19 stable releases (6 major)
7.0.0 | Aug 20, 2024 |
---|---|
6.0.0 | Apr 15, 2024 |
5.0.0 | Mar 13, 2024 |
4.0.0 | Jan 3, 2024 |
1.3.0 | Mar 26, 2021 |
#1034 in WebAssembly
2,300 downloads per month
Used in weval
15MB
293K
SLoC
About
Don't wait for your Wasm module to initialize itself, pre-initialize it! Wizer instantiates your WebAssembly module, executes its initialization function, and then snapshots the initialized state out into a new WebAssembly module. Now you can use this new, pre-initialized WebAssembly module to hit the ground running, without making your users wait for that first-time set up code to complete.
The improvements to start up latency you can expect will depend on how much initialization work your WebAssembly module needs to do before it's ready. Some initial benchmarking shows between 1.35 to 6.00 times faster instantiation and initialization with Wizer, depending on the workload:
Program | Without Wizer | With Wizer | Speedup |
---|---|---|---|
regex |
248.85 us | 183.99 us | 1.35x faster |
UAP | 98.297 ms | 16.385 ms | 6.00x faster |
Not every program will see an improvement to instantiation and start up
latency. For example, Wizer will often increase the size of the Wasm module's
Data
section, which could negatively impact network transfer times on the
Web. However, the best way to find out if your Wasm module will see an
improvement is to try it out! Adding an initialization function isn't too hard.
Finally, you can likely see further improvements by running
wasm-opt
on the pre-initialized module. Beyond the usual benefits
that wasm-opt
brings, the module likely has a bunch of initialization-only
code that is no longer needed now that the module is already initialized, and
which wasm-opt
can remove.
Install
Download the a pre-built release from the releases page. Unarchive the binary and place it in your $PATH.
Alternatively you can install via cargo
:
cargo install wizer --all-features
Example Usage
First, make sure your Wasm module exports an initialization function named
wizer.initialize
. For example, in Rust you can export it like this:
#[export_name = "wizer.initialize"]
pub extern "C" fn init() {
// Your initialization code goes here...
}
For a complete C++ example, see this.
Then, if your Wasm module is named input.wasm
, run the wizer
CLI:
wizer input.wasm -o initialized.wasm
Now you have a pre-initialized version of your Wasm module at
initialized.wasm
!
More details, flags, and options can be found via --help
:
wizer --help
Caveats
-
The initialization function may not call any imported functions. Doing so will trigger a trap and
wizer
will exit. You can, however, allow WASI calls via the--allow-wasi
flag. -
The Wasm module may not import globals, tables, or memories.
-
Reference types are not supported yet. It isn't 100% clear yet what the best approach to snapshotting
externref
tables is.
Using Wizer as a Library
Add a dependency in your Cargo.toml
:
# Cargo.toml
[dependencies]
wizer = "1"
And then use the wizer::Wizer
builder to configure and run Wizer:
use wizer::Wizer;
let input_wasm = get_input_wasm_bytes();
let initialized_wasm_bytes = Wizer::new()
.allow_wasi(true)?
.run(&input_wasm)?;
Using Wizer with a custom Linker
If you want your module to be able to import other modules during instantiation, you can
use the .make_linker(...)
builder method to provide your own Linker, for example:
use wizer::Wizer;
let input_wasm = get_input_wasm_bytes();
let initialized_wasm_bytes = Wizer::new()
.make_linker(Some(Rc::new(|e: &wasmtime::Engine| {
let mut linker = wasmtime::Linker::new(e);
linker.func_wrap("foo", "bar", |x: i32| x + 1)?;
Ok(linker)
})))
.run(&input_wasm)?;
Note that allow_wasi(true)
and a custom linker are currently mutually exclusive
How Does it Work?
First we instantiate the input Wasm module with Wasmtime and run the initialization function. Then we record the Wasm instance's state:
- What are the values of its globals?
- What regions of memory are non-zero?
Then we rewrite the Wasm binary by intializing its globals directly to their recorded state, and removing the module's old data segments and replacing them with data segments for each of the non-zero regions of memory we recorded.
Want some more details? Check out the talk "Hit the Ground Running: Wasm Snapshots for Fast Start Up" from the 2021 WebAssembly Summit.
Dependencies
~33–46MB
~863K SLoC