

High-end x86 do have it. Threadripper 3995WX for example.

Note that those are overclocked out of spec configurations today.

https://ark.intel.com/content/www/us/en/ark/products/134599/...

4800 MT/s is the actual maximum spec, anything beyond that is OC.

Agreed, I think they will use the 4C config to debut M2 and make a slash. They said in the keynote that M1 Ultra completes the M1 family. Timing works out well for November launch with the 2 year apple silicon transition timeline they gave themselves. Not sure what they are going to call it and if it will be A15 or A16 based.

A16 would give great performance, and I think it’s safe for them to have a two year iteration time on laptop/desktops vs one year for phone/tablet.

Hard to believe it’s already been almost 1.5 years!

I believe using Optane as fast swap could be great way to increase RAM capacity. Not completely fair, but enough for most usages.

It is strange Apple didn't cooperate with Intel in this area.

This went by so fast I'm not sure I heard it right, but I believe the announcer for the Ultra said it the last in the M1 lineup.

They just can't ship a Mac Pro without expansion in the normal sense, my guess is that the M2 will combine the unified memory architecture with expansion busses.

Which sounds gnarly, and I don't blame them for punting on that for the first generation of M class processors.

This is what I've been thinking as well, a M2 in a Mac Pro with 128/256gb soldered and up to 2TB 8 channel DDR5-6400 expandable, and do a tiered memory cache

A few points to make...

- the shared CPU+GPU RAM doesn't necessarily mean the GPU has to eat up system RAM when in use, because it can share addressing. So whereas the current Mac pro would require two copies of data (CPU+GPU) the new Mac studio can have one. Theoretically.

- they do have very significant video decoder blocks. That means that you may use less RAM than without since you can keep frames compressed in flight

Also, the memory model is quite different - with the ultra-fast SSD and ultra-fast on-die RAM. You can get away with significantly less RAM for the same tasks, not just because of de-duplication but because data comes in so quickly from the SSD that paging isn't nearly the hit it is on say an Intel based Mac.

I'd expect it to work more like a game console, streaming in content from the SSD to working memory on the fly, processing it with the CPU and video decode blocks, and insta-sharing it with the GPU via common address space.

All that is to say, where you needed 1.5TB of RAM on a Xeon, the architectural changes on Apple Silicon likely mean you can get away with far less and still wind up performing better.

The "GHz myth" is dead, long live the "GB myth."

> ultra-fast on-die RAM

The RAM is not on die. It’s just soldered on top of the SoC package.

> All that is to say, where you needed 1.5TB of RAM on a Xeon, the architectural changes on Apple Silicon likely mean you can get away with far less and still wind up performing better.

No, it does not. You might save a bit, but most of what you save is the transfers, because moving data from the CPU to the GPU is just sending a pointer over through the graphics API, instead of needing to actually copy the data over to the GPU’s memory. In the latter case, unless you still need it afterwards you can then drop the buffer from the CPU.

You do have some gains as you move buffer ownership back and forth instead of needing a copy in each physical memory, but if you needed 1.5TB physical before… you won’t really need much less after. You’ll probably save a fraction, possibly even a large one, but not “2/3rd” large, that’s just not sensible.

I agree, I got a 8GB mac mini (was really just curious to see the M1 in action and the 16GB model was backordered badly) and it performs extremely well memory wise. I often use 12GB+ and I never notice unless I check activity monitor.

That's what I based my assessment on too. I had an 8GB M1 MacBook Pro (swapped for the 16GB M1 Pro MacBook Pro 14 since) and I had no issues developing iOS and macOS apps with it using Xcode, and Rust development with VSCode. This brought my old 16GB Intel MacBook Pro to its knees.

Another thing to consider is memory compression. If Apple added dedicated hardware for that, it can effectively double the total memory with minimal performance hit.

Memory compression only works in certain scenarios. It requires your memory to actually have low entropy.

This is myth.

The Mac Pro will have replaceable RAM. It will use the RAM soldered onto the CPU as cache.

You’ll most likely also be able to buy dedicated GPUs/ML booster addon Cards and the likes for it.

It’s the most likely thing to happen or they won’t release another Mac Pro.

Why would they use soldered RAM as cache? It's not like it's faster then replaceable RAM. Unless they go HBM2 but I doubt that.

The bandwidth of the soldered ram is much higher which makes it much faster for code that accesses a lot of RAM like video editors.

I think they will unveil M2 which can probably at least double the 64GB max to 128GB max RAM of M1-series.

Then, on the highest configuration, I think they actually can put 6 M2-top-specced or more into the Mac Pro.

Does the mac studio potentially replace the mac pro concept? It seems targeted at exactly the audience that mac pros targeted (ridiculous amounts of video simul-editing)

No this looks like a modular replacement of the iMac Pro. If it was to replace the Mac Pro they wouldn't have said at the end of the event "the Mac Pro will have to wait until next time".

To me, this seems to have killed the iMac Pro not the Mac Pro.

The presentor very explicitly said they are not done and they will replace the Mac Pro.

But yes, I see a lot of folks replacing current Mac Pros with Studios.

The pro is most likely going to have ram and PCIe slots.

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