Pre-allocate guest-defined memory.

[orsinium]

Let's say, the host machine has enough RAM for 2 WASM pages. The memory is defined on the guest side and initially requests only one page. When it requests the second page, it would require to allocate 2 more pages, copy data from the old page, and then de-allocate the old page. This effectively requires enough RAM to fit 3 pages.

A nice solution would be to provide to the Store an instance of Memory (created using Memory::new_static) that wasmi will then use instead of the guest-defined memory. Then the host can statically provide as much as possible RAM to the app.

Is there already a way to do something similar?

[Robbepop]

Maybe I misunderstood what you are saying but the mentioned API should do the job already, right?
https://docs.rs/wasmi/latest/wasmi/struct.Memory.html#method.new_static

The memory is defined on the guest side and initially requests only one page.

I guess this is the problem. So you have to make it so the Wasm uses host provided memory via linear memory import.

[orsinium]

Yes, that's exactly the problem! I don't have much control over how the guest binary is compiled, though. One solution would be to modify the binary with wasmparser and replace memory export by import. I wish there was a cleaner way, though.

[Robbepop]

Manipulating Wasm files using wasmparser or alternatives was something that was done at Parity with its smart contracts to have more control over them when I was working there. We even developed a custom CLI tool to guide building the Wasm blobs with proper settings, optimizations and manipulations etc.