rayon/Lobby

Rayon: A data parallelism library for Rust -- https://github.com/rayon-rs/rayon

xmac94x

@xmac94x:matrix.org

Hi everyone :) I just noticed some weird behavior on our game velorem where we are waiting for some threads to finish in places where we wouldn't have thought they would. I opened a issue with the details: rayon-rs/rayon#969 . Not sure if this is a bug, but we noticed that a threadpool.spawn inside a parallel iterator will let the parallel iterator wait until the spawn completed. Which is not always what we want. Do you know any good workaround for that problem ? Maybe already others experienced it (:

Josh Stone

@jistone:fedora.im

xmac94x: I think this is the nature of work-stealing, but see my reply on the issue for gory details

(also, don't take my word as gospel!)

xmac94x

@xmac94x:matrix.org

I understand that rayon is using work-stealing, but not why its not having a check to not steal unreleated work inside a par_iter ? Or even steal work from other scopes

Josh Stone

@jistone:fedora.im

it has no concept of what is "related"

there's one global queue for each pool, and each thread within the pool has a local deque

Miguel Raz Guzmán Macedo

@miguelraz

Hello!
I'm looking for a good collection of serial vs Rayon's parallelization benchmarks snippets

Thinking of stuff like
Dot product calculation,
Independently processing many strings in a big file
or anything numeric/related to linear algebra

Josh Stone

@jistone:fedora.im

rayon-demo (in the rayon repo) has some, not sure if you'd call them "good" though ;)

Miguel Raz Guzmán Macedo

@miguelraz

@jistone:fedora.im yup, those seem just like what I wanted. What's the command to run them though? cargo bench in the rayon/rayon-demo dir?

Ah, yes...
https://github.com/rayon-rs/rayon/wiki/Guide-to-Development

:sweat:

whoknow

@whoknow:matrix.org

Hi, somewhere in my code I need to check a big vector (also some times it can be small) for finding some data in it (Im using data.iter().find(...) in old method) its work good, I didn't notice any kind of performance lost in it, but I was thinking changing it so it search in parallel using rayon make sense? I mean there is any cost for rayon to spawn the threads everytime Im calling it? (as I said it would so many times...)
at the end is it worth it for use it in this kind of situations?

Josh Stone

@jistone:fedora.im

rayon keeps a threadpool, so after the first time it won't be spawning any new threads

3 replies

(unless you explicitly create your own ThreadPool each time)

whoknow

@whoknow:matrix.org

also one other thing, is it possible to use rayon inside normal loops?

2 replies

WGH

@WGH:torlan.ru

No clue about Windows, never really used it for any development work

On Linux, I'd use perf, if that helps

No, you can't

Josh Stone

@jistone:fedora.im

the language for loops are hard-coded to the regular Iterator trait

but if you can rewrite that with Iterator::for_each, then this can translate to ParallelIterator::for_each

I have a crate that hacks that transformation with a procedural macro: https://crates.io/crates/rayon-macro

Miguel Raz Guzmán Macedo

@miguelraz

Let’s say I have a dir with many nested dirs (at different levels of , all containing .txt files that can be processed independently.
What’s the Rayon idiomatic way of processing them? (for argument’s sake, a wc on each file will suffice)

Miguel Raz Guzmán Macedo

@miguelraz

Answering my own question :)
https://rust-lang-nursery.github.io/rust-cookbook/concurrency/parallel.html#generate-jpg-thumbnails-in-parallel

WcaleNieWolny

@WcaleNieWolny

pub fn fast_split(data: &[i8], cache: Vec<usize>) -> Vec<i8>{
        let mut out = Vec::<i8>::with_capacity(data.len());
    //TODO: resize

        for i in 0..data.len(){
            out[cache[i]] = data[i]
        }

        out
    }

how would I make this run in parallel? Like using rayon or something like that

Josh Stone

@jistone:fedora.im

@WcaleNieWolny: arbitrary parallel writes are difficult in Rust, but the best way is probably to temporarily cast that out vector to &mut [AtomicI8].

you can do that on nightly with https://doc.rust-lang.org/std/sync/atomic/struct.AtomicI8.html#method.from_mut_slice

or with your own unsafe pointer cast the same way, https://doc.rust-lang.org/src/core/sync/atomic.rs.html#2064

then in rayon, (0..data.len()).into_par_iter().for_each(|i| /* atomic store */)

it may not parallelize well though -- memory bound and probably cache-unfriendly

mr.rusty

@mr.rusty:matrix.org

Hi everyone! Just wanted to know if things like the JoinHandle are possible with Rayon. Or are there alternatives to wait for a task to finish?

Josh Stone

@jistone:fedora.im

mr.rusty: most of the rayon api blocks for completion, but for spawn you can use a scope, as that waits for its spawns to complete before returning.

mr.rusty

@mr.rusty:matrix.org

Thank your, i think that'll help me :)

mr.rusty

@mr.rusty:matrix.org

:point_up: Edit: Thank you, i think that'll help me :)

Miguel Raz Guzmán Macedo

@miguelraz

Hello @jistone:fedora.im !

I'm thinking of using some of my Xmas break to implement this - rayon-rs/rayon#329

What's a good way to start?

Josh Stone

@jistone:fedora.im

I don't know how Niko thought that would work, and he never got back to that issue with a better write-up...

@miguelraz: you might have to do your own research on that one

maybe looking into what @wagnerf42 was doing

wagnerf42

@wagnerf42

yeah i do have a scan

https://github.com/wagnerf42/diam/blob/main/examples/scan.rs

each task has its own state

wagnerf42

@wagnerf42

i use it in several places but i think it's always in use with a par_windows

Miguel Raz Guzmán Macedo

@miguelraz

Oh sweet, thanks @wagnerf42!
What would you say are the big blockers before shaping that up into something that could land in Rayon?

Also - I'm a huge fan of your course. I've been scouring for HPC courses that exploited Rust and yours is the only one I've been able to find so far!

<3

JT

@jntrnr

I might be missing something - but if you do .par_bridge() on an iterator that's potentially infinite, is there a way to get the values back out without collecting? Can you chunk it? If not, is there another way to work with a potentially unbound iterator?

wagnerf42

@wagnerf42

@miguelraz hey thanks a lot. thanks to let me know. i have a lot of material i should upload

wagnerf42

@wagnerf42

@jntrnr https://is.gd/QFQpOF