What is the reason behind the 384GB RAM limit? I suspected it might be due to directory sizes, since address space shouldn't be a problem.
As an aside, what kind of considerations go in to deciding the directory sizes? If what I suspect is true, it seems like it might be a balance between supported RAM amount and economics (size, price, etc.); I don't necessarily think more RAM support would make the chip more powerful after some point. Are there aspects to this that I'm overlooking?
Officially, virtual addresses are limited to 48 bits, which limits the size of directories both for hardware support and in the operating system. 384GB seems to be instead related to the DDR memory DIMMs. With support for 6 DIMMs, each slot could then have a 64GB chip. However, 64GB is less than the maximum for a DDR4 DIMM, so I too am scratching my head as to the exact cause of this limit. I'll keep looking for you.
@bpr Thank you. I'll post it if I can find some explanation.
Although this is a 64bit architecture and virtual addressing supports up to 48 bits that doesn't necessarily mean that it is advantageous to support such large amounts of memory. By decreasing the amount of physical memory supported optimizations can be made within the chip itself. I would suppose that having more than 384 GB of memory has diminishing returns for the chip and as a result they optimized it to support no more than that amount.