[whatwg] ArrayBuffer and ByteArray questions

[Kenneth Russell]

On Wed, Sep 8, 2010 at 5:04 PM, Silvia Pfeiffer
<silviapfeiffer1 at gmail.com> wrote:
> On Thu, Sep 9, 2010 at 4:37 AM, Chris Marrin <cmarrin at apple.com> wrote:
>>
>> On Sep 8, 2010, at 11:21 AM, Oliver Hunt wrote:
>>
>> >
>> > On Sep 8, 2010, at 11:13 AM, Chris Marrin wrote:
>> >
>> >> Web Sockets is certainly another candidate, but I meant Web Workers.
>> >> There have been informal discussions on using ArrayBuffers as a way to
>> >> safely share binary data between threads. I don't believe anything has been
>> >> formalized here.
>> >
>> > You can't share data between workers.  There is no (and there cannot be)
>> > any shared state between multiple threads of JS execution.
>>
>> Right. I didn't mean literal sharing. But you can imagine some
>> copy-on-write semantics which would make it more efficient to pass data this
>> way.
>>
>
> Is this then similar to posting ImageData with Web Workers?
> (http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#imagedata)
> . I know that these can already be put into a postMessage and they are
> effectively arrays.

It's similar, but we want to define different semantics to achieve
higher performance. Copy-on-write does not work because in a
producer/consumer scenario the producer will always overwrite the same
buffer passed to the consumer, leading to a copy each time. We want to
make the source ArrayBuffer, and any ArrayBufferViews, zero length
upon posting them to a worker or back to the main thread. By
ping-ponging the same ArrayBuffer back and forth you can avoid
allocating new backing store each iteration.

-Ken