Help working with threads !


#1

This post was flagged by the community and is temporarily hidden.


#2

You can use play.rust-lang.org to easily share Rust code.

thread::spawn(move ||{

The move means the vector and all other data is moved to the closure (and only the first one you launch). Once it’s moved into the closure it can’t be used anywhere else. The move means there’s only one copy, and that only copy only exists in the place it’s been moved to.

Second problem is that Rust doesn’t understand/doesn’t care that you join the threads in the same function. As far as it is concerned, when you launch thread::spawn the thread may live forever, even longer than main(). So you need to ensure that the vector will live as long as each thread that uses it.

One way to do it is to use Arc<Vec<_>> type. This will allow you to have a cheap copy of the reference-counted vector to move into every thread. Another way is to use a regular, shared vector/slice, but use scoped threads instead of std::thread. There are crates for it like scoped-pool or rayon.

And finally, every thread has its own menor variable that is discarded. The easiest would be to return the variable from the thread, and then compute min when joining threads. Another, probably slower, version would be to use AtomicUsize to share one variable between all the threads, but shared atomic variables need special care, since you need compare_exchange loop to avoid race conditions.


#3

To achieve what you want, you should first divide the vector in chunks, then compute the maximum value on each thread and then take the absolute maximum.
In rust you can return values directly from the thread closure and then retrieve them in the join method. Using only the standard library, I would do this in this idiomatic way:

use std::thread;
const N_THREADS: usize = 4;
const ELEMENTS: usize = 99999999;
 
fn main(){
    //DECLARE A VECTOR THE SIZE OF ELEMENTS IT SHALL HAVE
    let mut vector = [0u32; ELEMENTS];
    //FILL VECTOR IN CRESCENT ORDER
    for i in 0..ELEMENTS{
        vector[i] = i as u32;
    }
    let mut min = vector[0];
    //------------------------------------------------
    let mut spawn_threads = vec![];
    
    for chunk in vector.chunks(ELEMENTS%N_THREADS) {
        let chunk = chunk.to_owned();
        spawn_threads.push(thread::spawn(move ||{
            //IN HERE I'M GONNA FIND THE MAXIMUM VALUE IN THE
            //INTERVAL BETWEEN INI AND FIM
            let mut menor = chunk[0];
            for elem in chunk{
                if menor < elem {
                    menor = elem;
                }
            }
            //---------------------------------------------
            menor
        }));
    }

    for f in spawn_threads{
        //espera as threads terminarem
        let t = f.join().unwrap();
        if min < t {
            min = t;
        }
    }
}

There are also crates intended to efficiently parallelize this kind of operations on vectors, like rayon.
Using rayon, this operation would have been as easy as

extern crate rayon;

use rayon::prelude::*;
const ELEMENTS:usize = 99999999;

fn main(){
    //DECLARE A VECTOR THE SIZE OF ELEMENTS IT SHALL HAVE
    let mut vector = [0u32; ELEMENTS];
    //FILL VECTOR IN CRESCENT ORDER
    for i in 0..ELEMENTS{
        vector[i] = i as u32;
    }

    let max = vector.par_iter().max();
}


#4

This is sort of tagential, but I suggest vec![0u32; ELEMENTS] instead. What you have is an array on the stack, which is unlikely to go well with 100M elements. (OP did use vec!)


#5

Somehow, this topic gives me a feeling of deja-vu


#6

Again? :grinning: