Baby Steps

A blog about programming, and tiny ways to improve it.

Purity in Parallel JavaScript

I can’t believe I’m saying this, but I’ve started to think that Parallel JS (nee Rivertrail) should not demand pure callbacks to functions like map() and so forth. Rather it should just accept arbitrary functions. Previously, I thought that it was important that ParallelArray methods should only accept functions which, at least in a perfect world, would be safely parallelizable. But I am no longer so sure why that is an important goal. Here is my reasoning.

First, we always retain the right to execute sequentially. If we’re executing sequentially, it’s actively harder to enforce purity than it is to permit mutation—we will have to put in place some kind of proxying or write monitoring. I would really like it if using ParallelArray methods was never slower than writing an equivalent for loop in JavaScript. That is, writing parallel_array.map(f) should not be slower than normal_array.map(f). But if we have to impose write monitoring, it will almost certainly be slower, in the event that we cannot parallelize. It will also be less general.

Second, it’s not clear to me what negative side effect (no pun intended) comes of permitting mutation in the callbacks. In cases where we can tell at compile time that the function is safe, we can permit it to execute in parallel. In cases where we cannot, we can speculatively run in parallel and then monitor the suspicious writes (as indeed we do today). If we detect a violation, it just means we fallback to sequential (just as we do today). It’s not like telling users they are not supposed to write to shared data means that they will not; ultimately, we must always be ready to enforce this contract dynamically, so the only question is what happens when we detect a violation.

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