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Taken to the extreme, does anyone know the engineering limitations on how large you could make a functioning CPU?

by cing 11y ago

PhasmaFelis 11y ago
I think it depends on what you mean by "functioning." There's no reason why a CPU lightyears across couldn't perform operations, albeit very slowly.
simias 11y ago
The bigger you make it the slower it'll end up working, basically. The time for the signal to propagate through all those discrete transistors will be quite big and that puts an upper bound to your clock frequency.
TheLoneWolfling 11y ago
There are a couple limitations, depending on if you mean "take a CPU and scale the components up" or if you mean "add more and more to a CPU until it becomes larger". The main limitations are heat, frequency, reliability, and thermal stresses. Heat being mainly for if you're adding more and more to a CPU. With just scaling a CPU up, the increasing size of everything counteracts the longer distances. But if you are trying to add more to a CPU the amount of power dissipation will continue to increase until eventually you can't get the heat out from the center of the CPU. Ditto, the larger the CPU is the more you will have problems with thermal stresses. Once you get into macroscopic stuff, that is. At current CPU sizes you actually seem to end up with more problems with thermal stresses the smaller you get, due to wires scaling "funny" (edge effects becoming more dominant the smaller you get). Ditto, if you're trying to add more and more to a CPU eventually you'll hit a point where you can't add more because you'll cost more time replacing bad components than actually running. Some of the early vacuum tube computers had actually hit this limitation, and it is only through heroic "throw more money at it" solutions that modern CPUs aren't limited by this. I mean: you have a billion transistors on a chip. And get decent yields. Frequency is the big limitation. CPUs propagate signals at ~0.1c already - which means if you scale them more than 10x the size (or ~2x for PCB-style traces) you cannot maintain the same frequency. Remember: your frequency dictates how long the longest wire through the CPU that gets updated in a single clock cycle is. If your CPU is larger than c/f in diameter, you cannot send a signal to the other side of the CPU and back in one clock cycle - at which point it may as well be multiple CPUs.

Taken to the extreme, does anyone know the engineering limitations on how large you could make a functioning CPU?

by cing 11y ago

Taken to the extreme, does anyone know the engineering limitations on how large you could make a functioning CPU?

I think it depends on what you mean by "functioning." There's no reason why a CPU lightyears across couldn't perform operations, albeit very slowly.

simias 11y ago

The bigger you make it the slower it'll end up working, basically. The time for the signal to propagate through all those discrete transistors will be quite big and that puts an upper bound to your clock frequency.

TheLoneWolfling 11y ago

There are a couple limitations, depending on if you mean "take a CPU and scale the components up" or if you mean "add more and more to a CPU until it becomes larger". The main limitations are heat, frequency, reliability, and thermal stresses. Heat being mainly for if you're adding more and more to a CPU. With just scaling a CPU up, the increasing size of everything counteracts the longer distances. But if you are trying to add more to a CPU the amount of power dissipation will continue to increase until eventually you can't get the heat out from the center of the CPU. Ditto, the larger the CPU is the more you will have problems with thermal stresses. Once you get into macroscopic stuff, that is. At current CPU sizes you actually seem to end up with more problems with thermal stresses the smaller you get, due to wires scaling "funny" (edge effects becoming more dominant the smaller you get). Ditto, if you're trying to add more and more to a CPU eventually you'll hit a point where you can't add more because you'll cost more time replacing bad components than actually running. Some of the early vacuum tube computers had actually hit this limitation, and it is only through heroic "throw more money at it" solutions that modern CPUs aren't limited by this. I mean: you have a billion transistors on a chip. And get decent yields. Frequency is the big limitation. CPUs propagate signals at ~0.1c already - which means if you scale them more than 10x the size (or ~2x for PCB-style traces) you cannot maintain the same frequency. Remember: your frequency dictates how long the longest wire through the CPU that gets updated in a single clock cycle is. If your CPU is larger than c/f in diameter, you cannot send a signal to the other side of the CPU and back in one clock cycle - at which point it may as well be multiple CPUs.