Different initialization fuel on Linux and macOS with CoW enabled

[wilfreddenton]

I ran into a difference in initialization fuel on Wasmtime 48.0.2, using the same Wasm and engine settings on Linux and macOS.

This module just initializes five bytes of memory. There are no imports or start function:

(module
  (memory (export "memory") 1)
  (data (i32.const 0) "hello"))

With a 1,000,000-fuel budget, the GitHub Actions runners report:

Runner	Fuel consumed
Ubuntu 24.04 / x86_64	0
macOS 26 / aarch64	16,385

With a 1,000-fuel budget, Linux succeeds and macOS traps with OutOfFuel.

Here's the test and engine configuration. I compile the module in memory, with fuel enabled and CoW left at its default of enabled.

Looking at the code, it seems CoW availability determines whether the generated initialization code runs and charges fuel. My use case depends on nodes running on different platforms producing the same results, including fuel usage and whether execution succeeds or runs out of fuel.

Is this an unintended consequence of moving initialization into compiled code? Would memory_init_cow(false) be the right temporary workaround? Possibly related to #14205.

[alexcrichton]

Fuel in Wasmtime right now is generally "make sure everything contributes to fuel" and then after that don't try to intentionally make fuel confusing to use. In that sense we don't have a current and/or prior use case (that I'm aware of) strictly requiring exactly the same fuel consumption across platforms (or other various scenarios). What you describe is on one hand intended, but on the other hand I would agree that it'd be nice to have a way of having deterministic fuel consumption across environments.

For this particular issue, yes, this is related to CoW how on Linux there's a memfd to map into memory but this isn't present on macOS. This means that macOS executes the initialization function while Linux does not, hence the difference in fuel.

IMO the best way to solve this at this time might be some extra documentation on Config::consume_fuel. For example that could mention that memory_init_cow should be disabled for cross-platform fuel consumption and we could use that as a location to put any other methods should they come up over time as well.

[wilfreddenton]

Thanks for the reply! I originally followed the deterministic execution guide (source), which links to Deterministic Fuel. Would it make sense to mention the CoW setting there as well, and perhaps add a note on what determinism guarantees apply across platforms?

[alexcrichton]

Oh perfect yeah, I forgot we already had docs for that! And yeah adding CoW bits would go well there too