How do I pass disjoint slices from a vector to different threads?

Question

I am new to Rust, and struggling to deal with all those wrapper types in Rust. I am trying to write code that is semantically equal to the following C code. The code tries to create a big table for book keeping, but will divide the big table so that every thread will only access their local small slices of that table. The big table will not be accessed unless other threads quit and no longer access their own slice.

#include <stdio.h>
#include <pthread.h>

void* write_slice(void* arg) {
    int* slice = (int*) arg;
    int i;
    for (i = 0; i < 10; i++)
        slice[i] = i;
        
    return NULL;
}

int main()
{
    int* table = (int*) malloc(100 * sizeof(int));
    int* slice[10];
    int i;
    for (i = 0; i < 10; i++) {
      slice[i] = table + i * 10;
    }
    
    // create pthread for each slice
    pthread_t p[10];
    for (i = 0; i < 10; i++)
        pthread_create(&p[i], NULL, write_slice, slice[i]);
    
    for (i = 0; i < 10; i++)
        pthread_join(p[i], NULL);
        
    for (i = 0; i < 100; i++)
        printf("%d,", table[i]);
}

How do I use Rust's types and ownership to achieve this?

Answer 1

Let's start with the code:

// cargo-deps: crossbeam="0.7.3"
extern crate crossbeam;

const CHUNKS: usize = 10;
const CHUNK_SIZE: usize = 10;

fn main() {
    let mut table = [0; CHUNKS * CHUNK_SIZE];

    // Scoped threads allow the compiler to prove that no threads will outlive
    // table (which would be bad).
    let _ = crossbeam::scope(|scope| {
        // Chop `table` into disjoint sub-slices.
        for slice in table.chunks_mut(CHUNK_SIZE) {
            // Spawn a thread operating on that subslice.
            scope.spawn(move |_| write_slice(slice));
        }
        // `crossbeam::scope` ensures that *all* spawned threads join before
        // returning control back from this closure.
    });

    // At this point, all threads have joined, and we have exclusive access to
    // `table` again.  Huzzah for 100% safe multi-threaded stack mutation!
    println!("{:?}", &table[..]);
}

fn write_slice(slice: &mut [i32]) {
    for (i, e) in slice.iter_mut().enumerate() {
        *e = i as i32;
    }
}

One thing to note is that this needs the crossbeam crate. Rust used to have a similar "scoped" construct, but a soundness hole was found right before 1.0, so it was deprecated with no time to replace it. crossbeam is basically the replacement.

What Rust lets you do here is express the idea that, whatever the code does, none of the threads created within the call to crossbeam::scoped will survive that scope. As such, anything borrowed from outside that scope will live longer than the threads. Thus, the threads can freely access those borrows without having to worry about things like, say, a thread outliving the stack frame that table is defined by and scribbling over the stack.

So this should do more or less the same thing as the C code, though without that nagging worry that you might have missed something. :)

Finally, here's the same thing using scoped_threadpool instead. The only real practical difference is that this allows us to control how many threads are used.

// cargo-deps: scoped_threadpool="0.1.6"
extern crate scoped_threadpool;

const CHUNKS: usize = 10;
const CHUNK_SIZE: usize = 10;

fn main() {
    let mut table = [0; CHUNKS * CHUNK_SIZE];

    let mut pool = scoped_threadpool::Pool::new(CHUNKS as u32);

    pool.scoped(|scope| {
        for slice in table.chunks_mut(CHUNK_SIZE) {
            scope.execute(move || write_slice(slice));
        }
    });

    println!("{:?}", &table[..]);
}

fn write_slice(slice: &mut [i32]) {
    for (i, e) in slice.iter_mut().enumerate() {
        *e = i as i32;
    }
}