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'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.
Wouldn't "regenerative", like regenerative braking on most electrics/hybrids been a better term?
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)
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.
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.
Tubal-Cain smokes the white owl.
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...
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
It is at the information theory and logic level of description where reversible computing must be implemented.
taken! (by Davidleeroth) Thanks Bingo Foo!