45 year CPU evolution – one law and two equations
arxiv.org
arxiv.org
- 22mn or 7nm are idle marketing. "The nodes were first defined according to the transistor channel length. The last nodes are more defined according to marketing criteria."
- the memory wall... "The huge difference between CPU and DRAM growth rates led to the increased complexity of microprocessor memory hierarchies. Different levels of caches are needed to balance the differences in the bandwidth and latency needs of the CPU with those of the DRAM main memory."
- Conclusion "When the end of the predicted end of the exponential evolution will be real or when non-Von Neumann architectures will prove to be more efficient for programmable applications, the situation will be totally different. Until that point, the two equations that have been discussed in this paper will be there to explain the evolution"
(When ((the end of (the predicted end of the exponential evolution)) will be real) or (when non-Von Neumann architectures will prove to be more efficient)) the situation will be totally different
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The way to "solve" this is well known. You build buffers between the CPU and the Memory, and then execute as much as possible out-of-order. And that's the purpose of L1, L2, and L3 caches: to provide this out-of-order code enough data to work (while the CPU is waiting on slow, slow main-memory).
However, contemporary RAM is not designed to coordinate with the CPU very well on this front, at least compared to designs like HMC (a type of "stacked RAM", a competitor to a GPU's HBM).
HMC is interesting because its a packet-based system. You tell the RAM a memory-address to access, but then the RAM may return that memory out-of-order compared to other requests !!!
After all: RAM also has physical "latency" issues. When a bank is open, it needs to close, incurring a tRAS, tCAS, and tRC delay. (A "hit" to the same bank would incur only a tCAS delay). If RAM could execute "out of order", then the memory-controller could allow more efficient orderings of memory.
Juggling which banks are open and closed has been traditionally a CPU's memory controller job. But communicating this information over a many-cm long PCB trace incurs latency. So it makes more sense to put this logic as close to the RAM as possible (speed of light, capacitance, inductance, etc. etc. slow down the signal).
Furthermore: since CPUs are incredible out-of-order machines already, it wouldn't be a major hassle to make the memory also out-of-order. I mean, yeah, its complicated, but its no different to what CPUs already do with L1, L2, and L3 caches (and the programming constructs built on top: Atomic Accesses and whatnot to control the effects of memory-reorderings).
I haven't heard of any major computer use HMC yet however. Only $10,000+ FPGAs on occasion. Still, there exists RAM today which can execute requests out-of-order. When this RAM becomes mainstream (if it ever becomes mainstream), I'd expect systems to be able to get much faster.
Indeed. So there's some parallelism that can be captured, but they're managed by the CPU instead of the Memory.
Because HMC abstracts away the protocol into a packet-based innately out-of-order protocol, and because the "memory controller logic" has been moved to an area physically as-close-as-possible to the RAM itself, it can achieve much higher degrees of parallelism.
Case in point: a single stack of HMC has 128 banks of parallelism across 4-vaults (I would argue that a "vault" is roughly equivalent to a DDR4 channel): https://www.micron.com/parts/hybrid-memory-cube/hmc-sr/mt43a...
Its very rare to get anywhere close to 100% bus utilization. HMC's real advance is switching to independent serial links which we know how to scale better than parallel busses. Its this improved bandwidth that allows you to pack in more parallelism. We could have more banks in traditional DDR memory but there isn't much point because the data bus couldn't feed it.
The problem with this is that putting 'complex control logic' on the DIMM inevitably brings up the cost of a component which has a shorter lifespan than the CPU. In many applications, the RAM DIMMs are almost a 'consumable resource', in the sense that they are expected to be regularly replaced as heavy read/write cycles lead to their deterioration and eventual demise.