Why Do Computers Use So Much Energy? • Hacker News Board

Re: Why Do Computers Use So Much Energy?

#111

Post by kragen » Thu, Oct 04, 2018, 7:29 PM UTC

I've been thinking about computer energy use for a while now, although I'm no expert in the area. Here's a comparison of nJ per instruction from my notes:

    |                   |          nJ/insn |
    | MSP430            |              0.9 |
    | PIC24             |               2? |
    | 1990s StrongARM   |                1 |
    | LPC1110           |              0.3 |
    | Pentium           |               10 |
    | STM32L0           |             0.23 |
    | Ickes DSP 2008    |             0.01 |
    | Subliminal 2006   |           0.0026 |

MSP430, PIC24: http://www.ti.com/general/docs/lit/getliterature.tsp?baseLit...

StrongARM: http://www.researchgate.net/profile/Kristofer_Pister/publica...

LPC1110: http://www.nxp.com/documents/data_sheet/LPC111X.pdf

Pentium: http://www.newscientist.com/blog/technology/2006/08/explodin...

Ickes DSP 2008: http://www-mtl.mit.edu/researchgroups/icsystems/pubs/confere...

Subliminal 2006: http://web.eecs.umich.edu/~taustin/papers/VLSI06-sublim.pdf 2009: https://web.eecs.umich.edu/~taustin/papers/TVLSI09-sublimina...

This is sort of comparing apples to oranges. The Pentium (all of them) uses wildly varying amounts of power for different instructions, and has 32-bit or 64-bit instructions, with hardware floating point. The STM32L0, LPC1110, and StrongARM are 32-bit processors with no hardware floating point. The MSP430 is a 16-bit CPU, while the PIC24 is an 8-bit CPU. The Ickes et al. device includes a 16-bit FFT accelerator and only runs at 4MHz, but it was only fabricated as a prototype; you can't buy it. The Zhai et al. Subliminal device, also only fabricated as a prototype, only runs at 833kHz, and it doesn't even include an integer multiply instruction, but its somewhat limited ALU is 32 bits.

However, _all_ of these numbers are far from the Landauer bound (kT ln 2). Suppose that we don't make any concessions to reversibility in our CPU design, like Metronome and its successors. A 32-bit instruction, then, erases the 32-bit register where its result is stored, costing 32 kT ln 2, or in some situations, an average of 16 kT ln 2. Supposing T = 300 K, 32 kT ln(2) ≈ 0.092 attojoules. That's _over seven orders of magnitude_ better than the prototype Subliminal processor mentioned above, and nine orders of magnitude better than the Cortex-M0-based commercial processors mentioned above.

Wolpert has published https://arxiv.org/pdf/1806.04103.pdf (mentioned in the article, but not linked AFAICT) which gives better expressions for the cost of computation than the Landauer bound. I haven't finished reading it, but it looks really interesting.

Re: Why Do Computers Use So Much Energy?

#112

Post by mcbits » Thu, Oct 04, 2018, 7:33 PM UTC

peeters wrote: post #105

cfallin wrote: post #88

Earlier quoted context omitted.

Why not, though? Energy is energy -- we (humans) oxidize food, basically burning it slowly, to power our bodies. Whether the energy is carried by glucose/ATP or electric current is just an implementation detail. Comparisons are frequently made in other domains as well -- e.g., cyclists measure power outputs in watts. The rule of thumb I've understood is that the brain runs on about 20 watts (peak power), and a human…

One is saying how much computers use of a global resource pool, the other is how much brains use of their host body. They're completely different things so it doesn't make sense to compare those two percentages. If you wanted to compare them, cite the percent that the CPU uses of its PSU, or cite the percent that human brains use of global energy consumption. Otherwise they're completely different things that have no…

One is viewing society as a "body" with computers being its "brain". The other is viewing the human body as a "body" with its brain being its "brain". The comparison is of the energy used by each "brain" relative to the total energy used by its "body".

The comparison is valid if you accept the analogy, although I think you could easily dispute the analogy. E.g. you could just as easily view the network of humans in a society as its "brain" and all the things we build (including computers) as its "body".

Re: Why Do Computers Use So Much Energy?

#113

Post by mmt » Thu, Oct 04, 2018, 7:36 PM UTC

cfallin wrote: post #88

simias wrote: post #45

Earlier quoted context omitted.

>So society's computer uses 5 percent and our own, evolved over millions of years, uses up to 20. This comparison doesn't make a lot of sense to me. It's not even comparing apples to oranges, it's comparing apples to the concept of taste. Furthermore these percentages don't really tell you much regarding efficiency, I'm sure there are less developed societies where computers count for less than 5% of all energy consu…

Why not, though? Energy is energy -- we (humans) oxidize food, basically burning it slowly, to power our bodies. Whether the energy is carried by glucose/ATP or electric current is just an implementation detail. Comparisons are frequently made in other domains as well -- e.g., cyclists measure power outputs in watts. The rule of thumb I've understood is that the brain runs on about 20 watts (peak power), and a human…

> Energy is energy

Even this isn't quite true, since it's not fungible, except in the narrow use-case of creating heat[1].

> Whether the energy is carried by glucose/ATP or electric current is just an implementation detail.

This implementation detail means that converting between these forms incurs a tremendous cost (in lost energy) and, perhaps more importantly, is nowhere near instantaneous.

[1] Even the slightly broader use case of heating (e.g. increasing the temperature of a home) can be done with a heat pump, and those don't tend to run off of fungible energy.

Re: Why Do Computers Use So Much Energy?

#114

Post by yunong » Thu, Oct 04, 2018, 7:38 PM UTC

flyGuyOnTheSly wrote: post #54

lucideer wrote: post #12

> Precise estimates vary, but currently about 5 percent of all energy consumption in the U.S. goes just to running computers 30% goes on transport. > the human brain is a computer. This particular computer uses some 10–20 percent of all the calories that a human consumes So society's computer uses 5 percent and our own, evolved over millions of years, uses up to 20. I'm not saying computers can't or shouldn't be made…

My thoughts exactly. When reading the title I thought "They do?" My laptop consumes the equivalent of approximately a single 60w incandescent lightbulb that used to be required just to write on a pad of paper in the dark. It's capable of quite a bit more than that, however. EDIT: Updated power specifications to exact amounts.

You're forgetting the vast network of much more energy-hungry computers your laptop is connected to.

Re: Why Do Computers Use So Much Energy?

#115

Post by hyperman1 » Thu, Oct 04, 2018, 7:38 PM UTC

hyperman1 wrote: post #110

kens wrote: post #72

Earlier quoted context omitted.

Those formulas seem highly unlikely to me. Power consumption is generally proportional to clock speed. And I don't know what m^n even means in this context.

I'm trying to understand what blattimwind is saying here, but this might be a case where deep understanding has suffered from a badly worded post. Just guessing here, by pulling apart what little is in there: * Total power usage has a big fixed parts, e.g. screen power usage going up linear with brightness and screen size. Or RAM power usage goes up linearly per bit just to keep the memory intact. So Let's assume OP…

Update: The link between power and frequency seems to be complicated. See e.g.

https://physics.stackexchange.com/questions/34766/how-does-p...

First answer says P goes linear with f, based on theory.

Second answer goes for cubic or worse based on some deep papers, but a comment says this is dated.

Third answer has a graph from real-world measurements which is clearly worse than linear, and a comment saying there is an exponential part somewhere there too, but it is swamped by linear factors.

WIkipedia, by the way, goes for linear - https://en.wikipedia.org/wiki/CPU_power_dissipation#Sources

I think the most realistic answer I can give is: It is complicated, and OP is a very good nerd sniper ;-)

Re: Why Do Computers Use So Much Energy?

#116

Post by gowld » Thu, Oct 04, 2018, 7:47 PM UTC

mikeash wrote: post #76

lucideer wrote: post #12

> Precise estimates vary, but currently about 5 percent of all energy consumption in the U.S. goes just to running computers 30% goes on transport. > the human brain is a computer. This particular computer uses some 10–20 percent of all the calories that a human consumes So society's computer uses 5 percent and our own, evolved over millions of years, uses up to 20. I'm not saying computers can't or shouldn't be made…

It would make more sense to compare absolute numbers. A brain uses about 20W. A modern computer has similar consumption, but is many orders of magnitude less powerful.

My brain (or even my whole body) can't play HD video.

Re: Why Do Computers Use So Much Energy?

#117

Post by JohnBerea » Thu, Oct 04, 2018, 7:50 PM UTC

peeters wrote: post #107

mikeash wrote: post #76

Earlier quoted context omitted.

It would make more sense to compare absolute numbers. A brain uses about 20W. A modern computer has similar consumption, but is many orders of magnitude less powerful.

A single modern CPU can crunch more numbers in a second than every person living on earth combined. Sure there may be things that it's not capable of, but most things it is capable of, it's usually 8-10 orders of magnitude more efficient at.

The brain still tops every CPU at visual processing, which may be its highest throughput interface.

Re: Why Do Computers Use So Much Energy?

#118

Post by derefr » Thu, Oct 04, 2018, 8:21 PM UTC

orev wrote: post #66

flyGuyOnTheSly wrote: post #54

Earlier quoted context omitted.

My thoughts exactly. When reading the title I thought "They do?" My laptop consumes the equivalent of approximately a single 60w incandescent lightbulb that used to be required just to write on a pad of paper in the dark. It's capable of quite a bit more than that, however. EDIT: Updated power specifications to exact amounts.

If your laptop consumed that much power it would be too hot to use on your lap and also would probably damage whatever surface you put it on. Just because a power supply has a max rating of X Watts doesn’t mean it’s always using that much. Even then, 200w for a laptop is an enormous amount. Maybe you meant LED bulbs that emit the equivalent light of a 100w bulb? That would be much closer to standard laptop power cons…

> If your laptop consumed that much power it would be too hot to use on your lap

A lot of laptops do actually get too hot (and are too heavy) to effectively be used on laps. Some even require two concurrent power connections from two separate power bricks in order to charge!

Re: Why Do Computers Use So Much Energy?

#119

Post by utopcell » Thu, Oct 04, 2018, 8:38 PM UTC

The energy consumed by the US is less than 0.1% of the energy the planet gets from the Sun, so I'm not sure the 5% comparison holds water. Besides, computers return their energy cost many times over.

I do agree with the article's conclusion though: the human brain runs at 20W, on par with a low-power laptop, but arguably the former does much more with that energy.

Re: Why Do Computers Use So Much Energy?

#120

Post by voxl_ » Thu, Oct 04, 2018, 8:50 PM UTC

flyGuyOnTheSly wrote: post #54

lucideer wrote: post #12

> Precise estimates vary, but currently about 5 percent of all energy consumption in the U.S. goes just to running computers 30% goes on transport. > the human brain is a computer. This particular computer uses some 10–20 percent of all the calories that a human consumes So society's computer uses 5 percent and our own, evolved over millions of years, uses up to 20. I'm not saying computers can't or shouldn't be made…

My thoughts exactly. When reading the title I thought "They do?" My laptop consumes the equivalent of approximately a single 60w incandescent lightbulb that used to be required just to write on a pad of paper in the dark. It's capable of quite a bit more than that, however. EDIT: Updated power specifications to exact amounts.

The contrast! My laptop can consume as little as ~3-4w when I'm just scrolling a webpage.