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Re: TAG feedback on Web Audio

From: Chris Wilson <cwilso@google.com>
Date: Tue, 6 Aug 2013 09:58:02 -0700
Message-ID: <CAJK2wqX=_WrNo0vZHj0v4e2t5Z2fuf78d6TSrcejE=wx4rrqFg@mail.gmail.com>
To: Marcus Geelnard <mage@opera.com>
Cc: Alex Russell <slightlyoff@google.com>, Noah Mendelsohn <nrm@arcanedomain.com>, Anne van Kesteren <annevk@annevk.nl>, Olivier Thereaux <Olivier.Thereaux@bbc.co.uk>, "robert@ocallahan.org" <robert@ocallahan.org>, "public-audio@w3.org" <public-audio@w3.org>, "www-tag@w3.org List" <www-tag@w3.org>
I still (obviously) disagree that a model that relies on passing copies
around will have the same memory/speed/latency performance.  It's possible
the actual copying can be minimized, at a cost in usability to the
developer, but even with that I don't think the cost will ever be truly
zero; so this is a tradeoff.  It may be that the group chooses that
tradeoff - but traditional audio APIs have not.  Of course, most of them
don't have the limited execution scope of JS, either.  (Note that I should
have said "glitch-free, reasonably-low-latency audio with good performance
and API usability." :)

Interestingly enough, if we'd not allowed JS in nodes (rather used a more
restricted audio processing language), I'm not sure we'd have this problem
to this scale.

On Tue, Aug 6, 2013 at 2:43 AM, Marcus Geelnard <mage@opera.com> wrote:

>  2013-08-06 04:20, Chris Wilson skrev:
> See, I read two different things from this.  Marcus, I heard you say the
> comment about not being describable in JS was in specific reference to
> threading and memory sharing not being part of the current model; from
> Alex, I heard "open your mind to what JS *could* be, and then describe
> everything in one model" - in part, the nodes' behaviors themselves should
> be described in JS, albeit of course not necessarily run in JS.
>  The latter (as I'd previously expressed to Alex) is fine with me; I'd be
> fine describing how delay nodes work in JS.  However, high-quality
> glitch-free audio requires a number of things that aren't available to JS
> today - notably, a separate thread with restricted execution capabilities
> (i.e. not arbitrary JavaScript) and no garbage collection that can have
> elevated priority, and access to the physical hardware, of course.  It's
> not at all impossible to imagine how to describe Web Audio if you have
> those capabilities; and the memory sharing we're discussing under the
> rubric of race conditions is really whether that somewhat different
> execution environment supports those or not.
> I have always been a strong supporter of being able to describe and
> implement as much as possible of the API in JS, and I agree that there are
> certainly some important aspects missing in the JS execution environment
> that make a practical implementation in JS impossible (including access to
> audio HW and low-level control over thread execution etc). You should,
> however, still be able to implement the *behavior* of the API in JS (as
> an exercise - imagine implementing an OfflineAudioContext in JS - I don't
> see why it shouldn't be possible).
> I might have misinterpreted Alex's point (I thought he was referring to
> the shared data in the API interfaces, but of course there may be more
> things to it). However, from the perspective of making the Web Audio API
> describable in JS terms, I still think that the issue of exposing shared
> data interfaces to JS is a key deficiency in the current design, mostly
> because:
> * It makes it impossible to express operations such as
> AudioBuffer.getChannelData() in JS terms.
> * Since the interfaces are exposed to JS, the *only* way to make them
> JS-friendly would be to change/extend the JS memory model (which is a much
> bigger task than to change the Web Audio API model).
> * It's not in any way necessary for achieving the goal "glitch-free,
> reasonably-low-latency audio".
> In fact, I believe it should be fully possible to implement the Web Audio
> API without using shared *mutable* data internally (provided that we drop
> the shared data model from the interfaces). Correct me if I'm wrong, but
> I'm pretty sure you could solve most things by passing read-only references
> to data buffers between threads (which would be semantically equivalent to
> passing copies around) and still have the same memory/speed/latency
> performance. That would make it a moot point whether or not shared data
> must be part of a potential fictional execution environment or not.
> /Marcus
> On Mon, Aug 5, 2013 at 1:49 PM, Alex Russell <slightlyoff@google.com>wrote:
>>  On Sun, Aug 4, 2013 at 6:46 PM, Chris Wilson <cwilso@google.com> wrote:
>>> (Sorry, on vacation, and beach > Web Audio discussions. :)
>>>  Alex, as we discussed a few weeks ago - glitch-free,
>>> reasonably-low-latency audio is something that I just don't believe JS -
>>> *as it is today - *can do.
>>  In the TAG feedback document you might have detected two lines of
>> attack on this argument:
>>    1. It's insufficient to say "JavaScript can't do this". The entire
>>    goal of some of the suggestions in the document related to the execution
>>    model are all about creating space in the model to change the constraints
>>    under which JS (or something else) runs such that JS* *(or ASMJS or
>>    NaCL in a worker, etc.) *could* do what's needed without breaking the
>>    invariants of either the platform or the conceptual semantic model that Web
>>    Audio presents. So that's simply an argument against an assertion *that
>>    hasn't been presented*. It fails on that ground alone, however...
>>    2. Nothing in my argument against breaking the platform invariants
>>    requires that you actually RUN the built-in node types in JS. To some great
>>    degree, I'm advocating for JS-as-spec-language when I discuss de-sugaring.
>>    Not an implementation strategy (except perhaps in naive cases for
>>    newly-bootstrapping impls).
>> The net is that discussing main-thread-JS-as-we-know-it-today and not the
>> framework for execution/specification is arguing against an
>> implementation-level straw-man.
>>  Regards
>>    On Thu, Aug 1, 2013 at 5:42 PM, Alex Russell <slightlyoff@google.com>wrote:
>>>> Let me be clearer, then: the issue is that it introduces effects to JS
>>>> that can't be described *in terms of JS*. It violates the
>>>> run-to-completion model of the language without appealing to the turn-based
>>>> concurrency model we use everywhere else in the platform.
>>>> On Wed, Jul 31, 2013 at 10:43 AM, Chris Wilson <cwilso@google.com>wrote:
>>>>> In addition, I'd ask that you be more explicit than calling this
>>>>> problem "data races", because there's clearly some explicit effect you're
>>>>> trying to prevent.  Any asynchronously-programmable or event-driven system
>>>>> can enable developers to introduce race conditions.
>>>>> On Mon, Jul 29, 2013 at 10:09 PM, Noah Mendelsohn <
>>>>> nrm@arcanedomain.com> wrote:
>>>>>> On 7/29/2013 7:05 PM, Anne van Kesteren wrote:
>>>>>>> On Sat, Jul 27, 2013 at 9:22 AM, Noah Mendelsohn<
>>>>>>> nrm@arcanedomain.com>  wrote:
>>>>>>>> >Again, I have no informed opinions on the specific merits, just
>>>>>>>> suggesting a
>>>>>>>> >useful role the TAG might play to clarify for the many members of
>>>>>>>> the
>>>>>>>> >community who are less expert on this than you are. Thank you.
>>>>>>  I'm not sure we call out data races anywhere, it's something we just
>>>>>>> don't do.
>>>>>>  Well, my recollection may be faulty, but I think that one of the
>>>>>> reasons the TAG took the trouble to formalize things like the architecture
>>>>>> document was the belief that it's easier to ask skeptics to stick to rules
>>>>>> that have been written down, and especially those that have garnered formal
>>>>>> consensus through something like the Recommendation track.
>>>>>> Whether it's worth taking a guideline on data races all the way to
>>>>>> Rec I'm not sure, but it seems that it would be worth setting it down
>>>>>> formally, perhaps in a TAG Finding/blog post/Recommendation or whatever
>>>>>> will get the right level of discussion, consensus building, and eventually
>>>>>> attention.
>>>>>> Certainly, of the many things that have come up recently relating to
>>>>>> APIs, this one seems deeply architectural and very much within the TAG's
>>>>>> remit.
>>>>>> Noah
> --
> Marcus Geelnard
> Technical Lead, Mobile Infrastructure
> Opera Software
Received on Tuesday, 6 August 2013 16:58:29 UTC

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