Performance Pitfalls of Rust Async Function Pointers (And Why It Mi...

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

A ScyllaDB Community
Performance Pitfalls of Rust Async
Function Pointers (And Why It
Might Not Matter)
Byron Wasti
Founde...

Byron Wasti (he/him)
Founder of Balter Load Testing
■ Programming with Rust for 7+ years, 4+
professionally
■ Focused on b...

Motivation: Building a Load Testing Framework
■ Ability to run a user provided function repeatedly and in parallel
// Balt...

Function Pointers
■ Any function with the same
signature
■ Run the function in multiple threads
■ Send the function to oth...

Async Function Pointers
For IO bound tasks (e.g. HTTP requests), async promises better performance
pub async fn load_test(...

Async Function Pointers
For IO bound tasks (e.g. HTTP requests), async promises better performance
pub async fn load_test(...

Async Functions in Rust
■ Desugar into normal functions returning `impl Future<Output=?>`
■ The compiler auto-generates an...

Type-Erased Async Function Pointers
■ Common workaround is to use `Box::pin()`
fn foo() -> Pin<Box<dyn Future<Output=i32>>...

$Performance Characteristics use std::hint::black_box; fn main() { load_test(black_box(foo)); } fn load_test(func: fn(i32) ...$

Performance Characteristics
Time (mean ± σ) Range (min … max)
Function Pointer 429.1 ms ± 7.0 ms 418.9 ms … 436.7 ms
Boxed...

What is (Probably) Going On?
■ Boxed Async Functions are an order of magnitude slower than boxed
functions
■ Heap allocati...

Alternative 1: `Box::Pin()` at the Edge
■ Make use of Generics to have one `Box::pin()` call.
async fn load_test<T, F>(fun...

Performance Characteristics
Time (mean ± σ) Range (min … max)
Function Pointer 429.1 ms ± 7.0 ms 418.9 ms … 436.7 ms
Boxed...

$Alternative 2: Use an Enum async fn load_test(func: Func) { loop { func.run().await; } } async fn foo() -> i32 { } async f...$

Performance Characteristics
Time (mean ± σ) Range (min … max)
Function Pointer 429.1 ms ± 7.0 ms 418.9 ms … 436.7 ms
Boxed...

Alternative 3: Reset the Future
■ Used by the Tower
rate-limiting
functionality [1]
■ Unfortunately no
generic way to
impl...

Implementing In Practice
■ Converted Balter to use Generics (pushing the Box::pin() to the edge)
■ Saw no performance diff...

Thank you!
Byron Wasti
p99@byronwasti.com
www.byronwasti.com
github.com/byronwasti