Comment by nutjob2

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?

Threadripper have that extra bandwidth and M5 still is faster

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.

Except strix halo.

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

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.

More channels or soldered memory? Channels are basically RAID 0 so it depends what you're measuring. Soldering memory down was the only way to use LPDDR5X so if you wanted the best memory you had to solder it down. LPCAMM2 exists though so newer devices can use that instead of soldering them down, but not all devices would be able to fit the required LPCAMM2 slots.