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So, what was the reason CPUs could not scale vertically (higher frequency)? Temperature? Stability?
by kevin2r 10y ago
So, what was the reason CPUs could not scale vertically (higher frequency)? Temperature? Stability?
- gaze 10y agoIf I understand correctly, Intel CPUs around that time were super deeply pipelined so you could meet timing closure at such high frequencies. So yeah you can keep pushing the clock-speeds up and up but now a mispredicted branch really shoots you in the foot. Makes sense at the time because people associated clock speed directly with performance. Higher frequency more better. Intel kinda gave up on that at some point and started over with the core architecture design which has a lower clock speed but does more each pipeline stage.
- monocasa 10y agoHaha, yeah P4 had multiple pipeline stages just for propagation delay.
- largote 10y agoThose very deep pipelines are the reason NetBurst was very good at some tasks but not so great for general use. They made branching mispredictions be really expensive as a pipeline flush would result in a large number (~20 IIRC) cycles to be "wasted".
- dzhiurgis 10y agoNot enough time for electrons to move. Photons - another story.
- aisofteng 10y agoThis comment makes no sense. Neither electrons nor photons are what move through wires to propagate electrical signals.
- gaze 10y agoI'd definitely say electrons propagate through wires to carry signals. The electrons charge the gates on the fets which opens or closes the channel, which allows electrons to flow through another wire. I wouldn't say it's terribly explanatory tho
- kushan2020 10y agoWell that is not correct @gaze. Although your light turns on very quickly when you flip the switch, and you find it impossible to flip off the light and get in bed before the room goes dark, the actual drift velocity of electrons through copper wires is very slow. It is the change or "signal" which propagates along wires at essentially the speed of light. refer https://en.m.wikipedia.org/wiki/Drift_velocity https://en.m.wikipedia.org/wiki/Drift_velocity
- xanton 10y agoAll the theory behind microelectronics is all about about electrons, "holes" and how fast and where they move.
- monocasa 10y ago> Temperature? Stability? Yes. Notice how after it was clear that P4 was a dead end they went and dusted off their P3 mobile line that they had been making more power efficient the whole time?
- iamaaditya 10y agoTechnically it was heat but mostly it was due to (i) Economics, there was less demand for faster clock speed. Otherwise more research could have gone towards solving heat problem. (ii) Each cycle of CPU was more efficient with ability to execute multiple instructions in a single cycle and with more efficient instruction sets. Surprisingly, power consumption also made huge impact. As tablets and laptops got more popular than desktop battery life became a major concern and thus TDP played major role in research. Try this fun experiment: Underclock your CPU by half a GHz and see if you notice the difference in your day to day work.
- monocasa 10y ago> Otherwise more research could have gone towards solving heat problem. (ii) Each cycle of CPU was more efficient with ability to execute multiple instructions in a single cycle and with more efficient instruction sets. Dude, Intel spends something like $80B/yr on R&D. This is closer to hitting fundamental laws of physics barriers. They killed off their P4 line and developed their mobile line for a reason.
- kashkhan 10y agothe $80B a year in R&D is off by an order of magnitude. https://www.fool.com/investing/2017/02/05/intel-corporation-struggles-to-rein-in-spending-as.aspx https://www.fool.com/investing/2017/02/05/intel-corporation-...
- knowaveragejoe 10y agoIndeed, that's more than Intel earned in total revenues in 2016...
- ma2rten 10y agoIsn't an order of magnitude 10x? According to your link they spent $12.74 billion in 2016.
- slededit 10y ago
- deleted 10y ago[deleted]
- camus2 10y agoheat mainly and the shift toward mobile devices which implies good autonomy and power efficiency.
- adt2bt 10y agoMostly heat dissipation. Assuming you hold voltage constant, heat increases linearly with frequency. Also, as process nodes get smaller, you increase the # of transistors per square inch, so a jump to a new node can also increase heat generation quadratically if all else stays constant. As other commenters mentioned, you can counteract this with better cooling, but eventually the marginal $/watt of cooling isn't worth the tradeoff of lower speeds & multicore processors.
- DontGiveTwoFlux 10y agoEach switch of a gate causes energy to be expended. When a transistor switches, there is some current flowing and some voltage across the source and sink. That voltage * current is greater than zero, and therefore requires power. This is opposed to the stable non-switching state where there is no current and therefore no power. So, because each switch consumes the same amount of power, the power consumed is directly proportional to the switching frequency. Double the frequency = double the power. As other posters have alluded, Intel tried to work on the power issue by decreasing the voltage. Unfortunately, they were not able to figure out how to decrease the voltage as they had in the past. With the high power required to run high frequency circuits, cooling would have been more of a problem than could be overcome economically.
- baq 10y agoit's even worse - transistors require more voltage to reliably switch at higher frequencies, e.g. http://images.anandtech.com/reviews/cpu/intel/22nm/powersm.jpg http://images.anandtech.com/reviews/cpu/intel/22nm/powersm.j...
- kec 10y agoAnd to get even more pedantic, it's even worse than that alone implies because switching power increases with the square of voltage. We're also at the point where transistor leakage current matters. All transistors leak a tiny amount of current even without switching. Alone this might only be a few µA, but pack enough of them together on a die and that might add up to hundreds of mA. Combine that with higher voltages so the chip can run at high clock speeds, and the fact that leakage increases with temperature (which will be driven by higher voltages and frequencies), and all of a sudden you have a feedback loop which can cause you to burn a few extra Watts before you've even done anything.
- deleted 10y ago[deleted]
- inlineint 10y agoThere is also another fundamental limit, the speed of light. Because of the special relativity theory nothing including electrical current can transmit information faster than the speed of light (c = 3*10^10 cm/s). So if maximum size dimension of your CPU is say 2cm (I'm ignoring that they actually can have multiple smaller cores), then the maximum physically achievable frequency is upper bounded by size/c = 15 GHz. Although it has not been reached yet, it has the same order of magnitude as the record frequencies achieved by overclockers using liquid nitrogen cooling (something a little below 9 GHz). Also it shows that reducing transistor size and as a consequence overall CPU size increases maximum physically allowed frequency.
- unixhero 10y agoThank you for your synaptic sugar this morning. It saved an incredibly boring meeting during a long track implementation project.
- Dylan16807 10y agoIt's very easy to design a CPU such that signals do not have to cross it in one cycle. The speed of light is unlikely to ever cap the speed of a CMOS processor.
- phoenix3200 10y agoThey were scaling vertically because the GHz game was a marketing strategy and not an engineering strategy. But when the pipeline got so deep that it didn't make sense (branch misses would incur huge penalties) and AMD stole the spotlight, marketing finally gave way for the engineers.