[eigenspace]

Framework said that when they built a Strix Halo machine, AMD assigned an engineer to work with them on seeing if there's a way to get CAMM2 memory working with it, and after a bunch of back and forth it was decided that CAMM2 still made the traces too long to maintain proper signal integrity due to the 256 bit interface.

These machines have a 512 bit interface, so presumably even worse.

[zamadatix]

Current (individual, not counting dual socketed) AMD Epyc CPUs have 576 GB/s over a 768 bit bus using socketed DIMMs.

[eigenspace]

My understanding is that works out due to the lower clock speeds of those RAM modules though right?

It's getting that bandwdith by going very wide on very very very many channels, rather than trying to push a gigantic amount of bandwidth through only a few channels.

[zamadatix]

Yeah, "channels" are just a roundabout way to say "wider bus" and you can't get too much past 128 GB/s of memory bandwidth without leaning heavily into a very wide bus (i.e. more than the "standard" 128 bit we're used to on consumer x86) regardless who's making the chip. Looking at it from the bus width perspective:

- The AI Max+ 395 is a 256 bit bus ("4 channels") of 8000 MHz instead of 128 bits ("2 channels") of 16000 MHz because you can't practically get past 9000 MHz in a consumer device, even if you solder the RAM, at the moment. Max capacity 128 GB.

- 5th Gen Epyc is a 768 bit bus ("12 channels") of 6000 MHz because that lets you use a standard socketed setup. Max capacity 6 TB.

- M3 Ultra is a 1024 bit bus ("16 channels") of "~6266 MHz" as it's 2x the M3 Max (which is 512 bits wide) and we know the final bandwidth is ~800 GB/s. Max capacity 512 GB.

Note: "Channels" is in quotes because the number of bits per channel isn't actually the same per platform (and DDR5 is actually 2x32 bit channels per DIMM instead of 1x64 per DIMM like older DDR... this kind of shit is why just looking at the actual bit width is easier :p).

So really the frequencies aren't that different even though these are completely different products across completely different segments. The overwhelming factor is bus width (channels) and the rest is more or less design choice noise from the perspective of raw performance.

[jsheard]

Yeah, but AMDs memory controllers are really finnicky. That might have been more of a Strix Halo issue than a CAMM2 issue.

[eigenspace]

Entirely possible. Obviously Apple wouldn't have been interested in letting you upgrade the RAM even if it was doable.

I'd love to have more points of comparison available, but Strix Halo is the most analogous chip to an M-series chip on the market right now from a memory point of view, so it's hard to really know anything.

I very much hope CAMM2 or something else can be made to work with a Strix-like setup in the future, but I have my doubts.