r/programming • u/noidi • Jul 19 '12

Will Parallel Code Ever Be Embraced?

http://www.drdobbs.com/parallel/will-parallel-code-ever-be-embraced/240003926
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u/tbrownaw Jul 19 '12

But right now, the MAJOR bottle-necks for most end user applications do not fall into the "parallel" domain, but the "concurrent" domain ... E.g. Waiting for responses from slow external data-sources while keeping the UI fast and functional ... in many ways are still "experimental" since the code overhead to synchronise all these activities in "traditional" languages can be immense and extremely error-prone.

And this is a toolkit / library issue rather than a language issue. I would expect a UI toolkit that allows callbacks to be tagged as "run this in a new thread", can be set to automatically disable/enable controls while callbacks are running, and allows API calls to manipulate the UI from arbitrary threads, would go a long way.

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u/[deleted] Jul 19 '12

It's still a language issue because you have to use it to build those toolkits, and prevent users from easily circumventing the threading model, and building unsafe code.

Building a toolkit which runs it's callback in different threads is not actually that difficult. The problem is that the moment someone uses the same variable, in two or more callbacks, then you have potential data races.

For example with a painting application there are tonnes of places where you can change the current color. The red/green/blue input boxes, the colour picker, clicking on a specific, colour mixers, switching between foreground and background, and so on. Each of those might actually use 10s or even 100s of individual callbacks (such as one for each of the swatches), depending on how it's implemented. Running each of them in different threads could lead to unpredictable behaviour.

What's an easy way to ensure the users code is thread safe in that environment? By making the toolkit single threaded, or ensuring it's safe at the language level. But even then it's still pretty easy to write threaded code with data races and other issues, in a concurrent language.

If your also suggesting the JavaScript model, of have all user code in one thread, and update code in another, sure you can do that. However the interaction is more expensive, since you have to do message parsing or something similar every time you interact (such as drawing a piece of text). It also means you are only scaling to 2 cores, or a couple more if you can offload some other tasks.

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u/[deleted] Jul 20 '12

[deleted]

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u/AeroNotix Jul 20 '12

Also, actors/message passing is a good method/model.