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Comment by jsheard | Hacker News Reader

SRAM scaling also hit a wall a while ago, so you can't really count on new processes allowing for significantly higher density in the future. That's more of a longer-term issue with the SRAM gambit that'll come into play after the DRAM shortage is over though - logic and DRAM will keep improving while SRAM probably stays more or less where it is now.

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You can still scale SRAM by stacking it in 3D layers, similar to the common approach now used with NAND flash. I think HBM DRAM is also directly stacked on-die to begin with, apparently that's the best approach to scaling memory bandwidth too.

It'll be interesting to see if we get any kind of non-NAND persistent memory in the near future, that might beat some performance metrics of both DRAM and NAND flash.

NAND is built with dozens of layers on one die. HBM DRAM is a dozen-ish dies stacked and interconnected with TSVs, but only one layer of memory cells per die. AMD's X3D CPUs have a single SRAM die stacked on top of the regular CPU+SRAM, with TSVs in the L3 cache to connect to the extra SRAM. I'm not aware of anyone shipping a product that stacks multiple SRAM dies; the tech definitely exists but it may not be economically feasible for any mass-produced product.

The issue is size, SRAM is 6 transistors per bit while DRAM is 1 transistor and a capacitor. Anyone who wants density starts with DRAM. There’s never been motivation to stack.

> AMD's X3D CPUs have a single SRAM die stacked on top of the regular CPU+SRAM, with TSVs in the L3 cache to connect to the extra SRAM.

Just FYI, the latest X3D flipped the stack; the cache die is now on the bottom. This helps transfer heat from the compute die to the heatsink more effectively. In armchair silicon designer mode, one could imagine this setup also adds potential for multiple cache dies stacked, since they do interpose all the signals, why not add a second one ... but I'm sure it's not that simple, for one: AMD wants the package z-heights to be consistent between the x3d and normal chip.

I agree with your description and conclusion. Additionally the companies that can make chip stacks like HBM in volume are the HBM manufacturers. As they are bottlenecked by the packaging/stacking right now (while also furiously building new plant capacity) I can't see them diverting manufacturing to stacking a new SRAM tech.

Every time I read about D|S-RAM scaling I'm reminded of https://www.besang.com/

Ever heard of them? What do you think? Vaporware?

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