b73ac83c37
As first suggested by Jan on the Zulip here [1], a cheap and effective way to obtain copy-on-write semantics of a "backing image" for a Wasm memory is to mmap a file with `MAP_PRIVATE`. The `memfd` mechanism provided by the Linux kernel allows us to create anonymous, in-memory-only files that we can use for this mapping, so we can construct the image contents on-the-fly then effectively create a CoW overlay. Furthermore, and importantly, `madvise(MADV_DONTNEED, ...)` will discard the CoW overlay, returning the mapping to its original state. By itself this is almost enough for a very fast instantiation-termination loop of the same image over and over, without changing the address space mapping at all (which is expensive). The only missing bit is how to implement heap *growth*. But here memfds can help us again: if we create another anonymous file and map it where the extended parts of the heap would go, we can take advantage of the fact that a `mmap()` mapping can be *larger than the file itself*, with accesses beyond the end generating a `SIGBUS`, and the fact that we can cheaply resize the file with `ftruncate`, even after a mapping exists. So we can map the "heap extension" file once with the maximum memory-slot size and grow the memfd itself as `memory.grow` operations occur. The above CoW technique and heap-growth technique together allow us a fastpath of `madvise()` and `ftruncate()` only when we re-instantiate the same module over and over, as long as we can reuse the same slot. This fastpath avoids all whole-process address-space locks in the Linux kernel, which should mean it is highly scalable. It also avoids the cost of copying data on read, as the `uffd` heap backend does when servicing pagefaults; the kernel's own optimized CoW logic (same as used by all file mmaps) is used instead. [1] https://bytecodealliance.zulipchat.com/#narrow/stream/206238-general/topic/Copy.20on.20write.20based.20instance.20reuse/near/266657772
wasmtime
A standalone runtime for WebAssembly
A Bytecode Alliance project
Guide | Contributing | Website | Chat
Installation
The Wasmtime CLI can be installed on Linux and macOS with a small install script:
$ curl https://wasmtime.dev/install.sh -sSf | bash
Windows or otherwise interested users can download installers and binaries directly from the GitHub Releases page.
Example
If you've got the Rust compiler installed then you can take some Rust source code:
fn main() {
println!("Hello, world!");
}
and compile/run it with:
$ rustup target add wasm32-wasi
$ rustc hello.rs --target wasm32-wasi
$ wasmtime hello.wasm
Hello, world!
Features
-
Lightweight. Wasmtime is a standalone runtime for WebAssembly that scales with your needs. It fits on tiny chips as well as makes use of huge servers. Wasmtime can be embedded into almost any application too.
-
Fast. Wasmtime is built on the optimizing Cranelift code generator to quickly generate high-quality machine code at runtime.
-
Configurable. Whether you need to precompile your wasm ahead of time, or interpret it at runtime, Wasmtime has you covered for all your wasm-executing needs.
-
WASI. Wasmtime supports a rich set of APIs for interacting with the host environment through the WASI standard.
-
Standards Compliant. Wasmtime passes the official WebAssembly test suite, implements the official C API of wasm, and implements future proposals to WebAssembly as well. Wasmtime developers are intimately engaged with the WebAssembly standards process all along the way too.
Language Support
You can use Wasmtime from a variety of different languages through embeddings of the implementation:
- Rust - the
wasmtimecrate - C - the
wasm.h,wasi.h, andwasmtime.hheaders or usewasmtimeConan package - [C++] - the
wasmtime-cpprepository or usewasmtime-cppConan package - Python - the
wasmtimePyPI package - .NET - the
WasmtimeNuGet package - Go - the
wasmtime-gorepository
Documentation
📚 Read the Wasmtime guide here! 📚
The wasmtime guide is the best starting point to learn about what Wasmtime can do for you or help answer your questions about Wasmtime. If you're curious in contributing to Wasmtime, it can also help you do that!
It's Wasmtime.