'Reversible' Computers More Energy Efficient

James Clark writes "As Congress continues work on a federal energy bill, a group of University of Florida researchers is working to implement a radical idea for making computers more energy efficient -- as well as smaller and faster." Reversible computing rears its head again.

Comments

Vaporware?

[Carnildo]

Has anyone ever built even a very simple reversible computer? Or is this like quantum computers: all theory, no practice?

Re:Vaporware?

[nestler]

This is more practical than quantum computers because it is much easier to build and can be used for general purpose things other than search and factoring.

The idea is to (down at the gate level) keep everything reversible. For example, current OR gates are not reversible (given a true output you can't definitively tell what either input was individually). If you have two outputs on the gate instead of one, you make the gate reversible. However, since you are just using it for OR, you are free to ignore the second bit you added on to make it reversible.

The bit doesn't help your computation in the sense of the answer you are looking for, but it can make things more energy efficient at the gate level.

Re:Vaporware?

[stoolpigeon]

The answers to your questions are in the article - here:

Frank, who first worked on reversible computing as a doctoral student at the Massachusetts Institute of Technology, heads UF's Reversible & Quantum Computing Research Group. Among other recent publications and presentations, he presented three papers dealing with topics related to reversible computing this summer, including "Reversible Computing: Quantum Computing's Practical Cousin" at a conference in Stony Brook, N.Y.

and here:

Frank currently is trying to persuade major chipmakers to direct more of their research-and-development resources toward reversible technologies.

Re:Vaporware?

[randyest]

I think you completely misunderstood the article, though in your defense it didn't do a very good job of explaining. The idea is not to be able to reverse logical operations -- that is of little value to anyone. Rather, they're trying to make the electrical changes (the energy transfer) reversible. That's a fundamentally differeent thing. A decent analogy, mentioned in the article is:

The concept is somewhat analogous to hybrid cars now on the market that take the energy generated during braking and recycle it into electricity used to power the car.

So, the logical realm is no different here. Physically, and electrically, there is a big difference from existing computers. Now, when a bit changes from 1->0, the voltage (accumulated charge) is simply shorted to ground (via resistive path that dissipates heat). That energy is lost. In a reversible computer, that charge would be stored, in the electrical equivalent of a spring or flywheel in a mechanical system. So, next time it needs to go 0->1, the energy is sitting there, ready to be re-used(stored in the spring's compression or flywheel's rotation).

I assume these electrical "springs or flywheels" need to be phsycally close to the transistors they're storing energy for. If all transistor's storage were common, the heat loss (and time delay) to get the energy back to where it's needed would defeat the entire purpose.

In the article, they mention that current prototypes use oscillators to store the energy (which are more like a flywheel than a spring, to continue the mechanical analogy), but the efficiency is not quite good enough to be called "reversible". Too much energy is lost in storing and un-storing the energy. The current work is focused on improving the efficiency of storing and un-storing energy from state changes.

However, as a chip designer, I know that oscillators are usually (1) much much bigger than simple logic gates and (2) much more difficult to design with (it's analog design stuff, really). So, my concerns are (1) how much bigger will dice need to be to use this system (linear increase in die size equals exponential increase in manufacturing cost) and (2) how much longer is it going to take to close a design with all those little analog cells all over the place.

I don't even want to think about the implications for STA (static timing analysis) or LVS (layout versus schematic verification) -- it makes my head hurt. :)

Re:Vaporware?

[cgb8176]

I think you completely misunderstood the article, though in your defense it didn't do a very good job of explaining. The idea is not to be able to reverse logical operations -- that is of little value to anyone. Rather, they're trying to make the electrical changes (the energy transfer) reversible. That's a fundamentally differeent thing.

Actually, you are wrong, in that the two things are very intimately related. I will assume that, as a chip designer, you are aware of what AND, OR, and NOT gates are, and that NAND is an example of a universal gate. NAND, however, is not reversible; you cannot in general determine the inputs by looking at the output.

The Fredkin Gate is an example of a reversible gate. As it happens, it is impossible to do reversible computing with two input gates. The Fredkin Gate (a controlled swap; two inputs, two outputs, and a control wire that passes through) has the property that it is

reversible (Fredkin inverts Fredkin), and

it has the same number of non-zero outputs as it does non-zero inputs.

To achieve reversible computing, you need reversible gates. Furthermore, with reversible gates, you can perform any computation with an arbitrarily small amount of energy; the catch is that you need more time (see adiabatic circuits, Carnot engines).

Re:Vaporware?

[Bingo Foo]

Sorry, but reversible computing is about having N distinct ouputs for N distinct inputs in any logical operation. Think thermodynamics and statistical mechanics, where reversibility is intimately coupled with "no production of entropy" which means "no loss of information."

It is at the information theory and logic level of description where reversible computing must be implemented.

Re:Vaporware?

[Chris Burke]

Furthermore, with reversible gates, you can perform any computation with an arbitrarily small amount of energy; the catch is that you need more time (see adiabatic circuits, Carnot engines).

Hey, thanks for the keywords. Google turned up lots of nice stuff.

Though that catch is rather a big one. According to the links, as E->0, T->infinity, which I don't like one bit. Arbitrarily low power, but arbitrarily lengthy computation.

So I've now got my own low-power logic idea. I call it the "apathetic circuit", and it works by not doing the computation at all. Same zero energy/infinite time tradeoff, but with the advantage that the basic "meh" gate can be arbitrarily small even to the point of zero area! :)

Re:Vaporware?

[tho 1234]

Are you really a chip designer? 1- In CMOS technology (or any other logic type used in the last 20 years) there is absolutely no resistive path to ground. (except for gate leakage) Two complementary (the C in cmos) PMOS and NMOS transistors are used to eliminate the need for any resistive branch. 2-voltage is not "accumulated charge", its a difference in potential energy. Changing voltage levels in itself does not cause any power to be lost, and a logic level 0 certainly isn't produced by shoring vcc to ground. Power is consumed in a logic circuit precisely because it stores energy- the capacitance of the transistor causes charge to be stored and later released to ground whenever the voltage level changes. Power consumtion in chips has been reduced over the last 30 years by reducing the amount of stored energy- by making transistors smaller and reducing capacitance. Technically, you can create an oscillator by adding an inductor to the circuit, but that would increase complexity/cost with little benefit in itself. I am not familiar with reversable computing, but i would expect they would need a substantial change in logic structure to extract the stored energy. Also, without a change in logic structure, it would simply be a process improvement, not an entirely new branch of computing. Yes, analog design is more involved than digital design, but the layout and composition of transistors, capacitors, interconnects, etc on any digital logic circuit are analog in themselves. THe only reason you don't have to deal with them is they are generated automatically with CAD tools. If resonators were found to be a viable way to decrease power consumtion, they could be easily added to your CAD tools and make their design just as simple as what you're used to. However, i'm sure its not as simple as adding a resonator to the circuit, and most likely requires an entire new method of computing to extract the stored energy.

Re:Vaporware?

[randyest]

0) Yes, for the last 10 years. 1) There is leakage through the substrate, and that path is resistive -- though you're correct that the majority of the current flow is not really to "ground" (which, BTW, is a relative thing), rather use in charging/discharging parasitic capacitors -- the relevent fact is that switching dissipates power in the form of heat. Sometimes simplifications are necessary to get a point across. If you want to be a pedant, we can note that there are no 1's or 0's, whip out our detailed SPICE models to explain an AND gate, and worry that there are no "holes", just electrons, though we assume there are in our models, etc. Were this "EEdot", I might have been more precise, but probably not, since it doesn't matter for the understanding of this issue. 2) You can't have a potential difference without some charge accumulation -- chicken: meet egg. This is semantic.

Bah, I can't keep up that formatting. :). As for inductors: they're really hard to make out of silicon. I'd love to hear about how you'd implement that without using way more wiring tracks than it's worth.

You're close on that last point -- the reason I don't have to deal with analog design (too much) is not that my cad tools do it, rather that we can (safely) simplify our models of logic gates down from the complex analog circuits they are and treat them as digital logic. Accounting for crosstalk, transition times, wire delays, signal integrity, etc. outside of the primary design flow allows us to maintain this gross simplification throughout most of the design flow. EDA tools that can handle a new idea usually follow initial real-world implementations by at least a few years. Check out the hierarchical design tools available over the last 5 years, then look at the hierarchical tapeouts for the last 5 years. The tools were (and are, largely) crap. But the chips work (each of mine included).

One of my favorite design tools is Perl. :)

Re:Vaporware?

[onomatomania]

1- In CMOS technology (or any other logic type used in the last 20 years) there is absolutely no resistive path to ground. (except for gate leakage) Two complementary (the C in cmos) PMOS and NMOS transistors are used to eliminate the need for any resistive branch.

Yeah, no shit sherlock. Just because there are no explicit resistors drawn in the circuit doesn't mean that the stored charge isn't dumped to ground through a resistive path. When the NMOS gates turn on, they're effectively shorting the stored charge in the load capacitance to ground through the ON resistance of the gate. And similarly, when the PMOS gates turn on, they charge the load capacitance through the supply rails in an analogous manner.

So just because there aren't explicit resistors (thanks to complimentary logic) doesn't mean that the charge isn't effectively being just supplied to a temporary store and then dumped to ground though resistive paths, which is what the original poster was saying.

Reversing entropy?

[oGMo]

IANAP, but this sounds like trying to reverse entropy as much as possible to me. Won't it take more energy to do a reverse computation than you'll save? Where does the lost energy from that go?

Re:Reversing entropy?

[Carnildo]

There are two theories about energy usage during computation. One is that moving and transforming data requires energy. The other is that the only operation that requires energy is destroying data. Reversible computing subscribes to the second theory, so a reversible computer would not actually use energy to do computations (apart from the inevitable inefficiencies). Since there is no net energy usage, the net entropy neither increases nor decreases, and the second law of thermodynamics doesn't apply.

Re:Reversing entropy?

[plastik55]

It's not strictly "data" as you're accutomed to thinking about it as a bunch of ones and zeroes, but "information" in the sense of information theory--more or less another word for entropy.

The actual measurement of entropy has to do with counting the possible states that a system could be in. A computer containing a list of numbers in its memory could be in any of a large number of states depending on what you know about the list of numbers and the contents of the rest of its memory. If you instruct it to go replace the list of numbers with its sum, the number of states it could be in afterwards is decreased. So its entropy has gone down.

Thermodynamics says that the only way to reduce a system's entropy is to expend energy on it. So if you worked out a way to juggle the bits in the list of numbers around so that you would get the sum, but in such a way that you could back out the operation afterwards to recover the original list, and do it without overwriting any of the other information on the system, then you could do the operation without reducing the entropy of the computer, and wouldn't be forced to expend energy.

Now, a lot of confusion comes from the fact that Shannon decided to call mutual entropy "information" when he was working out coding theory. The concept has a lot of parallels to how we ordinarily think of information, but the correspondence isn't exact, and trying to think of mutual entropy as "information" informally will lead you to a lot of wrong conclusions. It's one of those all-too-common instances where picking a commonly understood name to stand for a subtle concept can do more harm than good.

Re:Reversing entropy?

[kfg]

You are not a real idiot. You have asked a deep question. Pity those around you choose to cover their own ignorance by being arrogant.

Me, I like to display my ignorance, so here I go:

The computer is just a big abacus really, a physical model of the data. When you shake up an abacus it still has just as many beads on it, only their state has changed, and the now random ordering of the beads can still be read as representing a number. What has been lost is meaning.

Is "2" data? (bearing in mind that we're talking about the logical number here and not the numeral that serves as its physical model)

No. It's just a number. Unless the number relates to a logical model (the number of quarts of milk in my refridgerator) it isn't data.

So the state of your all shook up (uh uh huh, uh uh huh, yeah, yeah)abacus is still a physical reprentation of a number, but it isn't data.

When we run code we keep just as many "beads on our abacus," only their state changes, but data is the physical state and what they mean in the logical model. So if we lose meaning we have lost data or if we lose the proper state to reflect that meaning we have lost data.

So to "destroy" data means to dispose of the physical model and/or its meaning in the logical model.

When we change the state of a computer we certainly don't change the total amount of data, but we certainly change its state and that state's relationship to the logical model.

Take Linux in a Nutshell off your shelf (a physical representation of data) and burn it. Have you not destroyed data? Now fill the same space on the shelf with a gardening book.

You are now still in possession of the same amount of data as you had before, but both its state and meaning have changed. You aren't going to be able to use that book to look up a vi command because the gardening book doesn't contain that meaning.

You destroyed that data.

KFG

"Reversible" a bad name?

[cant_get_a_good_nick]

Wouldn't "regenerative", like regenerative braking on most electrics/hybrids been a better term?

Re:"Reversible" a bad name?

[Anonymous Coward]

This is an area where information theory and physics meet. To minimize heat you must minimize entropy production and that means you must in fact make your computations reversible, as much as possible.

Sounds good, but...

[Tin Foil Hat]

I have to admit that I'm no chip designer, but I have to wonder why this hasn't been done before? What are the problems with this technic?

It sounds good, but what's the catch?

Re:Sounds good, but...

[pclminion]

Because it seemed totally pointless. It was a theoretical curiosity.

People started looking at reversibility in earnest when quantum computing came on the scene. A quantum computer HAS to be reversible in order to function. That made it a very important field of study.

We only recently realized that reversible circuits are also more energy efficient. So basically, we didn't do it before because we didn't know. There is no "catch."

How exactly am I supposed to ...

[burgburgburg]

fry eggs if this sort of thing becomes the norm?

Insensitive clod!

sort of like recycling....

[smd4985]

your computer could spit out: "these CPU cycles made of 75% post-CPU-consumed waste" :)

Something to worry about...

[Nom du Keyboard]

In fact, unless reversible computing is achieved, computer chips are expected to reach their maximum performance capabilities within the next three decades

Boy, that's something to worry about today. I'll just have to find a spot to insert it on my Worry List. Maybe I can drop Global Warming to make space.

Re:Something to worry about...

[avgjoe62]

So, what do you plan to do with your 3 exahertz Pentium 17?

Why, play DukeNuke'em Forever, of course!

Sigh... I knew I shouldn't RTFA

[winkydink]

Here I thought it was an Intel box that, when turned inside out, became a Mac.

Sigh.

Wrong tagline...

[Anonymous Coward]

Should be "Reversible computing rears its butt again"

From the article...

[TamMan2000]

While he was at MIT, Frank worked on a team that built several simple prototypes of reversible chips.

It has at least gotten to the chip level so far...

Thermodynamics 101

[majid]

You get the most energy efficiency from a machine when it works in a thermodynamically reversible way, for instance the most efficient thermal motor possible is one that uses a Carnot cycle. Most real-world engines use different, less efficient cycles like the Otto or Stirling cycle because they yield higher speeds or torque.

Losing the ability to reverse computations means increasing entropy and thus lower efficiency. Interestingly, there is a whole class of functional programming methods that is intrinsically reversible (because evaluating expressions without side effects is reversible).

The best explanations of the issues involved is in Richard Feynman's "Lectures on Computation", that show how thermodynamics constrain what is ultimately possible with a computer.

Re:Thermodynamics 101

[iggymanz]

funny this topic has popped up again after I saw it featured in Scientific American over 10 years ago....the real problem is that no one wants to halve the number of useful gates on a chp in order to bulid all the extra circuitry required to reduce (of course, not eliminate, entropy still will increase though at lessened rate) the thermodynamic cost of "forgetting" data.

I attack instead the basic premise, that there is a shortage of energy, or that we must accept lower standard of life or lower capability in our machinery. What we DO need to do is get smarter about where we get our energy - instead of adding to net heat budget and pollution budget of earth getting really serious about solar energy (which might just mean making hydrocarbon fuel out of plant & suitable waste materials)

Re:Thermodynamics 101

[Theaetetus]

I attack instead the basic premise, that there is a shortage of energy, or that we must accept lower standard of life or lower capability in our machinery. What we DO need to do is get smarter about where we get our energy - instead of adding to net heat budget and pollution budget of earth getting really serious about solar energy (which might just mean making hydrocarbon fuel out of plant & suitable waste materials)

Um, that's not the basic premise. The basic premise is that with each bit of information lost, that bit is converted to heat. More bits lost = more heat = limit on how fast a processor can be due to temperature-caused failures. Removing that problem results in much faster theoretical limits on processors.

-T

Re:Thermodynamics 101

[greg_barton]

I attack instead the basic premise, that there is a shortage of energy, or that we must accept lower standard of life or lower capability in our machinery.

Then you are attacking a straw man, at least with respect to the article. A reversible chip would be no less capable. In fact, in the long run, it would be more capable. Less energy heat dissapated means we can continue to use the same materials far into the future with faster and faster chips. As it stands silicon will become ununsable once the heat dissapation reaches the melting point of silicon. (Far sooner, actually.)

Now, I'd love to see chips made from artificial diamond, but I think reversible chips will come sooner.

Read the Feynman book

[rarose]

http://www.amazon.com/exec/obidos/tg/detail/-/0738 202967/103-2222180-5559862?v=glance

He has a great deal of info about how reversable computers work and why they save energy.

Re:Read the Feynman book

[MillionthMonkey]

I second this. Feynman's lectures on computation are at a very fundamental level, so they are impractical for day to day use, but the theory is solid. Thousands of years from now, computers will undoubtedly have changed a lot, but the principles in this book will still apply to them since they merely describe how the laws of physics affect any computational system.

Here is an interesting excerpt on pages 149-150 that explains Maxwell's demon in terms of reversible computing:

The demon has a very simple task. Set into the partition is a flap, which he can open and shut at will. He looks in one half of the box (say, the left) and waits until he sees a fast-moving molecule approaching the flap. When he does, he opens the flap momentarily, letting the molecule through into the right side, and then shuts the flap again. Similarly, if the demon sees a slow-moving molecule approaching from the right side of the flap, he lets that through into the side the fast one came from. After a period of such activity, our little friend will have separated the fast- and slow-moving molecules into the two compartments. In other words, he will have separated the hot from the cold, and hence created a temperature difference between the two sides of the box. This means that the entropy of the system has decreased, in clear violation of the Second Law!

This seeming paradox, as I have said, caused tremendous controversy among physicists. The Second Law of Thermodynamics is a well-established principle in physics, and if Maxwell's demon appears to be able to violate it, there is probably something fishy about him. Since Maxwell came up with his idea in 1867, many people have tried to spot the flaw in his argument. Somehow, somewhere, in the process of looking for molecules of a given type and letting them through the flap, there had to be some entropy generated.

Until recently, it was generally accepted that this entropy arose as a result of the demon's measurement of the position of the molecules. This did not seem unreasonable. For example, one way in which the demon could detect fast-moving molecules would be to shine a demonic torch at them; but such a process would involve dispersing at least one photon, which would cost energy. More generally, before looking at a particular molecule, the demon could not know whether it was moving left or right. Upon observing it, however this was done, his uncertainty, and hence entropy, would have reduced by half, surely accompanied by the corresponding generation of entropy in the environment.

In fact, and surprisingly, Bennett has shown that Maxwell's demon can actually make its measurements with zero energy expenditure, providing it follows certain rules for recording and erasing whatever information it obtains. The demon must be in a standard state of some kind before measurement, which we will call S: this is the state of uncertainty. After it measures the direction of motion of a molecule, it enters one of two other states- say L for "left-moving", or R for "right-moving". It overwrites the S with whichever is appropriate. Bennett has demonstrated that this procedure can be performed for no energy cost. The cost comes in the next step, which is the erasure of the L or R to reset the demon in the S state in preparation for the next measurement. This realization, that it is the erasure of information, and not measurement, that is the source of entropy generation in the computational process, was a major breakthrough in the study of reversible computation.

Re:Cool

[Have Blue]

Energy is equivalent to WORK. Information is a static property of a configuration of elements.

What's going on here is a circuit implementation detail. In a normal chip, when you have a bit set to 1 and a bit set to 0 and you flip them both, the bit set to 0 is charged with fresh energy from the power supply and the energy in the bit set to 1 is converted to heat. In this proposed system, the charges would be moved from the 1 to the 0 with no loss and no additional draw on the power supply. Less work, same informational content.

HOW does it make it more efficent?

[autopr0n]

I mean, say you have a CMOS OR gate. If both of the inputs are high, then the NMOS transistors will close and the PMOS transistors will open. Energy is lost only when electrons 'leak through' when the gate changes (and of course, electrons that leak through but don't affect the computation, which I guess happens all the time). How would reversing the computation affect this? Maybe if you were using plain PMOS or something...

Re:HOW does it make it more efficent?

[mikeee]

Well, there isn't exactly a how. Thermodynamics guarentees us that non-reversable gates will use energy. It doesn't guarentee that it's possible to build a reversable one that doesn't; it does guarentee that any gate which doesn't lose energy is reversable.

Re:HOW does it make it more efficent?

[randyest]

Enegy is lost always (leakage current) because the gate is not a perfect insulator. The smaller the gates, the more ther leakage. This is called static power.

Energy is also lost during switching, as the charge needed to switch is moved around. This is called dynamic power.

Reversible computing endeavors to reduce/eliminate dynamic power. It does nothing for static power. A long time ago, dynamic power was dominant and static power was negligble. Now, gates are so small, static power is approaching the same order of magnitude as dynamic.

So, even though they're only addressing about 1/2 of the problem, it would be great to have the magnitude of that big problem halved.

Re:What about cars?

[Jerf]

And if it is realistic, then how much more powerful are oil companies politically than electric power companies that the latter are going to just stand by and let this happen?

You mean, the power companies are going to force Intel to make their chips more wasteful, causing progress to halt and people to buy fewer Intel chips? Yeah, sure.

I mean, there's paranoia, and there's paranoia.

Come on, wake up. I won't claim that kind of thing never happens but by and large capatalism is too powerful; Intel isn't going to act against its own best interests for any mere money the power companies can throw at it, because it won't be worth it. Growth is worth more then mere money to Intel. (If you don't understand why, go learn about business; the explanation is too complicated for a Slashdot posting.)

The power company is made of people like you and me; far too busy to hover over various scientific journals and swoop around like super-villians repressing "dangerous" information.

Re:What about cars?

[Carnildo]

Even a perfect internal combustion engine can't be more than about 25% efficient, because of the nature of heat engines. Cars are already getting pretty close to this limit, so any improvements to fuel efficiency will come from techniques like lighter-weight vehicles, better aerodynamics, and techniques like hybrid engines that let the engine run at top efficiency all the time.

a practical use

[MrLint]

Imagine a computer that ran on heat and got colder the more you used it.. then i could pay video games and have ice cold beer.

Oh thats not what they mean by reversible? Damn

Photographs of "a very simple reversible computer"

[Dlugar]

Universiteit Gent has some pictures of reversible logic gates, including a four-bit adder composed out of Feynman's "NOT, the CONTROLLED NOT, and the CONTROLLED CONTROLLED NOT" reversible logic gates, and some other circuits they've built.

They also have links to other sites about reversible logic and reversible computing, such as Ralph Merkle's Reversible Computing page (from Xerox).

Also note the bottom of the page: there's a vacancy in the research group, for all those just aching for a chance to work on reversible computing! (Looks like you'll have to speak Dutch, though.) ;-)

Dlugar

Re:Cool

[hchaos]

Yet more evidence that information is in fact a quantifiable property. We're starting to see hints that information and energy are flip sides of the same coin.

I'm not just spewing. There are serious theoretical problems associated with how information "disappears" when it falls into a black hole. Fortunately, you get the information back again from Hawking radiation, as the hole converts mass into energy. From a theoretical standpoint it's really starting to look like "information == energy," or to put it more precisely, there is a specific equivalence between information and energy like the equivalence between matter and energy.

Actually, you are just spewing, at least kind of. As long as the Second Law of Thermodynamics holds true, there is no "conservation of information" law in this universe.

What's really happening here is a lot more simple. In a digital computer, information is stored as a series of energy states. A bit is either 1 or 0, with a 1 meaning that a circuit is energized, while a 0 means that the circuit is not energized. The important thing is that both energized and non-energized circuits hold exactly the same amount of information.

The only thing that this article is talking about is storing the energy from the energized bits in an "energy cache" once the 1 has been switched back to 0, so it can then be used to power other bits. It's really not a very radical idea at all. The only semi-radical thought here is that it would be worthwhile to recover this energy, and that chip manufacturers would benefit from investing in this research.

Basic Problems with Reversible Computing

[reaperbean]

Reversible computing is severly limited in terms of normal processor operations. This means that operations such as modular multiplication start to build up a lot of data since you need to 'remember' the two number multiplied in order to undo the operation.

Consider multiplying two numbers, a and b. So a * b = c. Now to undo the operation you only need c and either a or b. So with normal multiplication (or addition, etc) you have two inputs as such and you need to remember two outputs. This gets worse with modular multiplication (depending on the exact set up) since you may need to remember a, b, and c to undo the operation.

When you think of standerd computer operations, most of them are lossy. The problem with reversible computing is coming up with algorithms that are reversible and still useful. This is the case with quantum computers -- quantum operations are not allowed to lose info, so they are reversible. The most famous quantum algorithm, Shor's Algorithm, will factor very large integers quite easily on a quantum computer. It is actually a probabalistic algorithm, and quite complicated (and interesting). Although the entire opeartion is not reversible (and hense not all quantum), the key components are indeed reversible. Other than Shor's Algorithm, there are not a whole lot of algorithm's for quantum computers becuase they are reversible by nature, and, as such, are limiting to work with.

I agree with the author of the article that more research should be done on reversible chips, algorithims, etc. However, I feel that people should understand the limitations inherent in such a system.

Huh?

[autopr0n]

Thermodynamics also says that you lose non-heat energy in reversable systems as well. If you throw a ball into the air, you lose some energy from wind resistance, from converting chemical energy in your arm into mechanical energy, etc.

Re:Huh?

[Aardpig]

Thermodynamics also says that you lose non-heat energy in reversable systems as well. If you throw a ball into the air, you lose some energy from wind resistance, from converting chemical energy in your arm into mechanical energy, etc.

Sure, but mechanical losses can always be recovered and put back into a system. Heat losses can't, which is the point of the second law of thermodynamics.

For more info...

[Niten]

You can find more information about Dr. Frank's research on his homepage.

Stirling engine

[bs_02_06_02]

Isn't it just easier to use the excess heat to power a Stirling engine to recapture waste energy?
Maybe the Stirling idea is going too far.
How about a more efficient circuit? It's been awhile since college, but isn't excess heat a sign that the circuit is inefficient?
While it's not completely frivolous research, it's not the first avenue I would approach when looking at this problem. It seems more difficult and time-consuming to add in circuitry to re-use the energy to perform other actions inside of a CPU. It seems like you'd have a better chance at compounding the problem, rather than helping it.
However, make the circuit more efficient, you'll generate less heat. That would be my first goal. What kind of efficiency do they get with today's CPUs?

With this reversible thinking, I have an idea. I need a little help from the anti-SUV crowd... wouldn't all gasoline engines be better off with really big flywheels?

Re:Stirling engine

[zymano]

1. Stirling engines are too bulky . While efficient heat engines they are not practical . They have been used once in project by detroit but didn't produce much power. Power is key ,what everyone wants.

2. Circuits compared to reverse logic are probably inefficient but the heat comes from Clock speed and thinner interconnects and poor insulators(dielectrics at present sizes).

This is the reason for the push for spintronic transistors . google that.

Asynchronous Logic will be here first.

[Cordath]

Asynchronous Logic (i.e. no clock) has many of the same benefits, as well as potentially increasing the speed of processors significantly.

A rather large portion of the heat genreated by a processor is just from the clock signal propagating to every bloody logic gate in the mess including the parts not in use. With asynchronous logic, if a part isn't in use, it gets no current. Of course, clock signals have been used for the last half century for a reason. Clock signals are used to time signals so that you don't have 3 digits of a number showing up before the rest, etc. With asynchronous logic you have to worry about path lengths down to the picometer so you don't need the clock to act like a traffic warden. The biggest holdup to asynchronous logic has been the immense design difficulty involved, but that is changing as new design tools are developed.

Anyways, the big reason why Asynchronous logic is going to arrive on the processor scene long before reversable logic is that it already has. Intel and other manufacturers are already incorporating asynchronous logic into their designs, and plan to increase the ammount used as time goes by. The different manufacturing techniques required are slowly being phased in. Reversible computing, on the other hand, has virtually no chance of showing up within the decade.

My point is that the article linked made no allowance for the increasing use of asynchronous logic. It's going to have a significant impact on heat dissipation in the neBuew years.

Re:Asynchronous Logic will be here first.

[mrtroy]

Asynchronous blows for non-trivial computation.

You are correct, the clock signal needs to get stronger/faster as speed increases.

But try designing a whole motherboard using asynchronous design...it would be VERY hard.

Hence why nobody has (that I am aware of)

Clocked is much simpler...

Another benefit of asynchronous would be speed benefits...instead of something taking 1/3 of a clock cycle having to wait, it just finishes when its done.

Re:Asynchronous Logic will be here first.

[Yobgod Ababua]

"Asynchronous blows for non-trivial computation."

Please cite your references or evidence to this statement if you wish to be taken seriously.

Several companies are currently working on complex and high-performance designs using asynchronous techniques. It's true that it is currently more difficult, predominantly because current design tools are all geared towards generating and testing "standard" clocked logic, but it is being done and it does not by any stretch "blow".

It will be quite some time before all of the components on a motherboard are asynchronous, but groups -have- designed processors, memory controllers, and other components in asynchronous.

For but the briefest of examples... check out this article or this article. No, it isn't the answer to everything... but it's much farther along than you seem to realize

don't know exact details

[nestler]

I don't know the exact details of how it is more efficient. It was explained to me once in a quantum computation course (where among other things they were using equations to relate energy to information).

So I don't know how to explain in terms of currents and transistors, but it is similar to what mikee is saying in this thread (that thermodynamic laws say that destroying information will always consume energy).

The reason quantum computation guys tend to know about this area is because all logical operations on a quantum computer (except for the measurement at the end) are reversible operations.

Wait a minute!!! (was Re:For more info...)

[BabyDave]

Frank ... or Frink?

Feynman Lectures on Computation

[John Sokol]

This book by Richard Feynman is based on a series of lectures given at CalTech in the mid 1980s.

In it he discusses Reversible Computation and the Thermodynamics of Computing and quantum computing.

As usual, Feynman was way ahead of his time.

I highly recomend this book.

The basic idea is heat is only generated when information is destroyed. So don't destroy information when performing computations.

How this relates to something actualy practical is hard to say, but it didn't strike me as something that would apply to silicon very easily.

John

Re:Feynman Lectures on Computation

[Alomex]

This book by Richard Feynman is [from the ] 1980s. In it he discusses Reversible Computation and the Thermodynamics of Computing and quantum computing.

As usual, Feynman was way ahead of his time.

Reversible computing had been proposed twenty years earlier by an IBM engineer and widely recognized as an important idea, so one can hardly credit Feynman for this one.

There has been steady research on reversible computation over the last ten years or so. In fact the best paper award at one of the major CS conferences in 1993 was for a reversible computing paper.

Size penalty

[toybuilder]

The problem with reversability is that for any given semiconductor process, it effectively doubles the number of gates that need to be built on the chip, and manufacturers are currently more interested in cramming more features into the chip; not to make them more efficient.

It might be theoretically possible to build smaller and faster chips by reducing the energy/thermal issues, but I suspect most companies are not willing to take that leap of faith.

I bet the first places we'll see reversible gates being used in a full-fledged MCU/CPU would be for a mobile/handheld processor running reversified version of an older (less gates) core using latest processes...

Re:Size penalty

[foniksonik]

What about elegant design? From reading the summary it sounds like they want to do more than just add more gates to 'reverse' the computations... they want to use new design methods such as oscillators and springs to capture and hold the energy as potential which would then be reused when needed in an alternate process.

The point is not to over-engineer for this but to intelligently engineer. it will take more R and D time but will hopefully gain enough to justify the expense.

Re:WTF is reversable computing?

[merlin_jim]

This means, for one thing, that a reversible computer has no concept of boolean AND. Or OR, for that matter

Actually, you can add one additional output to any binary logic gate in order to make it reverseable; most reverseable computing designs focus on that and the logic circuits themselves ignore the secondary output...

Re:Resistance == Heat

[anubi]

Interesting...

So it looks as if we could get rid of the resistance, we could have essentially "perpetual" computing, much like we have essentially "perpetual" current flow in a superconducting ring... except this time with switches directing the electron flow amongst many parallel channels.

I think you are onto the superconducting computer.

I read the main article and was kinda confused about the use of "resonators" to store energy with much less loss. I design a lot of switching power supplies, and I use those techniques a lot to boost the efficiency, as well as reduce stress, on my power supply components. By doing resonant designs, I can use stray capacitances to my advantage, storing their energy in inductors during switching intervals, then re-introduce the stored energy back into the circuit at the proper time to make some really cool power converters.. ( pun intended ).

But here's the problem.. my frequencies are determined by the laws of physics and are either sinusoidal or sinusoidal derivatives. Data is not. I would find it hard to store energy is some sort of inductor, as the energy will bounce back at me in a given time... and if I am not prepared to route the energy in a constructive way when it comes back at me, its wasted, only thing it does then is expend its energy heating up and stressing my switch. I have looked at enough core-dump to know data is not periodic.

It doesn't look like an easy thing to do to try to recover energy from the edges of many switching lines that are all switching at asynchronous times. I would have to know a lot more about this before I could really generate a cogent comment.

Reversable versus Probabilistic Computation

[John.P.Jones]

If you are interested in reversable circuits read what Feynman had to say about them in his lectures on computation.

While they may be helpful for certain things, especially quantum computers (but that is a whole different story) there is a snag. They are deterministic; great CS people like Rabin have taught us the value of probabilistic turing machines and today we use them as the basis of determining what is computationally efficient (BPP, see Michael Sipser's intro to computation and complexity). Every once in a while you have to take a non-reversable step to pick a random number (as well as through away garbage you don't want to store any more) and this negates the thermodynamic advantages of reversible computing.

No Free Lunch

Re:Theory

[TeknoHog]

Entropy S = k ln W
(k = Boltzmann's constant, W = number of states)
Information (in bits) I = log_2 W = ln W / ln 2
Hence S = kI/ln 2 or roughly S = kI.

Heat Q = ST = kTI.

Let's say we destroy 100 gigabits of information at a temperature of 300 K. Since k = 1.38E-23 J/K, this means a heat of about 4E-10 Joules. Which is not very much, and does not really contribute to the heat produced by CPUs.

In fact, I think this is the way to find a theoretical minimum for the heat produced from information processing. We can try and make more efficient processors to get closer, just like we can increase the efficiency of engines to get closer to the thermodynamic limit.

is this possible?

[Major_Small]

i don't know much about processors, but is it possible to send all the unused electricity to a capacitor somewhere nearby on the motherboard, and then draw some of the power for something else (a smaller chip?) from that capacitor?

what I'm thinking is that the CPU does billions of calculations/second, but some other chips don't run as fast and don't need as much power, so they can take what's left over from the CPU and other chips and use some outside energy.

is that possible? like i said, I don't know much about electrical engineering, so I don't even know if it's practical to map a ground pin to a capacitor...

Worst Article Ever

[alexq]

After reading that article very closely, thinking "that doesn't make any sense", thinking some more, reading some other posts, and _finally_ getting the basics of this technology (as badly named as it is)...

I can only suggest that this James Clark seek a career working for SCO's legal department. With his ability to confuse an issue and their desire to do so, it's a match like peanut butter and jelly.

words I understood in the above post.

[Anonymous Coward]

you
both
close
guess

You want to save *how* much electricity???

[stienman]

You want to cut back on the 100W of heat being released by today's processors?

100W?

I piss 100W when I get up in the morning.

100W will cost you $79 [US] a year if you run it hard and constant every second 24/7/365. ($0.09 per KWH)

In the US, each average family has more power, more cheaply than some cities in other parts of the world.

Furthermore, the energy is still going to be released as heat at some point. Where else does it go??? Sure, you might be able to switch a given transister 3-4 times with the same energy, but once it drops in voltage and current, the transister no longer switches. Furthermore the chips are already being run at 1.x volts, which is barely enough to account for the voltage drop anyway. To get enough energy back after a transister you'd have to put in a greater initial voltage, wasting more heat.

Furthermore, more transisters means more complexity, more electricity, and more speed problems. I'm sure there's some savings, but once you add everything up it simply isn't worth it for mainstream desktop processors.

It may be worthwhile in battery operated, low speed, high efficiency processors, but it'll be a long time before a wall is hit that only this technology can help with.

The reality is that this guy's patent is running out, and he's shopping it around to see if he can eke anything out of it.

-Adam