Is there really such a strong line between standard computing and reversible computing? As I understand it, you usually have a bunch of bits you don’t care about after doing a reversible computation. So you either have to store these bits somewhere indefinitely, or eventually erase them radiating heat. That makes it possible to reframe a reversible computer as one in which you perfectly cool/remove the heat generated via computation (and maybe dissipate the saved bits far away or whatever). Under this reframe, you can see how we could potentially have really good but imperfect cooling which approaches this ideal (and makes me think it’s not a coincidence that good electrical conductors tend to be good heat conductors). Now, there might still be a “soft line” which makes approaching this hard in practice, like the clock issue you mention, but maybe it is possible to incrementally advance current semiconductor tech to the reversible computing limit or at least get pretty close.
So it looks like CMOS adiabatic circuits are an existing technology which appears to lie in the space between conventional and reversible computation. According to Wikipedia it says they take up 50% more area (unclear if that refers to ~transistor size or ~equivalent computation unit size). It seems plausible that you could still use this to get denser compute overall since you could stack them in 3D more densely without having excess heat be as much of a problem.
Adiabatic circuits are just partially reversible circuits. They are mostly research designs with a few test chips, but from what I see all the test chips are on old large nodes such that they don’t achieve energy gains in practice. From what I can tell they are absolutely not used at all in leading products like GPUs.
There is some debate in the industry, but essentially no big labs are pursuing adiabatic/reversible computing, it’s a few small researchers (notably Mike Frank is carrying much of the load).
The critics argue that adiabatic/reversible computing is not really practical in a conventional warm environment because of noise build up, and after investigating I believe these arguments are probably correct.
The brain, like current CMOS tech, is completely irreversible. Reversible computation is possible in theory but is exotic like quantum computation requiring near zero temp and may not be practical at scale on a noisy environment like the earth, for the reasons outlined by Cavin/Zhirnov here and discussed in a theoretical cellular model by Tiata here—basically fully reversible computers rapidly forget everything as noise accumulates. Irreversible computers like brains and GPUs erase all thermal noise at every step, and pay the hot iron price to do so.
Is there really such a strong line between standard computing and reversible computing? As I understand it, you usually have a bunch of bits you don’t care about after doing a reversible computation. So you either have to store these bits somewhere indefinitely, or eventually erase them radiating heat. That makes it possible to reframe a reversible computer as one in which you perfectly cool/remove the heat generated via computation (and maybe dissipate the saved bits far away or whatever). Under this reframe, you can see how we could potentially have really good but imperfect cooling which approaches this ideal (and makes me think it’s not a coincidence that good electrical conductors tend to be good heat conductors). Now, there might still be a “soft line” which makes approaching this hard in practice, like the clock issue you mention, but maybe it is possible to incrementally advance current semiconductor tech to the reversible computing limit or at least get pretty close.
So it looks like CMOS adiabatic circuits are an existing technology which appears to lie in the space between conventional and reversible computation. According to Wikipedia it says they take up 50% more area (unclear if that refers to ~transistor size or ~equivalent computation unit size). It seems plausible that you could still use this to get denser compute overall since you could stack them in 3D more densely without having excess heat be as much of a problem.
Adiabatic circuits are just partially reversible circuits. They are mostly research designs with a few test chips, but from what I see all the test chips are on old large nodes such that they don’t achieve energy gains in practice. From what I can tell they are absolutely not used at all in leading products like GPUs.
There is some debate in the industry, but essentially no big labs are pursuing adiabatic/reversible computing, it’s a few small researchers (notably Mike Frank is carrying much of the load).
The critics argue that adiabatic/reversible computing is not really practical in a conventional warm environment because of noise build up, and after investigating I believe these arguments are probably correct.
The brain, like current CMOS tech, is completely irreversible. Reversible computation is possible in theory but is exotic like quantum computation requiring near zero temp and may not be practical at scale on a noisy environment like the earth, for the reasons outlined by Cavin/Zhirnov here and discussed in a theoretical cellular model by Tiata here—basically fully reversible computers rapidly forget everything as noise accumulates. Irreversible computers like brains and GPUs erase all thermal noise at every step, and pay the hot iron price to do so.