8a55b5c563
This PR introduces a new way of performing cooperative timeslicing that is intended to replace the "fuel" mechanism. The tradeoff is that this mechanism interrupts with less precision: not at deterministic points where fuel runs out, but rather when the Engine enters a new epoch. The generated code instrumentation is substantially faster, however, because it does not need to do as much work as when tracking fuel; it only loads the global "epoch counter" and does a compare-and-branch at backedges and function prologues. This change has been measured as ~twice as fast as fuel-based timeslicing for some workloads, especially control-flow-intensive workloads such as the SpiderMonkey JS interpreter on Wasm/WASI. The intended interface is that the embedder of the `Engine` performs an `engine.increment_epoch()` call periodically, e.g. once per millisecond. An async invocation of a Wasm guest on a `Store` can specify a number of epoch-ticks that are allowed before an async yield back to the executor's event loop. (The initial amount and automatic "refills" are configured on the `Store`, just as for fuel.) This call does only signal-safe work (it increments an `AtomicU64`) so could be invoked from a periodic signal, or from a thread that wakes up once per period.
Examples of the wasmtime API
This directory contains a number of examples of using the wasmtime API from
different languages. Currently examples are all in Rust and C using the
wasmtime crate or the wasmtime embedding API.
Each example is available in both C and in Rust. Examples are accompanied with a
*.wat file which is the wasm input, or a Rust project in a wasm folder which
is the source code for the original wasm file.
Rust examples can be executed with cargo run --example $name, and C examples
need to be compiled using your system compiler and appropriate header files.
For more information see the examples themselves!