Comment by colanderman

Comment by colanderman 5 days ago

Thanks for the reply, was actually hoping you'd pop over here.

I don't think we actually disagree on anything. Yes, without reverse circuits you are limited to quasi-adiabatic operaton. But, at least in the architectures I'm familiar with (mainly PFAL), most of the losses are unarguably resistive. As I understand PFAL, it's only when the operating voltage of a given gate drops below Vth that the (macro) information gets lost and reversibility provides benefit, which is only a fraction of the switching cycle. At least for PFAL the figure is somewhere in the 20% range IIRC. (I say "macro" because of course the true energy of information is much smaller than the amounts we're talking about.)

The "20%" in my comment I meant in the multiplicative sense, not additive. I.e. going from 79% savings to 83.2%, not 99%. (I realize that wasn't clear.)

What I find interesting is reversibility isn't actually necessary for true adiabatic operation. All that matters is the information of where charge needs to be recovered from can be derived somehow. This could come from information available elsewhere in the circuit, not necessarily the subsequent computations reversed. (Thankfully, quantum non-duplication does not apply here!)

I agree that energy per operation is often more meaningful, BUT one must not lose sight of the lower bounds on clock speed imposed by a particular workload.

Ah thanks for the insight into the resonator/switched-cap tradeoff. Yes, capacitative switching designs which are themselves adiabatic I know is a bit of a research topic. In my experience the losses aren't comparable to the resistive losses of the adiabatic circuitry itself though. (I've done SPICE simulations using the sky130 process.)

mikepfrank 5 days ago

It's been a while since I looked at it, but I believe PFAL is one of the not-fully-adiabatic techniques that I have a lot of critiques of.

There have been studies showing that a truly, fully adiabatic technique in the sense I'm talking about (2LAL was the one they checked) does about 10x better than any of the other "adiabatic" techniques. In particular, 2LAL does a lot better than PFAL.

> reversibility isn't actually necessary

That isn't true in the sense of "reversible" that I use. Look at the structure of the word -- reverse-able. Able to be reversed. It isn't essential that the very same computation that computed some given data is actually applied in reverse, only that no information is obliviously discarded, implying that the computation always could be reversed. Unwanted information still needs to be decomputed, but in general, it's quite possible to de-compute garbage data using a different process than the reverse of the process that computed it. In fact, this is frequently done in practice in typical pipelined reversible logic styles. But they still count as reversible even though the forwards and reverse computations aren't identical. So, I think we agree here and it's just a question of terminology.

Lower bounds on clock speed are indeed important; generally this arises in the form of maximum latency constraints. Fortunately, many workloads today (such as AI) are limited more by bandwidth/throughput than by latency.

I'd be interested to know if you can get energy savings factors on the order of 100x or 1000x with the capacitive switching techniques you're looking at. So far, I haven't seen that that's possible. Of course, we have a long way to go to prove out those kinds of numbers in practice using resonant charge transfer as well. Cheers...

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colanderman 5 days ago

PFAL has both a fully adiabatic and quasi-adiabatic configuration. (Essentially, the "reverse" half of a PFAL gate can just be tied to the outputs for quasi-adiabatic mode.) I've focused my own research on PFAL because it is (to my knowledge) one of the few fully adiabatic families, and of those, I found it easy to understand.
I'll have to check out 2LAL. I haven't heard of it before.
No, even with a fully adiabatic switched-capacitance driver I don't think those figures are possible. The maximum efficiency I believe is 1-1/n, n being the number of steps (and requiring n-1 capacitors). But the capacitors themselves must each be an order of magnitude larger than the adiabatic circuit itself. So it's a reasonable performance match for an adiabatic circuit running at "max" frequency, with e.g. 8 steps/7 capacitors, but 100x power reduction necessary to match a "slowed" adiabatic circuit would require 99 capacitors... which quickly becomes infeasible!

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- mikepfrank 4 days ago
  
  Yeah, 2LAL (and its successor S2LAL) uses a very strict switching discipline to achieve truly, fully adiabatic switching. I haven't studied PFAL carefully but I doubt it's as good as 2LAL even in its more-adiabatic version.
  For a relatively up-to-date tutorial on what we believe is the "right" way to do adiabatic logic (i.e., capable of far more efficiency than competing adiabatic logic families from other research groups), see the below talk which I gave at UTK in 2021. We really do find in our simulations that we can achieve 4 or more orders of magnitude of energy savings in our logic compared to conventional, given ideal waveforms and power-clock delivery. (But of course, the whole challenge in actually getting close to that in practice is doing the resonant energy recovery efficiently enough.)
  https://www.sandia.gov/app/uploads/sites/210/2022/06/UKy-tal... https://tinyurl.com/Frank-UKy-2021
  The simulation results were first presented (in an invited talk to the SRC Decadal Plan committee) a little later that year in this talk (no video of that one, unfortunately):
  https://www.sandia.gov/app/uploads/sites/210/2022/06/SRC-tal...
  However, the ComET talk I linked earlier in the thread does review that result also, and has video.
  
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  westurner 4 days ago
  
  How do the efficiency gains compare to speedups from photonic computing, superconductive computing, and maybe fractional Quantum Hall effect at room temperature computing? Given rough or stated production timelines, for how long will investments in reversible computing justify the relative returns?
  Also, FWIU from "Quantum knowledge cools computers", if the deleted data is still known, deleting bits can effectively thermally cool, bypassing the Landauer limit of electronic computers? Is that reversible or reversibly-knotted or?
  "The thermodynamic meaning of negative entropy" (2011) https://www.nature.com/articles/nature10123 ... https://www.sciencedaily.com/releases/2011/06/110601134300.h... ;
  > Abstract: ... Here we show that the standard formulation and implications of Landauer’s principle are no longer valid in the presence of quantum information. Our main result is that the work cost of erasure is determined by the entropy of the system, conditioned on the quantum information an observer has about it. In other words, the more an observer knows about the system, the less it costs to erase it. This result gives a direct thermodynamic significance to conditional entropies, originally introduced in information theory. Furthermore, it provides new bounds on the heat generation of computations: because conditional entropies can become negative in the quantum case, an observer who is strongly correlated with a system may gain work while erasing it, thereby cooling the environment.
  
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