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Comment by reissbaker | Hacker News Reader

I can't speak to the BananaBread codebase, since I haven't read it — although BananaBread performs poorly as compared to commercial engines, regardless — but if you look at even the published benchmarks in that blog post, the Worker-based implementation is slower in all cases except for Firefox with two bots. Chrome is always slower when using Workers, sometimes massively so, and Firefox with ten bots is slower multithreaded than single-threaded.

Regardless, shared memory isn't only useful for WebGL. It's useful for any kind of UI where you don't want to block the main thread, and if you don't have DOM access it's tough to make that work. If copying is fine then current Workers are good enough; if copying isn't fine, then this doesn't change that.

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I agree with

> If copying is fine then current Workers are good enough; if copying isn't fine, then this doesn't change that.

I am saying that copying is fine (for most apps).

Copying is fine because BananaBread is indeed less performant than commercial engines, as you said, and that actually makes it a better test candidate here. It does far more GL commands than a perfectly optimized engine would, which means much more overhead in terms of sending messages to the main thread.

Despite that extra overhead, it does very well. 2 bots, a realistic workload, is fast in Firefox, and the slowdown in Chrome (where message-passing overhead is higher) is almost negligible. 10 bots, as mentioned in the blogpost, is a stress test, not a realistic workload. As expected, things start to get slower there. (Game is still playable though!)

And large amounts of GL commands, as tested there, are much more than what a typical UI application would need. So for UI applications, that just want to not stall the main thread, I think a single worker proxying operations to the main thread could be enough. Copying is fine.

The proposal in the blogpost here is for other cases. Workers + copying already solve quite a lot, very well. What this new proposal focuses on are computation-intensive applications, that can multiply their performance by using multiple threads. For example, an online photo-editing application needs this. This might be just a small fraction of websites, but they are important too.

Here are some things that BananaBread doesn't test that I suspect would break larger games in commercial engines:

* High-poly models. This is one area where copying breaks down: that can be a ton of data. BananaBread has a single, low-poly model that it uses for NPCs, and it presumably rarely (if ever?) needs to be re-copied.

* Large, seamless worlds. If you can transfer the entire world into a single static GL buffer, sure, copying isn't a problem since it only happens once on boot. If you need to incrementally load and unload in chunks, you're going to be paying that cost again and again.

* Multi-pass rendering. In fact, the proxying approach makes multi-pass rendering impossible, as noted in the blog post.

By all means, if your application already works single-threaded, or within the confines of the existing spec, you're going to be fine. But memory copies aren't free, and UI offloading is one of the biggest reasons to use shared memory.

Shared memory in Workers is nice — it doesn't make anything worse, and it makes some things better! — but it's a little disappointing that the main thread can't access the shared memory buffers. That's all.

I understand your disappointment, clearly more opportunities are opened up on the main thread. As the article says, this is a proposed first step, and the main thread is trickier, so it can be considered carefully later on. Meanwhile, for a large set of use cases, the current proposal can provide massive speedups.

> if copying isn't fine, then this doesn't change that.

Good point. I imagine it would be possible to transfer ownership of a SharedArrayBuffer to the main thread from a worker, just as it is done today with zero-copy transferable ArrayBuffers.

References to the SharedArrayBuffer in all workers would be made unusable. When the code on the main thread is done it can transfer ownership of the SharedArrayBuffer back to the workers.

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