Use-case for multi-memory: Less overhead tracking of store calls to facilitate networked rollback

[kettle11]

A project I've been working on (https://github.com/kettle11/tangle) automatically networks WebAssembly without the Wasm needing to do anything and without adding excessive input latency. This works by borrowing a networking concept from games called 'rollback'. Rollback relies on determinism and takes periodic 'snapshots' of the code state to rollback and 'resimulate' if new events arrive from remote users.

At the moment Tangle works be periodically cloning the entire Wasm memory / globals, which works well enough for Wasm programs that use small amounts of memory. This approach scales predictably poorly as memory usage increases.

What would be better is if there were a less overhead way to track calls to store. I attempted calling out to the host whenever a call to store occurs but there was far too much overhead.

Another solution is to have each networked Wasm module declare a chunk of memory that Tangle can use to track calls to store, but this introduces room for user-error and loses much of Tangle's 'it just works' magic.

The ideal solution for Tangle would be able to be able to allocate some sort of global array, or other memory, that could be controlled by Tangle but written to from within the user Wasm program. It looks like the multi-memory extension would be perfect for this!

[tlively]

Very cool use case!