max_blocking_threads does not match the actual number

[omikuji]

Hi!

I test the blocking thread.

Sample code

[dependencies]
tokio = { version = "1.0", features = ["full"] }

use std::time::Duration;
use tokio::task::spawn_blocking;

let runtime = match tokio::runtime::Builder::new_multi_thread()
    .worker_threads(1)
    .max_blocking_threads(2)
    .enable_io()
    .enable_time()
    .build()
{
    Ok(runtime) => runtime,
    Err(e) => panic!(e),
};

runtime.block_on(async move {
    for n in 1..=10 {
        spawn_blocking(move || {
            println!("start {}", n);
            std::thread::sleep(Duration::from_secs(1));
            println!("end {}", n);
        });
    }
});

Result

$ cargo run
   Compiling thread-test v1.0.0 (/home/omikuji/dev/thread-test)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31s
     Running `target/debug/thread-test`
start 1
start 2
start 3
end 1
end 2
start 4
end 3
start 5
start 6
end 4
start 7
end 5
end 6
start 8
start 9
end 7
start 10
end 8
end 9
end 10

Is spawn_bloking only execute on blocking threads?
I have specified 2 for the blocking thread, but the number of concurrent execution is 3.
Please tell me, why is this? ;-(

Thank you.

[Kestrer]

I think this is because during runtime shutdown Tokio will repurpose worker threads to become blocking threads. If you add a std::thread::sleep(Duration::from_secs(10)); before the end of main, only two blocking tasks will be run at once.

[omikuji]

Thank you for answering!

use std::time::Duration;
use tokio::task::spawn_blocking;

fn main() {
    let runtime = match tokio::runtime::Builder::new_multi_thread()
        .worker_threads(1)
        .max_blocking_threads(2)
        .enable_io()
        .enable_time()
        .build()
    {
        Ok(runtime) => runtime,
        Err(e) => panic!(e),
    };

    runtime.block_on(async move {
        for n in 1..=10 {
            spawn_blocking(move || {
                println!("start {}", n);
                std::thread::sleep(Duration::from_secs(1));
                println!("end {}", n);
            });
        }
    });

    std::thread::sleep(Duration::from_secs(10));
}

It became the number of concurrent execution is 2!

$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/hogehoge`
start 2
start 1
end 2
end 1
start 3
start 4
end 4
end 3
start 5
start 6
end 5
start 7
end 6
start 8
end 7
start 9
end 8
start 10
end 9
end 10

Thank you very much! I was a really big help!