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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)

#31

Post by jacquesm »

blackflame7000 wrote: post #23
jacquesm wrote: post #13

Earlier quoted context omitted.

You can't cool from the inside of that cube without having some way of transporting the heat out of it. All you'd end up doing is heating that inside up to the temperature of the dies and after that there would be no more cooling effect (and this would happen in a few seconds after starting the whole thing up). You could do an 'inverse' of this by cooling the dies from the outside and having the interconnects in the…

What if you used electronic cooling to chill a copper thermal conductor?

Electronic cooling? Do you mean Peltier elements? They have a 'hot' and a 'cold' side, and their capacity is handily outstripped by any modern CPU. If that worked we would be using Peltier coolers to make quiet PCs today, after all, that problem is only 1/5th as complex.

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

#32

Post by rwmj »

woah wrote: post #5
asfasgasg wrote: post #3

Earlier quoted context omitted.

If I understand correctly, neither heat dissipation nor existing manufacturing techniques are amenable to this approach. Also modern CPUs do have more than a dozen layers IIRC.

I think op is talking about 6 flat normal chips as the sides. This would allow for cooling stuff inside the cube. Maybe having only 5 of the sides as chips would make it even easier to have a heat sink. The center of the cube could be copper or something.

Another problem is that companies (though not necessarily users) love thinner and thinner phones and laptops, and a cubic CPU wouldn't fit in such a device.

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

#33

Post by wolfgke »

blackflame7000 wrote: post #2

Why don't they start making CPUs 3 Dimensional like a cube with 6 "processors" each with multiple cores as its "sides" with the pins on the opposite sides of the cube wall. Seems to me more internal volume might allow for more cleaver head distribution channels

> Why don't they start making CPUs 3 Dimensional like a cube with 6 "processors" each with multiple cores as its "sides" with the pins on the opposite sides of the cube wall.

Not directly an answer to this specific question, but a (somewhat) colleague who writes his PhD thesis about 3-dimensional chip design made a popular scientific lecture about this topic. As I understood it, the central problem is that it is very hard to produce chips with multiple (lots of) layers (where you want to have interconnects inbetween). In particular producing the interconnects between the layers is really hard if they can lie "everywhere" instead of only at the border. There exist multiple ideas how this might be done (e.g. drill holes with high-precission lasers into the substrate and try to fill them with something conductive), but none of them as of today "really works" (at least if we are talking about chips with somewhat more than 4 layers and interconnects everywhere inbetween).

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

#34

Post by Tuna-Fish »

a_wild_dandan wrote: post #15
Phillipharryt wrote: post #7

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?

(Intensity) = (Power) / (Unit Area) (Sun's Radius) = 695 x 10^6 m (Sun's Power) = 4 x 10^26 W (CPU Output) = 75 W (Die Size) = 37 mm x 37 mm (Sun Intensity) = (4 x 10^26 W ) / (4 x pi x (695 x 10^6 m)^2) ~ 66 x 10^6 (CPU Intensity) = (75 W) / (0.0014 m^2) ~ 53 x 10^3 I'm getting a several orders of magnitude higher W/m output for the sun. Perhaps I made an algebra mistake?

Your CPU is huge. For example, the highest-performance single-chip Pinnacle ridge (AMD Ryzen 2 2700X) is 213 mm² and has a TDP of 105W. That's roughly an order of magnitude difference.

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

#35

Post by ars »

Phillipharryt wrote: post #7

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?

From:

http://www.wolframalpha.com/input/?i=Sun+%7C+luminosity+%2F+...

The sun is: 6300 W/cm^2

> inside of a nuclear reactor

Inside means volume? That's not the same unit, so I'm not really sure how to calculate that.

Interestingly, on a per-volume bases, the sun's output is quite low. The sun is just very very large.

See: http://www.wolframalpha.com/input/?i=Sun+%7C+luminosity+%2F+...

And you find the sun is only: 1.383×10^-6 horsepowers per gallon :) Or if you insist: 0.2725 W/m^3, which as you can see is really really low.

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

#36

Post by ars »

andlier wrote: post #9
Phillipharryt wrote: post #7

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?

After a quick napkin calculation+google it seems the sun has around 20kW of power per square centimeter. So not entirely unfeasible that a cooler star, or a nuclear reactor is closer to the typical 10-100W/cm2 of a modern cpu/gpu. Still some orders of magnitude off from our closest star. (Hope calculation is correct)

> 20kW of power per square centimeter

I'm getting 6300 W/cm^2 (see: https://news.ycombinator.com/item?id=17589371)

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

#37

Post by Cybiote »

andlier wrote: post #9
Phillipharryt wrote: post #7

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?

After a quick napkin calculation+google it seems the sun has around 20kW of power per square centimeter. So not entirely unfeasible that a cooler star, or a nuclear reactor is closer to the typical 10-100W/cm2 of a modern cpu/gpu. Still some orders of magnitude off from our closest star. (Hope calculation is correct)

The temperature of the sun at its surface is ~5772 Kelvin. To get power per unit area, use Stefan-Boltzman: σ * T^4, σ * (5772 K)^4 ≈ 6294 W/cm^2. Dividing the sun's luminosity (power) by its surface area will also give a similar value.

A 815 mm^2, 250 W GPU will be 250 W / 8.15 cm^2 ≈ 31 W / cm^2.

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

#38

Post by frozenport »

Cybiote wrote: post #37
andlier wrote: post #9

Earlier quoted context omitted.

After a quick napkin calculation+google it seems the sun has around 20kW of power per square centimeter. So not entirely unfeasible that a cooler star, or a nuclear reactor is closer to the typical 10-100W/cm2 of a modern cpu/gpu. Still some orders of magnitude off from our closest star. (Hope calculation is correct)

The temperature of the sun at its surface is ~5772 Kelvin. To get power per unit area, use Stefan-Boltzman: σ * T^4, σ * (5772 K)^4 ≈ 6294 W/cm^2. Dividing the sun's luminosity (power) by its surface area will also give a similar value. A 815 mm^2, 250 W GPU will be 250 W / 8.15 cm^2 ≈ 31 W / cm^2.

Doesn't that only include radiated heat? If I touched it, it would be hotter.

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

#39

Post by bogomipz »

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?

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

#40

Post by bogomipz »

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?

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