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Comment by ghostpepper | Hacker News Reader

It's not just compression - also the unified memory architecture.

At the risk that I’ve missed a joke, the unified memory architecture reduces the amount of RAM you have available since “RAM” for any computer with discrete graphics doesn’t include VRAM.

I think they are saying it from the perspective rather than two separate banks which are rarely both full it's one bank which is only ever full when 100% of the hardware is full. That said I think many miss that's how integrated graphics memory management had been working on x86 systems for many many years already. I.e. the "dedicated RAM" slider in the boot firmware is a legacy holdover that should be set to a minimum token value, not something which determines the limit the iGPU has access to. macOS also works this way, there is a token amount of the unified memory reserved for the iGPU still, you just can't adjust that amount higher since there is no legacy holdover for it to make sense to do so.

They are saying it from the perspective of the RAM being on the same die as the CPU. This is one of the innovations of the Apple Silicon architecture as it SIGNIFICANTLY reduces memory access latency.

It's not just TSMC's 3nm process. It's also Apple engineering.

No, it doesn't reduce latency.

Also, the RAM isn't on the same die as the CPU cores. TSMC doesn't even make DRAM.

It's not on die, it's separate dies on chip.

This engineering was common in x86 CPUs by 2013 when AMD introduced Heterogeneous System Architecture which utilized Heterogeneous Uniform Memory Access. The approach has its upsides and downsides, the latter generally being a unified bus tends to have much lower overall bandwidth (even in the Max) and runtime scalability issues. Upsides are more obvious for the types of systems people want APUs for in the first place though so that's usually fine.

The main bit of engineering Apple should be lauded for in the memory department is the gumption to throw the hundreds of GB/s at the mid and high end models.

HSA was not “unified” in the modern sense. It still required designating memory as gpu or cpu side and these implied different cache coherency rules that meant memory couldn’t actually be shared, by default. To actually share memory you had to use a special “garlic bus” that guaranteed visibility and ordering and massively slowed down performance. Similarly, it was also impossible for the gpu to see cpu memory unless it was pinned and tagged for a special “onion bus”, but at least this was relatively fast iirc.

https://www.realworldtech.com/fusion-llano/3/

In contrast apple actually has everything tied into a single unified space with a single controller that immediately makes all writes visible regardless of where the happen.

They’ve also got enormously more memory bandwidth to play with. M1 Max is close to PS5 in both shader configuration and memory bandwidth.

https://x.com/LinaAsahi/status/1820947147312820497

"Sooo apparently there's this myth that only Apple has "unified memory"...?

Every single modern integrated graphics system works the same way. They even share the MM code in Linux.

I just had a reply guy try to argue otherwise with me even when I told him I wrote the driver"

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