[ineedasername]

Okay, probably a stupid question, but solid state memory can be pretty dense: why don't we have huge caches, like a 1GB cache? As I understand it, cache memory doesn't put off heat like the computational part of the chip does, so heat dissipation probably wouldn't increase much with a larger chip package.

[emn13]

Nand flash is pretty dense, but way too slow. Sram is fast but not at all dense, needing 6 transistors per bit.

For reference: https://en.m.wikipedia.org/wiki/Transistor_count lists all the largest cpu as of 2019 amd's epyc Rome at 39.54 billion MOSFETs, so even if you replaced the entire chip with Sram you wouldn't even quite reach 1GB!

Dram would be enticing, but the details matter.

[chemmail]

Nand is nonvolatile and the tradeoff with that is write cycles. We have an inbetween in the form of 3D Xpoint (Optane), Intel is still trying to figure out the best way to use it. It currently like an L6 cache after system DRAM.

[thu2111]

Well not just Intel. Optane is a new point in the memory hierarchy. That has a lot of implications for how software is designed, it's not something Intel can do all by itself.

[atq2119]

SRAM is 6 transistors per bit, so you're taking about 48 billion transistors there, and that's ignoring the overhead of all the circuits around the cells themselves.

DRAM is denser, but difficult to build on the same process as logic.

That said, with chiplets and package integration becoming more common, who knows... One die of DRAM as large cache combined with a logic die may start to make more sense. It's certainly something people have tried before, it just didn't really catch on.

[Pet_Ant]

> DRAM is denser, but difficult to build on the same process as logic.

What makes it so difficult to have on the same chip?

[elihu]

I don't know the details, but the manufacturing process is pretty different. Trying to have one process that's good at both DRAM and logic at the same time is hard, because they optimize for different things.

[janekm]

Or, well, how about putting all your ram on the package as Apple says they are doing with the M1?

[coryrc]

Cost. Area is what you pay for (at a given transistor size).

[gameswithgo]

the bigger the cache the slower it gets

so ram is your 1gb cache

[tomxor]

Are you are referring to latency due to propagation delay where the worst case increases as you scale?

Would you mind elaborating a bit? I'm not following how this would significantly close the gap between SRAM and DRAM at 1GB. Since an SRAM cell itself is generally faster than a DRAM cell, and I understand that circuitry beyond an SRAM cell itself is far simpler than DRAM. Am I missing something?

[chongli]

Think of a circular library with a central atrium and bookshelves arranged in circles radiating out from the atrium. In the middle of the atrium you have your circular desk. You can put books on your desk to save yourself the trouble of having to go get them off the shelves. You can also move books to shelves that are closer to the atrium so they're quicker to get than the ones farther away.

So what's the problem? Well, your desk is the fastest place you can get books from but you clearly can't make your desk the size of the entire library, as that would defeat the purpose. You also can't move all of the books to the innermost ring of shelves, since they won't fit. The closer you are to the central atrium, the smaller the bookshelves. Conversely, the farther away, the larger the bookshelves.

Circuits don't follow this ideal model of concentric rings, but I think it's a nice rough approximation for what's happening here. It's a problem of geometry, not a problem of physics, and so the limitation is even more fundamental than the laws of physics. You could improve things by going to 3 dimensions, but then you would have to think about how to navigate a spherical library, and so the analogy gets stretched a bit.

[Scaevolus]

Area is a big one. Why isn't L1 MB? Because you can't put that much data close enough to the core.

Look at a Zen-based EPYC core- 32KB of L1 with 4 cycle latency, 512KB of L2 with 12 cycle latency, 8MB of L3 with 37 cycle latency.

L1 to L2 is 3x slower for 8x more memory, L2 to L3 is 3x slower for 16x more memory.

You can reach 9x more area in 3x more cycles, so you can see how the cache scaling is basically quadratic (there's a lot more execution machinery competing for area with L1/L2, so it's not exact).

https://www.7-cpu.com/cpu/Zen.html

[MobiusHorizons]

I am sure there are many factors, but the most basic one is that the more memory you have, the longer it takes to address that memory. I think it scales with the log of the ram size, which is linearly with the number of address bits.

[psykotic]

Log-depth circuits are a useful abstraction but the constraints of laying out circuits in physical space imposes a delay scaling limit of O(n^(1/2)) for planar circuits (with a bounded number of layers) and O(n^(1/3)) for 3D circuits. The problem should be familiar to anyone who's drawn a binary tree on paper.

[Dylan16807]

With densities so high, and circuit boards so small (when they want to be), that factor isn't very important here.

We regularly use chips with an L3 latency around 10 nanoseconds, going distances of about 1.5 centimeters. You can only blame a small fraction of a nanosecond on the propagation delays there. And let's say we wanted to expand sideways, with only a 1 or 2 nanosecond budget for propagation delays. With a relatively pessimistic assumption of signals going half the speed of light, that's a diameter of 15cm or 30cm to fit our SRAM into. That's enormous.

[semi-extrinsic]

Well, the latest AMD Epyc has 256 MB L3 cache, so we're getting there.

[gameswithgo]

but any given core only has access to 16 soon to be 32mb

[fulafel]

in Zen 1 and Zen 2, cores have direct or indirect access to the shared L3 cache in the same CCX. In the cross-CCX case the neighboring CCX cache can be accessed over the in-package interconnect without going through system DRAM.

[samoa42]

this!

when i started with computers, they had a few KB of L2 cache, L3 did not exist. Main Memory was a few MB.