Comment by fooker
5 hours ago
Apple's "unified memory" marketing is so strong that even tech literate people seem to have this misconception!
They have managed to pull this sort of thing off many many times. https://en.wikipedia.org/wiki/Reality_distortion_field
Yup, the only reason Macs have higher memory bandwidth is because they use more memory channels, which gives them a wider bus. Both Intel and AMD only allow more than dual channel memory on server class processors these days.
The “Apple only does X better because they do Y” thing has been a meme for ages. I remember dismissals like “PowerPC is only faster at math because it has more integer units” or something along those lines, and thinking, uh, isn’t that a good thing?
There's a qualitative difference between "they're doing a different thing" and "they're doing the same thing, tuned differently". GP is saying that this is a case of "they just tuned it differently".
This distinction doesn't change what the performance numbers look like today, but it does inform what changes would be necessary for those numbers to look different tomorrow. E.g. Apple Silicon isn't fundamentally orders of magnitude more efficient than x86, they just used smaller features. Newer Intel and AMD chips made on equivalent processes _also_ get similar efficiency gains.
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Depends on the expense trade off.
If I get 10% more performance for 50% more cost it really depends on one's needs, for example.
"You're only better than me at sports because you practice more and try harder!"
I am just saying it's not magic and x86 is capable of doing the same. Quad channel memory used to be more common in consumer hardware but now it looks like you don't even have the option anymore for desktops. AMD's Strix Halo was the first sign to reversing that (it has quad channel memory!) and hopefully we see more of that in the future.
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The memory bandwidth is a small thing compared to the massive win you get by not having to move data between two memory pools at all.
Often you don't move memory around as a programmer, but that's exactly what happens in the background.
It's the address space that's unified, not always the physical hardware.
The data movement (when needed) is handled transparently in the background by page faults and other tricks.
Depends on your workload. And AMD has supported unified memory long before Apple Silicon existed anyway.
Threadripper have that extra bandwidth and M5 still is faster
Except strix halo.
Having unified memory is a real advantage though, it's not a reality distortion.
It isn't really, as long as you don't care about power consumption, physical constraints and money. Basically desktops <2025 (and hopefully >2027).
DDR is optimized for latency and stability at the cost of bandwidth whilst GDDR is optimized for bandwidth at the cost of latency and stability. GDDR is pushed so hard these days that a small percentage of errors is expected and corrected because this is still faster than running it slower but more accurate.
GDDR7 often has 10-20x the total bandwidth but 3x the latency of DDR5. Graphical workloads want as much bandwidth as possible but care relatively little for latency. Conversely, applications love low latency but don't really see any performance benefit from higher bandwidth.
So basicallyt you have workloads that are diametrically opposed and running unified memory forces you to compromise.
The price is that you essentially glue CPU and GPU together, which limits total compute, from a size and thermal perspective.
This is really not a limit because of unified memory -- in principle, PCIe GPUs could read/write main memory without the CPU. But it's a limit for /fast/ unified memory, because fast means close.
So unified memory is great as long as the integrated GPU is strong enough. Then it has two advantages: a) probably faster transfer CPU<->GPU (but that's an implementation choice for the non-unified case b) If you either need a lot of memory for the CPU or the GPU, but not for both at the same time, you pay for memory only once.
Agreed. That's why it's a good thing all computers made in the past 15 years have unified memory, not just macs.
https://x.com/Lina_Hoshino/status/1820947147312820497
In theory perhaps but the benefit is not as relevant with a weak iGPU. In practice all PC's with performance ambitions had a dGPU until Strix Halo and Panther Lake.
Is that Marcan's vtuber persona (Asahi Lina) that changed name since Marcan doesn't work on Asahi Linux anymore?
It is a real advantage.
The reality distortion is that people seem to believe it's HBM, or somehow it gives you extraordinary amounts of vram. Neither are really true.