WASM Multithreading

[BenMcLean]

For several years, I've been writing a .NET Standard 2.0 library (using PolySharp for newer C# language features) that does some graphics processing on the CPU, which means many operations it does are CPU-bound. When it came time to add multi-threading, I went through the codebase and just replaced a lot of Select queries with this:

/// <summary>
/// Parallelizes the execution of a Select query while preserving the order of the source sequence.
/// </summary>
public static List<TResult> Parallelize<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector) => [.. source
	.Select((element, index) => (element, index))
	.AsParallel()
	.Select(sourceTuple => (result: selector(sourceTuple.element), sourceTuple.index))
	.OrderBy(resultTuple => resultTuple.index)
	.AsEnumerable()
	.Select(resultTuple => resultTuple.result)];

Just replacing Select with Parallelize made the same operations take a small fraction of the time they used to. It works great whenever you have a series of CPU-bound tasks that you know in advance don't depend on each other or on anything that changes to get done.

Now I'd like to give my library a web front-end for people to use and host the web front-end for free on Github Pages. So naturally, the first tool I reached for was Blazor WASM because that would be the Microsoft way to do this. The Parallelize method I wrote works on Blazor WASM, except single-threaded because Blazor WASM only provides a single-threaded runtime. Being single-threaded means it blocks the UI which is bad ... but the user's patience is eventually rewarded with the result they requested so that's good.

But then I read that Uno Platform has multi-threading support on WASM but I'm not sure I fully understand the requirements. Would a PLINQ method like this be able to actually use multi-threading? Apparently it requires a special header to be added to the site: so is this something I could still host for free on Github Pages? When making multi-threaded code, what stuff is allowed vs not allowed on WASM?

The real point / heart of my question is that I am considering whether I should port my front-end from Blazor WASM to Uno or just start over coding the front-end in Uno solely in order to get the multi-threading feature. Is that worth it? I'd hate to invest many hours in learning the Uno platform just for this only to realize that multi-threading for my application would be untenable.

[jeromelaban]

Thanks for the question.

Wasm threading is experimental overall, is a best effort support in .NET 8, and is not supported in .NET 9 on Uno's side because of structural changes made by the runtime. We expect this to change in .NET 10, however.

[BenMcLean]

So if I were to do this, I'd need to make my Uno project in .NET 8. OK, that's good to know, but which multi-threading techniques in the code would (at least most likely) work and which wouldn't? Is this multi-threading support going through some kind of special JavaScript interop class to make the threads or is it just implementing it in the runtime to work with normal C# async/await methods? Would PLINQ like I showed work?

[jeromelaban]

Threading that does not use any javascript interop is generally accepted in net8, you'd otherwise need to go back to the UI thread first. You can use any async method that does actual parallel CPU-bound work.

The best is to try it out and see what works for you.