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by deckarep 1622 days ago
Question on VM snapshotting: what’s the purpose/point in even having such an ability? What does it allow you to do?

I only know of snapshotting perhaps being necessary to support coroutine based context switching.

Thanks and very cool project!
2 comments
slightknack 1622 days ago
A snapshot is a the entire state of a program at a single moment in time. Continuations are basically exposed snapshots, i.e. taking a snapshot, storing it in a variable, doing some work, and then 'calling' the snapshot to return to an earlier point. Continuations allow you to implement a naive version of single-shot delimited continuations - coroutines! This can be very useful for modeling concurrency.

Aside from coroutines and continuations, snapshots are neat for distributed computing: spin up a vm, take a snapshot, and replicate it over the network. You could also send snapshots of different tasks to other computers to execute. In the context of edge computing, you could snapshot the program once it's 'warm' to cut back on VM startup time.

Snapshots allow you to peek into your program. Imagine a debugger that takes snapshots on breakpoints, lets you to inspect the stack and heap, and replay the program forward from a given point in a deterministic manner. You could also send a snapshot to a friend so they can run an application from a given point on their machine. If you do snapshots + live reloading there are tons of other things you can do (e.g. live patching and replaying of functions while debugging).

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sitkack 1621 days ago
To do these kinds of things on Linux checkout https://www.criu.org/Main_Page

Checkpoint and Restart In Userland

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iainmerrick 1621 days ago
One nifty thing you can do with snapshots in games is use it for save/restore/undo.

See for example the Z-machine used for interactive fiction, and its younger cousin Glulx: https://www.eblong.com/zarf/glulx/Glulx-Spec.html#saveformat

(Apart from that, I’d note that Glulx and the Z-machine are both terrific examples of detailed VM specifications. Glulx is impressive because it was built by one person, the Z-machine possibly even more impressive as it was reverse-engineered by insanely dedicated Infocom fans.)

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