How to Think about Parallel Programming: Not! [video] (2021)

https://www.infoq.com/presentations/Thinking-Parallel-Programming/

by caned • 1 year ago

21 23 1 year ago

infoq.com

fifilura6• 1 year ago

Disclaimer: at work so I didn't watch the video.

For loops are the "goto":s of the parallel programming era.

Ditch them and the rest can be handled by the programming language abstraction.

Why? Because they 1. Enforce order of execution and 2. Allow breaking computation after a certain number of iterations.

bee_rider3• 1 year ago

I’ve always been surprised that we don’t have a really widely supported construct in programming that is like a for loop, but with no dependency allowed between iterations. It would be convenient for stuff like multi-core parallelism… and also for stuff like out of order execution!

Not sure how “break” would be interpreted in this context. Maybe it should make the program crash, or it could be equivalent to “continue” (in the programming model, all of the iterations would be happening in parallel anyway).

I vaguely feel like “for” would actually have been the best English word for this construct, if we stripped out the existing programming context. I mean, if somebody post gives you instructions like:

For each postcard, sign your name and put it in an envelope

You don’t expect there to be any non-trivial dependencies between iterations, right? Although, we don’t often give each other complex programs in English, so maybe the opportunity for non-trivial dependencies just doesn’t really arise anyway…

In math, usually when you encounter “for,” it is being applied to a whole set of things without any loop dependency implied (for all x in X, x has some property). But maybe that’s just an artifact of there being less of a procedural bias in math…

Weryj• 1 year ago

Sounds like you're talking about Realtime operating systems. I don't know if there are many/any programming languages that build those operational requirements into the syntax/abstraction.

mannykannot• 1 year ago

The tricky cases are the very many where there are dependencies between iterations, but not demanding the strict serialization that a simple loop enforces. We have constructs for that, but there's an irreducible complexity to using them correctly.

two_handfuls• 1 year ago

They're not in the language proper, but "parallel for" is a common construct. I've seen it in C# and Rust, but I'm sure other languages have it too.

It may be a good idea to use a framework with explicitly stateless "tasks" and an orchestrator (parallel, distributed, or both). This is what Spark, Tensorflow, Beam and others do. Those will have a "parallel for" as well, but now in addition to threads you can use remote computers as well with a configuration change.

epgui2• 1 year ago

we don’t have a really widely supported construct in programming that is like a for loop, but with no dependency allowed between iterations

Uhhh... we don't? It seems to me like we do. This is a solved problem. Depending on what you're trying to do, there's map, reduce, comprehensions, etc.

vlovich1235• 1 year ago

We actually do have the abstractions but the problem is that the vast majority of for loops don’t benefit - you need to have so much work that the overhead of coordinating the threads is sufficient. Additionally, you’ve got all sorts of secondary effects like cache write contention that will fight any win you try to extract out of for loops parallelism. What we’ve been learning for a long time as an industry is that you benefit most from task level parallelism with minimal to no synchronization.

mystified5016• 1 year ago

like a for loop, but with no dependency allowed between iterations

"Break" is a dependency between iterations, and really only makes sense in a sequential iteration. In a parallel for loop, you can break from the current iteration, but the next is probably already running.

If you want any iteration to be able to cancel all others, they have to be linked somehow. Giving every task a shared cancellation token might be simplest. Or you turn your for loop into a sort of task pool that intelligently herds threads in the background and can consume and relay cancellation requests.

But I agree, we need a new paradigm for parallel programming. For loops just don't cut it, despite being one of the most natrual-feeling programming concepts.

C#'s Parallel.For and ForEach are a step in the right direction, but very unergonomic and unintuitive. I think we could get by with just bolting parallelism onto for loops, but we need a fundamentally parallel concept. I assume it'd look something like cuda programming but I really don't know.

Jtsummers• 1 year ago

(2011) and was submitted back then: https://news.ycombinator.com/item?id=2105661

How to Think about Parallel Programming: Not! [video] (2021)

Related Stories