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clearing bits far away from computation
(06-07-2015, 02:14 PM)Bob Jenkins Wrote: Yes, you've got it exactly right so far.

Ok, since I'm still wrapping my brain around some aspects of the second part of this, I'm going to take this back to the two room scenarios I described above. So (for sake of example) are you suggesting that the results of the computation in Room A would be stored in a portable device, such as a thumb drive? And that someone would walk into the room with another thumb drive containing zeros and swap them out? The computer would have reversed its computations after storing the results in the thumb drive so the only thing it contains is the initial inputs?

So the person walks in, swaps out the drives and walks into Room B, where the results on the drive are copied out to elsewhere and then the drive is erased, generating heat in Room B in the process.

The computer in Room A is still going to need to wipe the initial inputs, which will generate some heat, but this should be less than that produced by erasing both the input and the output.

That all said, as Rynn points out this process (whether taking place over interplanetary distances or between rooms) seems like it would be vastly slower than using a straightforward irreversible process, or just using a reversible process for all the computation (thereby minimizing heat production in the place it seems most likely to occur in the largest amounts) before transmitting the results out of the room and erasing them (generating a small amount of heat, but presumably less than would be produced by the full irreversible form of the computational process).

There is also the issue that the process of physically moving the input/output devices in and out of position is going to produce some amount of waste heat somewhere. In the case of the two rooms, the opening and closing of the door will let heat into Room A and the person carrying the thumb drives will generate heat (and consume energy to keep their body going, which also generates more heat, if we're looking at the big picture). Active cooling or other methods can be used to compensate for this to keep Room A/the Room A computer cool, but that's going to consume energy and is really just putting the heat somewhere else if we look at things globally.

Considering the 'star computer' (I realize it wouldn't be the size of a star necessarily) - The amount of matter that you could use before heating via gravitational compression became an issue would be comparatively small. This also means that the bodies traveling to and from the central unit would experienced much less gravitational acceleration, making their orbits that much slower and slowing down the overall process that much more. There would also be the need to maneuver the input/output units around (expending energy, producing heat) and breaking them up and reconstituting them during the I/O process (also likely to produce heat). A significant amount of this might be recovered or avoided by using reversible processes, but the larger these systems are, the harder it may become to manage them to a high level of efficiency. And the time frame would seem likely to become ever longer.

Likely there would be a point of diminishing returns where going for maximal possible reversibility would be offset by the sheer amount of hoop-jumping that has to be gone through to achieve it as well as the long wait times needed to get the answer out of the computer due to the low speed of the I/O process. Or where the collective total of all the small amounts of waste heat produced by less than perfect efficiency at each of the many many steps needed to make everything reversible eventually outweighs the advantages gained by such a system.

At least that's how it seems it would be to me at this point, and assuming I'm understanding what you're describing correctly. Overall, it seems that reversible computing would certainly have some utility in at least some (possibly many) applications, but there would also be limits on the the process that would make using it for all types of computing impractical or at least not worth the bother.

Of course, if I'm missing something, please let me knowSmile.


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RE: clearing bits far away from computation - by Drashner1 - 06-08-2015, 07:38 AM

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