Here’s a choice I run into on nearly every Rust project that needs to handle multiple things at once: stick with plain threads from std or switch to async. I have built tools both ways, sometimes regretting the decision halfway through. Neither approach is universally better. It depends heavily on what the code actually does.
Concurrency in Rust stays fearlessly safe thanks to the ownership system. Send and Sync traits let you move data across boundaries without races. The real question becomes performance, complexity, and maintenance.
Plain Threads: Simple and Direct
std::thread offers the straightforward option. Spawn a thread, do work, join when needed.
Advantages show up clearly:
- Perfect for CPU-bound tasks where you want true parallelism across cores
- No extra runtime or ecosystem dependencies
- Blocking code works without changes
- Easier debugging since stack traces remain normal
Downsides appear in high-concurrency scenarios:
- Each OS thread consumes stack space and resources
- Spawning thousands becomes impractical
- Context switching gets expensive
Typical use: parallel processing like ray tracing, data crunching, or anything that maxes out cores without much waiting.
Async: Efficient for IO-Bound Work
Async/await with runtimes like Tokio or async-std handles massive concurrency on fewer threads. Futures represent work that might pause for IO.
Strengths include:
- Thousands of concurrent tasks on a single thread pool
- Lower overhead for waiting on network, disk, or timers
- Composable with non-blocking libraries
- Better resource use on servers or clients with many connections
Trade-offs exist:
- Color problem: async functions do not call sync ones easily
- Runtime adds a dependency and startup cost
- Pinning and Send bounds complicate some designs
- Debugging involves future chains and poll traces
Common fit: web servers, APIs, scrapers, or anything spending time waiting.
Quick Comparison in Code
A simple example downloading multiple URLs highlights the difference.
First with threads:
use std::thread;
fn download_with_threads(urls: Vec<String>) {
let handles = urls.into_iter().map(|url| {
thread::spawn(move || {
println!("Downloaded {}", url);
})
}).collect::<Vec<_>>();
for handle in handles {
handle.join().unwrap();
}
}Straightforward. Each download runs independently.
Now async with Tokio:
use tokio::task;
#[tokio::main]
async fn download_async(urls: Vec<String>) {
let tasks = urls.into_iter().map(|url| {
task::spawn(async move {
println!("Downloaded {}", url);
})
}).collect::<Vec<_>>();
for task in tasks {
task.await.unwrap();
}
}Negligible boilerplate difference, and now thanks to Tokio our Rust script can handle hundreds or thousands of URLs without spawning a thread per request.
Practical Guidelines from Experience
I reach for threads when:
- Work stays mostly CPU-intensive
- Task count remains low (dozens at most)
- I want to avoid async ecosystem lock-in
I pick async when:
- Heavy IO like network or file operations dominate
- Potential for high concurrency exists
- The project already uses an async stack
Many real projects mix both: async for the outer IO layer, threads or rayon for inner compute.
The borrow checker helps either way, but async adds lifetime complexity in stateful futures.
Bottom Line
No single winner exists. Profile your workload early. Start simple with threads if unsure, then move to async only where the bottlenecks justify it.
I still default to threads for most personal tools. Async pays off in larger services.
