[webkit-dev] Parallel JavaScript: Why a separate ParallelArray types

Filip Pizlo fpizlo at apple.com
Fri Apr 12 14:13:48 PDT 2013

On Apr 12, 2013, at 1:59 PM, Ryosuke Niwa <rniwa at webkit.org> wrote:

> On Fri, Apr 12, 2013 at 1:50 PM, Filip Pizlo <fpizlo at apple.com> wrote:
> On Apr 12, 2013, at 1:39 PM, Jarred Nicholls <jarred.nicholls at gmail.com> wrote:
>> On Fri, Apr 12, 2013 at 2:54 PM, Filip Pizlo <fpizlo at apple.com> wrote:
>> For as little worth as it is, I agree with you Filip that providing low-level primitives would be best in terms of a foundation for many parallel programming models.  In terms of actually supporting shared mutable memory and locks, it would be a challenge for the engines to become thread-safe (internally) across contexts that share memory cells.  I'd envision the sharing of mutable memory being an opt-in semantic, marking a piece of memory as being shared/mutable from multiple contexts.
> Fixing the engines is a matter of typing. ;-)
> I don't think we need to add language features for opt-in, though this is an intriguing idea.
> Without opt-in, the one biggish challenge would be DOM accesses from threads other than the main thread; I suspect for those the initial implementation would have to throw an exception from non-main-threads if you try to access a DOM node.  This is akin to what some UI toolkits do: they let you have threads but prohibit access UI things from anything but the thread on which the runloop sits.  Of course, they don't do the thread-check; we would have to do it to preserve integrity and security.
> We already have Web workers for this kind of stuff, no?  Is your proposal significantly different from what Web worker offers?

Web workers don't have shared memory.  They instead have a really expensive message passing model.

Yes, my proposal is significantly different from Web workers.

> This is just a thought but is it possible to infer semantics of what Web workers and use GPU or SIMD instructions instead of starting a new thread as appropriate?

Probably that would be hard, because the Web Worker semantics are so bizarre: they require copying things all over the place.  It's probably an even worse match for SIMD/GPU/multicore than just Array.prototype.forEach().


> - R. Niwa

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