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Re: Web Workers

From: Jussi Kalliokoski <jussi.kalliokoski@gmail.com>
Date: Tue, 13 Mar 2012 15:38:27 +0200
Message-ID: <CAJhzemULQLZ69onX2gC7Q8HAHH5uc15+Dm7C27U4S6Vtvbiwkw@mail.gmail.com>
To: robert@ocallahan.org
Cc: Dmitry Lomov <dslomov@google.com>, Chris Rogers <crogers@google.com>, Alistair MacDonald <al@signedon.com>, public-audio@w3.org
To come back to the subject of doing audio processing outside workers, I
was almost convinced that it's OK to force people to use a worker for it,
because it's a good practice. But then I realized that if we do so, we make
a few use cases inherently harder. The person writing the code that
interfaces with the Audio API may not have the luxury of choice in the
matter. A few examples:

 * A game console emulator. If the console runs everything in a single
thread, you're going to be in serious trouble if you're trying to isolate
the audio processing into a separate thread. Game console emulation is hard
enough as it is.
 * We mustn't forget that the web is increasingly becoming a compilation
target, so the code running may be a port of existing code designed for a
completely different environment.


On Fri, Mar 9, 2012 at 2:10 AM, Robert O'Callahan <robert@ocallahan.org>wrote:

> On Thu, Mar 1, 2012 at 9:49 PM, Dmitry Lomov <dslomov@google.com> wrote:
>> On Wed, Feb 29, 2012 at 3:23 PM, Robert O'Callahan <robert@ocallahan.org>wrote:
>>> The writeAudio method still does a copy. Maybe that copy can be avoided
>>> with an alternative design using transfers, but simply preallocating an
>>> output buffer as Dmitry suggested isn't quite the right solution since an
>>> onprocessmedia handler can output any amount of audio, including none.
>>> Instead we probably should have writeAudio take ownership of the buffer and
>>> neuter it, or alternatively keep the current copy semantics for writeAudio
>>> and have a separate transferAudio method that uses the transfer semantics.
>> So these two proposals handle transfer-out but not transfer-in. As you
>> say, it will either be a copy in a writeAudio, or, if we have a
>> transferAudio method, an allocation on every callback (because you cannot
>> reuse a transferred-out array buffer).
>> If event has a perallocated output buffer, we will avoid any copies and
>> allocations.
> The allocation overhead can be practically eliminated by recycling buffers
> internally in the browser.
> The preallocated output buffer approach still has the problem of not
> handling situations where the processing engine isn't outputting the same
> amount of data that was input.
>>> Both current proposals suggest that any worker can handle only one
>>>> JavaScriptAudioNode/ProcessedMediaStream (since the pocessing nodes run
>>>> a
>>>> predefined callback on the worker).
>>> This is definitely not the case for ProcessedMediaStream. There is no
>>> problem with using the same Worker for several ProcessedMediaStreams. If
>>> you want to have different types of processing performed in the same
>>> Worker, you could use a field of the 'params' object in each onprocessmedia
>>> event to distinguish them.
>> so you would have something like:
>> self.onprocessmediaevent = function(event) {
>>    if (event,param.routine == "sinGenerator") { ... }
>>    else if (event.param.routine == "fobarEffect") { ... }
>>    ....
>> }
>> so you would basically have a dispatch in JavaScript?
>> This has several drawback:
>> 1) dispatch in JavaScript takes precious cycles that might have been used
>> processing audio
>> 2) methods structured like this are harder to optimize by JavaScript
>> engine JIT
>> 3) this requires param to have a value and be transferred to worker on
>> every call - this takes valuable cylces.
>> 4) the code looks ugly :)
> I'd write it like this:
> var dispatchTable = {
>   sinGenerator: function(event) {
>     ...
>   },
>   foobarEffect: function(event) {
>     ...
>   }
> };
> self.onprocessmedia = function(event) {
>   dispatchTable[event.params.type](event);
> };
> ...
> which reduces those drawbacks. I expect the overheads introduced are
> negligible.
>  My proposal, that lets you specify a callback name, solves all of that.
>> Moreover, I can see other benefits:
>> a. no need to add extra methods to WorkerContext.idl
>> b. ease of code use and re-use. The user can package her library of audio
>> effects, where all audio effects have different names but conform to the
>> same signature, and then easily import those into worker and    call them -
>> no need for any boilerplate in the worker code itself.
> On the other hand, it reduces isolation of the worker since it gives page
> script the ability to call any global function in the worker instead of
> going through predefined entry points (onmessage, onprocessmedia). I
> suspect that's not a good idea. It's also more work to implement, for a
> case that maybe doesn't even matter that much.
>>> However, I think we should not encourage authors to multiplex processing
>>> nodes onto Workers by hand, since the optimal assignment could depend on
>>> the details of the browser and platform (e.g. how heavyweight Workers are,
>>> and how many cores are available to the browser).
>> It looks like a simple UA-independent guideline that can be given is this:
>> - if your audio processing nodes are sequential in the graph, dispatch
>> them onto the same worker
>> - if they are parallel, dispatch them on different workers.
> That's good but it may not be optimal. In some situations you might get
> better throughput by placing sequential nodes on different cores.
>> Instead of making params a field of event object, maybe it will be cleaner
>>>> to make params an extra argument to callback, and also pass initial
>>>> value
>>>> on node/stream construction?
>>> I don't understand the benefits of this. Having an event callback with a
>>> single event object parameter is a standard pattern for Web APIs and I
>>> don't see a need to deviate from it here.
>> This just makes for nicer looking and more understandable code, both on
>> node creation and in processing routine.
> OK, it's not a big deal one way or another.
> My biggest problem with the params approach right now is this... It's
> natural to write:
> stream.params = {x:1, y:2};
> stream.params.x = 3;
> Unfortunately the second statement does not trigger any change in the
> worker, because only assignments to the params attribute itself trigger
> structured cloning and dispatch to the worker. I haven't thought of a
> reasonable fix for this yet. Any suggestions appreciated.
> Rob
> --
> “You have heard that it was said, ‘Love your neighbor and hate your
> enemy.’ But I tell you, love your enemies and pray for those who persecute
> you, that you may be children of your Father in heaven. ... If you love
> those who love you, what reward will you get? Are not even the tax
> collectors doing that? And if you greet only your own people, what are you
> doing more than others?" [Matthew 5:43-47]
Received on Tuesday, 13 March 2012 13:39:05 UTC

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