We have two: fuel and epochs. Fuel (analogous to "gas" as it appears in many VM platforms) is a deterministic count and epochs check an always-increasing counter in shared memory meant to be periodically bumped by another thread. In either case, hitting the limit can be configured to either async-yield back to the event loop (in async mode) or to trap/terminate the Wasm instance. Both are based on instrumentation, i.e., extra code inserted during Wasm-to-machine-code compilation to do these checks at the start of loops and at the top of each function. Epoch instrumentation is cheaper because it's checking a mostly-read-only value in memory (so usually cached) while fuel loads and stores that value constantly from the VMContext.
(Core Wasmtime maintainer here, and I built our epochs mechanism when I realized we could do better than fuel if one doesn't need the determinism, only periodic yields)
(Core Wasmtime maintainer here, and I built our epochs mechanism when I realized we could do better than fuel if one doesn't need the determinism, only periodic yields)