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The Dark Silicon Problem and What It Means for CPU Designers (2013)

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Re: The Dark Silicon Problem and What It Means for CPU Designers (2013)

#61

The author states "For every watt of power the CPU consumes, it must dissipate a watt of heat." It was always my understanding that that in electrical devices, (with the exception of heaters) the amount of heat produced was inversely proportional to the efficiency of the device. So is it really true that all energy provided to the CPU or SOC is dissapated as heat?

Yep, it all ends up as heat. Energy is conserved, so if it didn't end up as heat, it would have to go somewhere, and there isn't really anywhere else it could go.

Related topic: reversible computing. You might wonder if a computation need take any energy at all: it turns out that an irreversible computation (e.g. NOR, XOR, AND etc) is physically guaranteed to waste energy, but if you make sure your compute steps are always reversible (i.e. each input maps to one and only one output) then you can theoretically compute for free (the Feynmann lectures on computation cover this well).

But these energies are negligible compared to the wattage that goes through a standard CPU.

Re: The Dark Silicon Problem and What It Means for CPU Designers (2013)

#62
post #45

I had a question - the author states: >"The most obvious is the instruction decoder, which is near the start of the pipeline, and is responsible (in the loosest possible terms) for passing the inputs to each of the execution units." Why would it "in the loosest possible terms"? Isn't this "precisely" the job of the decoder?

IIRC in many chips the decoder stage translates native instructions into micro-ops, which are RISC-like and the main food for execution units. The translation is not necessarily a simple one (one native instruction is often more than one micro-op, and it's possible to collapse multiple native instructions -- especially stuff like prefixes -- into one or more micro-ops).

Ah OK that make sense. This is likely what the authors means here by "loosely." Cheers.

Re: The Dark Silicon Problem and What It Means for CPU Designers (2013)

#63

I had a couple of question about this bit of history mentioned in the article. I'm hoping someone could shed some light on this: >"You can emulate floating-point arithmetic by using integer instructions—but taking 10–100 times as long." Exactly how is/was floating point arithmetic emulated using only integers? Why is that range given an order of magnitude? Is this dependent on the precision I'm guessing?

We had floating point when programming in BASIC on old 6502 8bit computers. There were software routines for doing the math. You know, multiply the mantissa, add the exponents... If someone gave you pointers to a couple 4-byte chunks of data and told you to write code to do floating point multiply on the contents using only C-char variables, what would you write? That's why it's 100 times slower than a nice modern fm…

Thanks these are all good reads and explanations. I guess I just have never had to think about FPU emulation before and it kind of threw me for a loop. It's amazing what we can take for granted now I guess :)

Cheers.

Re: The Dark Silicon Problem and What It Means for CPU Designers (2013)

#64
post #43

Earlier quoted context omitted.

We had floating point when programming in BASIC on old 6502 8bit computers. There were software routines for doing the math. You know, multiply the mantissa, add the exponents... If someone gave you pointers to a couple 4-byte chunks of data and told you to write code to do floating point multiply on the contents using only C-char variables, what would you write? That's why it's 100 times slower than a nice modern fm…

Yep, here's the old Woz/Roy Rankin 6502 code for log, exp, conversions, and basic math: http://www.6502.org/source/floats/wozfp1.txt

Wha a wonderful bit of history your link is. Thanks for sharing.

Re: The Dark Silicon Problem and What It Means for CPU Designers (2013)

#65

From the article "The heat generation per unit area of an integrated circuit passed the surface of a 100-watt light bulb in the mid 1990s, and now is somewhere between the inside of a nuclear reactor and the surface of a star. " I can't tell if this is hyperbole or not, it amazes me but no amount of googling is coming up with a useful answer. Is anyone able to confirm or deny it for me?

keep in mind that 'the surface of a star', for various definitions of 'surface' is actually "cooler than you might think", because it's rather diffuse and more than made up for in total output by the sheer size of the star.

Re: The Dark Silicon Problem and What It Means for CPU Designers (2013)

#66
post #59

The author states "For every watt of power the CPU consumes, it must dissipate a watt of heat." It was always my understanding that that in electrical devices, (with the exception of heaters) the amount of heat produced was inversely proportional to the efficiency of the device. So is it really true that all energy provided to the CPU or SOC is dissapated as heat?

The author's sentence is essentially a truism, because fiddling with information _as such_ and in theory doesn't use up any energy. As practically implemented in current CPU electronics, though, information needs to be communicated from point A to point B as a change in voltage. To convey that voltage change means having to move some amount of electrical charge into or out of the tiny capacitor that is a transistor's…

> fiddling with information _as such_ and in theory doesn't use up any energy.

that's not true at all. Any time you destroy information (for example an and gate can destroy information) you use energy.

Re: The Dark Silicon Problem and What It Means for CPU Designers (2013)

#67

The author states "For every watt of power the CPU consumes, it must dissipate a watt of heat." It was always my understanding that that in electrical devices, (with the exception of heaters) the amount of heat produced was inversely proportional to the efficiency of the device. So is it really true that all energy provided to the CPU or SOC is dissapated as heat?

That's true only of devices that don't convert the energy into work(motors), light(LEDs) or other voltages(transformers). All other electronics convert all the power into heat. Basically any computer is a heater that so happens is able to do math as well. Energy is consumed for doing a computation but it's 6-7 orders smaller than what the heat produced

Re: The Dark Silicon Problem and What It Means for CPU Designers (2013)

#68
post #44
post #21

Earlier quoted context omitted.

Yes. You can handle 100W on a 300mm^2 chip. Or 300W on a large one, compare AMD Ryzen/ Nvidia Volta (MXM/V100).

this gives 3.46e+5, probably comparable to brown dwarf figures in the tread.

Sounds higher than a nuclear reactor, for which you can find data on nucleate boiling without forced flow to support about 10W/cm^2 without much risk of reaching critical heat flux (in the strict sense, as you then transition to a vapor isolation due to the surface rapidly heating beyond the temperature where nucleate boiling is sustained, and from what I remember/believe, you won't transition back without reducing the surface temperature to be below that at critical heat flux, possibly even lower due to the lack of agitation/uniformity of the vapor layer).

I did some research into what can be archived with suitable dielectric fluids and nucleate boiling to allow much higher heat flux on the surface of the silicon without requiring any energy input to actually separate hot and cold, but allowing the use of pumps and such to provide forced flow on the silicon. It seems that if you have a nozzle to provide sufficient flow speeds across the die you can get higher critical heat flux than can be reasonably handled through the power pins on an AMD EPYC socket, which has some limitations due to the LGA technology used for the contacts. A zero-insertion-force PGA socket should not be bound by this limit of temperature rise at the contact of the LGA spring/pin and the chip contact pad due to resistive heating resulting in the contact pressure declining (due to the spring weakening), which yields a feedback loop that can jump over to a domino effect on the other nearby power pins.

I remember the flow rate being proportional to the critical heat flux as well as the distance the flow from the nozzle has to cross and provide cooling for. There were speeds of iirc. about 20m/s if one were to cool a delidded AMD EPYC at maximum power draw @4GHz, or rather, extrapolating from what can be archived with a die surface temperature of 50 degree Celsius.

So, yeah, the heat flux is actually a problem, but I think one could integrate some sensors and sufficiently fast switches that shut the section of the die off if the temperature reaches dangerous levels (and do so fast enough to not damage anything, i.e. microseconds or what time there is), and use such a test chip to engineer a forced-flow direct-die (or maybe even with a heat spreader on top, but that costs you due to the nucleate boiling not going below 20 kelvin temperature loss, and additional system losses in the radiator/heat exchanger as well as pipes likely resulting in costs of about 5 additional kelvin). This could open the door to operating CPU dies at much, much higher power densities than currently normal. It's like a heat pipe on steroids. One would want circuity in the CPU to rapidly shut down in case the temperature rises quickly, as regardless of why this happens, not doing so will literally blow a hole in the chip before you can drain the inductors providing smooth power to the socket.

Re: The Dark Silicon Problem and What It Means for CPU Designers (2013)

#69
post #59

Earlier quoted context omitted.

The author's sentence is essentially a truism, because fiddling with information _as such_ and in theory doesn't use up any energy. As practically implemented in current CPU electronics, though, information needs to be communicated from point A to point B as a change in voltage. To convey that voltage change means having to move some amount of electrical charge into or out of the tiny capacitor that is a transistor's…

> fiddling with information _as such_ and in theory doesn't use up any energy. that's not true at all. Any time you destroy information (for example an and gate can destroy information) you use energy.

Chances are you know more about this than I do, and I'd be interested to learn more.

If you're willing to explain, I wonder how an AND gate destroys information. Certainly the output of a 2-input AND gate carries less information than both inputs together. But unless the gate is destroying the input signals, which it isn't, I don't see how infomation is being destroyed.

Re: The Dark Silicon Problem and What It Means for CPU Designers (2013)

#70
post #69

Earlier quoted context omitted.

> fiddling with information _as such_ and in theory doesn't use up any energy. that's not true at all. Any time you destroy information (for example an and gate can destroy information) you use energy.

Chances are you know more about this than I do, and I'd be interested to learn more. If you're willing to explain, I wonder how an AND gate destroys information. Certainly the output of a 2-input AND gate carries less information than both inputs together. But unless the gate is destroying the input signals, which it isn't, I don't see how infomation is being destroyed.

https://en.wikipedia.org/wiki/Landauer%27s_principle
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