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Comment by PragmaticPulp | Hacker News Reader

The M1 Ultra has a significant memory bandwidth and latency advantage due to the type of memory used and the way the memory is connected. They use the same memory for the CPU and GPU, so the memory interface was optimized more like a GPU and the CPU benefits in a few memory-constrained benchmarks (machine learning, AES-XT streaming)

The flip side is that you're limited to 128GB combined memory for the CPU and GPU on the M1 Ultra, whereas a comparable Threadripper Pro system will take up to 16 times as much (2TB) and you can upgrade it whenever you feel like.

It will be interesting to see how much RAM Apple offers on the upcoming M1 Mac Pro parts.

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Bandwidth yes, latency no.

Wiring out 32 channels of DDR5 to 16 slots might not be feasible, but latency-wise Anandtech's measurements suggest the M1 Max actually has a bit higher latency to memory than e.g. Icelake-SP

> significant memory bandwidth and latency advantage

This is to cache or the main memory?

I remember x86 based PCs taking 100ns for RAM access. Is it faster in ARM?

> They use the same memory for the CPU and GPU, so the memory interface was optimized more like a GPU and the CPU benefits in a few memory-constrained benchmarks (machine learning, AES-XT streaming)

This would mean each core having some dedicated RAM section better connected. Wouldn't this be more like L3 section in x86 but bigger? Maybe this is the advantage of having everything on the same die.

Beyond a certain size gains should also flat out, no?

> Is it faster in ARM?

Not in the general case, but specifically with how Apple has brought the ram chips so physically close to the cpu cores

The physical distance confers no latency advantage.

Is this because of signal speed anyway reaching almost speed of light?

Maybe 70% the speed of light. That's a handful of cycles at a few GHz for any SDRAM channel. The latency limitation is caked in to the scanning, strobed nature of SDRAM. It's dense and cheap, but it will never respond faster than ~20 ns (100+ cycles).

I wonder what stops AMD, Intel in trying this, except loss of modularity. It is not like the architecture will change by bringing it into the die. As far as I remember, CPU is the only thing that does RAM access even in x86

I would be surprised if they’re not evaluating this. The only difference I can see is that it complicates the number of SKUs they’d need to provide to their customers. It’s a little easier for Apple in this regard because they’re also building the final machines, whereas Intel/AMD are making chips that are going in a wider range of devices.

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