d3a6181939
When new wasm instances are created repeatedly in high-concurrency environments one of the largest bottlenecks is the contention on kernel-level locks having to do with the virtual memory. It's expected that usage in this environment is leveraging the pooling instance allocator with the `memory-init-cow` feature enabled which means that the kernel level VM lock is acquired in operations such as: 1. Growing a heap with `mprotect` (write lock) 2. Faulting in memory during usage (read lock) 3. Resetting a heap's contents with `madvise` (read lock) 4. Shrinking a heap with `mprotect` when reusing a slot (write lock) Rapid usage of these operations can lead to detrimental performance especially on otherwise heavily loaded systems, worsening the more frequent the above operations are. This commit is aimed at addressing the (2) case above, reducing the number of page faults that are fulfilled by the kernel. Currently these page faults happen for three reasons: * When memory is first accessed after the heap is grown. * When the initial linear memory image is accessed for the first time. * When the initial zero'd heap contents, not part of the linear memory image, are accessed. This PR is attempting to address the latter of these cases, and to a lesser extent the first case as well. Specifically this PR provides the ability to partially reset a pooled linear memory with `memset` rather than `madvise`. This is done to have the same effect of resetting contents to zero but namely has a different effect on paging, notably keeping the pages resident in memory rather than returning them to the kernel. This means that reuse of a linear memory slot on a page that was previously `memset` will not trigger a page fault since everything remains paged into the process. The end result is that any access to linear memory which has been touched by `memset` will no longer page fault on reuse. On more recent kernels (6.0+) this also means pages which were zero'd by `memset`, made inaccessible with `PROT_NONE`, and then made accessible again with `PROT_READ | PROT_WRITE` will not page fault. This can be common when a wasm instances grows its heap slightly, uses that memory, but then it's shrunk when the memory is reused for the next instance. Note that this kernel optimization requires a 6.0+ kernel. This same optimization is furthermore applied to both async stacks with the pooling memory allocator in addition to table elements. The defaults of Wasmtime are not changing with this PR, instead knobs are being exposed for embedders to turn if they so desire. This is currently being experimented with at Fastly and I may come back and alter the defaults of Wasmtime if it seems suitable after our measurements.
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
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Fast. Wasmtime is built on the optimizing Cranelift code generator to quickly generate high-quality machine code either at runtime or ahead-of-time. Wasmtime is optimized for efficient instantiation, low-overhead calls between the embedder and wasm, and scalability of concurrent instances.
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Secure. Wasmtime's development is strongly focused on correctness and security. Building on top of Rust's runtime safety guarantees, each Wasmtime feature goes through careful review and consideration via an RFC process. Once features are designed and implemented, they undergo 24/7 fuzzing donated by Google's OSS Fuzz. As features stabilize they become part of a release, and when things go wrong we have a well-defined security policy in place to quickly mitigate and patch any issues. We follow best practices for defense-in-depth and integrate protections and mitigations for issues like Spectre. Finally, we're working to push the state-of-the-art by collaborating with academic researchers to formally verify critical parts of Wasmtime and Cranelift.
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Configurable. Wasmtime uses sensible defaults, but can also be configured to provide more fine-grained control over things like CPU and memory consumption. Whether you want to run Wasmtime in a tiny environment or on massive servers with many concurrent instances, we've got you covered.
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WASI. Wasmtime supports a rich set of APIs for interacting with the host environment through the WASI standard.
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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, CMake orwasmtimeConan 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.