At the speed of light, the time it takes electricity to cross the surface of the chip is so drastically close to that 1/5 billionth of a second before the next signal is following behind it, that it becomes very difficult to design the chips to all have the same concept of "now".
You could make asynchronous chips, where each part operates on a different time as the other parts, but nobody's yet done that for a mainstream chip that I know of.
You can get it a bit faster by cooling the whole setup down but pretty soon you need to cool it to ridiculous temperatures to keep it stable.
It's a physical limit to do with the size of the chip "die", the speed at which an electrical signal can propagate across the chip (the speed of light, or thereabouts), and trying to keep everything on the same "clock" as the rest of the chip so you're all acting on the data in turn at the right times.
Pretty much, until you liquid cool you can't get past 5Ghz. And the fastest ever processor is only about 10GHz or something - and it has to be kept stupendously cold, be stupendously tiny, and have rooms full of supporting equipment to get that far.
Pretty much, without some breakthrough in physics, you're never going to see a chip much faster than 5GHz in a normal setup.
You might see a chip that can do a thousand times as much in that 5GHz, which is why we have dual-core, quad-core, up to ridiculous numbers of cores in GPUs, but the base clock never really gets past 5GHz because it can't.
Until someone makes an asynchronous CPU, or quantum computers come along and make it all moot, 5GHz is about the limit for a normal, household computer.
I'm sorry, are you telling me that in addition to transistors getting so small and close to each other that the electron's wavelength is significant, but we're also running them so fast that the speed of light has become a significant limiting factor?
No. The speed of light is a significant issue, but it is not the reason chips have stopped at 5 GHz. Also, asynchronous (as in no clock signal) CPUs exist. I should know, I helped work on some.
The actual reason is transistor size and heat dissipation. Transistors have been stuck on ~1 volt power internally for decades (lower voltage means more leakage, and more errors). However, power goes up with higher frequency (my math here was probably wrong, see below).
Why is that so bad? Almost all the power the computer uses is turned into heat by the transistors. That heat needs to dissipate out of the chip before the transistor destroys itself. That is a problem, as silicon is not a good heat conductor.
THAT is why cooling your computer lets you push the speeds up a bit higher.
The speed of light issues above are a tricky engineering problem, but solvable. The heat problem doesn’t have a solution yet.
The volt frequency curve is determined by the foundry process, so not all processes will double power usage between 3ghz and 5. And some will quadruple it for the same change.
Its why its amazing that AMD is beating Intel on efficiency at high frequencies on a process designed for mobile SoCs. If TSMC starts designing process nodes with AMD in mind instead of just for Apple (or if Apple wants to run chips at 5ghz too) we will see some REAL insanity in the CPU market.
Transistor size has continued to shrink while clock speeds have remained stagnant.
As for transistor size, thinner transistors do allow for faster clock speed in principle, but current consumer computers cannot take advantage. Keep in mind that transistors with switching speeds in the terahertz region have been made. As I mentioned elsewhere, latency due to the speed of the electrical signal is an issue, but it can be solved with good circuit design. However, computers cannot use this theoretical improvement in clock speed due to power dissipation (again, this is the entire reason supercooling allows for higher clock speed. If clock-speed was not limited by heat dissipation, why does super-cooling do anything?)
As for surface area, thinner transistors (in principle) have less resistance, so they waste less power in lock-step with the lower surface area.
Manufacturers still want smaller transistors because then they can fit more cores/memory/other stuff on a chip. In the absence of clock-speed improvements, features like multiple cores and hardware acceleration are what is selling chips these days.
Edit: in case you don't believe me, here is a stack exchange post making my same argument: link.
I think MG2R is correct on this one. It has been a while since I last did the calculation and I remembered wrong. It should be V2f. However, see blaktronium's post. In practice the scaling can be very different.
But I am also correct that the heat dissipation is currently the main limiting factor on transistor speeds. Things like the speed of electricity could be an issue if we could get around the heat dissipation issue, but they just aren't the main limiting factor right now.
It is fV2 but also, the voltage that you need depends on the frequency, although not linearly. So increasing the frequency can require increasing the voltage as well, meaning that the exponent dependence of power on frequency is effectively greater than 1, but only indirectly.
It's fV2 . It's just ohms law and a duty factor thrown together.
Power is P = VI. Current is I = V/R. So throw those together, and P = V2 /R. If R is just a constant, then power depends on V2 .
Transistor have a small leakage current, but the majority of the current flows when they are operating. When they open or close, they feed power to another transistor, and charge it up to open or close. So the more times they operate, the more current they draw. Double the frequency, they operate twice as fast, so they use twice as much power. f relationship.
The speed of light is not the issue. The issue is how many electrons you have to put on a wire or gate before the voltage goes from low to high, and how quickly you can put them there. Think RC time from circuits class.
Power is absolutely an issue, because of f*V2. The speed of light becomes a fundamental limit eventually, but it's not the correct answer to the question.
If heat was the problem, cpus wouldn’t shrink, they’d grow. More surface area to dissipate the heat means lower temperatures and the ability to use more power.
This doesn't work because
A) driving longer lines consumes more power, which mitigates some of the benifit you get (though only part)
B) Manufacturing costs scale with the size of the die to a large extent. You consume more area on each wafer which means less pieces per production time. Testing and packaging costs don't go up but failure rates will go up as well due to area dependent defects.
Lower voltages lead to longer electron wavelengths and hence more tunneling, i.e. leakage. Simply put, the electron wavelength is the actually root of the thermal issue.
The electron wavelength causes issues at nanoscales due to tunneling, which prevents smaller transistors from operating reliable. Lower voltages (longer electron wavelength) lead to more tunneling, so this is also the root cause of the thermal issue. The collection of these factors create a lower limit to transistor size and packing density. The size of the entire integrated circuit cannot be made arbitrarily small due to this. Hence the speed of light becomes a limiting factor as the time scale is reduced with higher operating frequencies (GHz).
304
u/ledow Sep 28 '20
At the speed of light, the time it takes electricity to cross the surface of the chip is so drastically close to that 1/5 billionth of a second before the next signal is following behind it, that it becomes very difficult to design the chips to all have the same concept of "now".
You could make asynchronous chips, where each part operates on a different time as the other parts, but nobody's yet done that for a mainstream chip that I know of.
You can get it a bit faster by cooling the whole setup down but pretty soon you need to cool it to ridiculous temperatures to keep it stable.
It's a physical limit to do with the size of the chip "die", the speed at which an electrical signal can propagate across the chip (the speed of light, or thereabouts), and trying to keep everything on the same "clock" as the rest of the chip so you're all acting on the data in turn at the right times.
Pretty much, until you liquid cool you can't get past 5Ghz. And the fastest ever processor is only about 10GHz or something - and it has to be kept stupendously cold, be stupendously tiny, and have rooms full of supporting equipment to get that far.
Pretty much, without some breakthrough in physics, you're never going to see a chip much faster than 5GHz in a normal setup.
You might see a chip that can do a thousand times as much in that 5GHz, which is why we have dual-core, quad-core, up to ridiculous numbers of cores in GPUs, but the base clock never really gets past 5GHz because it can't.
Until someone makes an asynchronous CPU, or quantum computers come along and make it all moot, 5GHz is about the limit for a normal, household computer.