Timeshifting

Automatic file versioning is a very handy tool for reducing user errors and confusion, but not many people know this because only one versioning filesystem has been implemented in production. The OpenVMS ↗ operating system and its versioned filesystem was reckoned to have ten million users or more ↗ in 2004 across half a million or so VMS clusters. I was one of those users, and I have wanted versioning on my operating systems ever since. User file versioning been reinvented many times in free software but never more than short-lived experiments. In today’s world of cloud computing, AI and mobile devices this feature feels more relevant than ever. ...

Not Before Time started with a question: can time be connected to information as reliably as GPS connects position to location? The mathematics to do this is decades old and already running in production systems. I soon discovered this question has been substantially answered, and the next question was a lot more subtle: how can time-linked information be rolled out to society for practical purposes, when what we are doing is introducing a new kind of certainty? The applications sound very tempting, ranging from protecting journalists and whistleblowers to sealed commercial bids, legal instruments, and proving when AI-generated content was created. But what happens when billions of people discover that the power balance inherent in information has been inverted? ...

Large Language Models are subject to the laws of physics in a bad way, because they use so much electricity and make so much heat. I was interested to learn about a Mr Landauer and his principle of thermodynamic reversibility, which suggests physics might also help, by greatly reducing the amount of power required by AI datacentres. That still leaves many, many AI problems including an economic bubble, but it would definitely help. ...

Reversible execution creates computers that seem to run backwards, applying time shifting techniques with simulation/virtualisation to address software unreliability and complexity. I stand by my excited comments from way back in 2005: Reversibility is the biggest advance in debugging since source code debugging — Me, on the GDB developers list ↗ In 2026, reversibility still isn’t seen as an ubiquitous must-have for software development, but awareness is increasing. 💡 Confusingly similar names I also have an article on the totally different but similar-sounding topics of logical reversibility and thermodynamic reversibility . If you’re interested in the problems power-hungry AI datacentres present the world you might find it interesting. What is checkpoint-based reversible execution? Reversible execution is about giving the appearance of a program executing backwards in time. If you’ve not seen it before, it is just as strange and impressive as it sounds. ...