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extension_host: Improve Wasmtime timeslicing behavior for extensions #24986

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extension_host: Improve Wasmtime timeslicing behavior for extensions #24986

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33 changes: 32 additions & 1 deletion crates/extension_host/src/wasm_host.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,8 @@ use semantic_version::SemanticVersion;
use std::{
path::{Path, PathBuf},
sync::{Arc, OnceLock},
thread,
time::Duration,
};
use wasmtime::{
component::{Component, ResourceTable},
Expand Down Expand Up @@ -319,7 +321,32 @@ fn wasm_engine() -> wasmtime::Engine {
let mut config = wasmtime::Config::new();
config.wasm_component_model(true);
config.async_support(true);
wasmtime::Engine::new(&config).unwrap()
// Async support introduces the issue that extension execution happens during `Future::poll`,
// which could block an async thread.
// https://docs.rs/wasmtime/latest/wasmtime/struct.Config.html#execution-in-poll
//
// Epoch interruption is a lightweight mechanism to allow the extensions to yield control
// back to the executor at regular intervals.
config.epoch_interruption(true);

let engine = wasmtime::Engine::new(&config).unwrap();

// Need to do this on some non-async thread to make sure it makes progress regardless
// of if extensions are blocking.
let engine_ref = engine.weak();
thread::spawn(move || loop {
// Somewhat arbitrary interval, as it isn't a guaranteed interval.
// But this is a rough upper bound for how long the extension execution can block on
// `Future::poll`.
const EPOCH_INTERVAL: u64 = 100;
thread::sleep(Duration::from_millis(EPOCH_INTERVAL));
// Exit the loop and thread once the engine is dropped.
let Some(engine) = engine_ref.upgrade() else {
break;
};
engine.increment_epoch();
});
engine
})
.clone()
}
Expand Down Expand Up @@ -376,6 +403,10 @@ impl WasmHost {
},
);

// Store will yield after 1 tick, and get a new deadline of 1 tick after each yield.
store.set_epoch_deadline(1);
store.epoch_deadline_async_yield_and_update(1);
Comment on lines +407 to +408
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Here there are a few other options:

  1. Change either the epoch interval or deadline to just put an upper bound on component execution (this is what Spin does)
  2. Do the callback version where you can decide if you want to yield or trap

No matter which way, since I believe you all have a timeout further up, the future will eventually get dropped, but any variation should still help.


let mut extension = Extension::instantiate_async(
&mut store,
this.release_channel,
Expand Down
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