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Re: Proposal for fixing race conditions

From: Chris Rogers <crogers@google.com>
Date: Wed, 17 Jul 2013 14:40:57 -0700
Message-ID: <CA+EzO0mwEFudmAz6=UANJxhW_dm78QK4G13NsX96Vc7gtXvY5w@mail.gmail.com>
To: Marcus Geelnard <mage@opera.com>
Cc: Srikumar Karaikudi Subramanian <srikumarks@gmail.com>, "K. Gadd" <kg@luminance.org>, Jer Noble <jer.noble@apple.com>, Olivier Thereaux <Olivier.Thereaux@bbc.co.uk>, WG <public-audio@w3.org>
On Wed, Jul 17, 2013 at 2:34 PM, Marcus Geelnard <mage@opera.com> wrote:

>
>
>
> On Wed, Jul 17, 2013 at 8:41 PM, Chris Rogers <crogers@google.com> wrote:
>
>>
>>
>>
>> On Tue, Jul 16, 2013 at 7:24 PM, Srikumar Karaikudi Subramanian <
>> srikumarks@gmail.com> wrote:
>>
>>> Your hypothetical test case merely demonstrates the difference; my point
>>> is that it is silly to optimize for imaginary edge cases at the cost of
>>> real-world use cases where developers will get unexpected results due to
>>> leaving race conditions in this API. I should also note that it has come up
>>> in past discussions that we could always introduce new no-copy APIs that
>>> don't contain races, if the cost of memcpy is so severe.
>>>
>>>
>>> It is not inconceivable to make an audio editor which plays an audio
>>> file from a specific sample onwards by assigning the buffer to an
>>> AudioBufferSourceNode and using start(t,offset,duration) ... possibly
>>> followed by effects. Large files (even 5mins?) would be unusable with such
>>> an editor if a copy were involved and clients/devs will be forced to do
>>> crazy optimizations just to get it to work. Now shift that situation to an
>>> iPad with limited memory and it can get worse. DAWs are a use case for the
>>> API.
>>>
>>> With Jer's example code, it would be possible to simulate such a
>>> (reasonable) case.
>>>
>>> What might, I think, be acceptable is a one-time copy provided the copy
>>> can be reused without additional cost. As far as I can see, immutable data
>>> structures are the best candidates to solve the race conditions.
>>>
>>> That said, I do find the argument (I think Rogers') that the worst thing
>>> that can happen with these race conditions is unexpected audio output and
>>> hence they are not very important an interesting stand.
>>>
>>
>> You're simplifying my position a bit.  What I'm saying is there are no
>> sensible or normal calling patterns where this type of race conditions is
>> even a possibility.  As the API is designed and has been used for over 2
>> years, these calling patterns are not used and so simply are not an issue.
>>  We do have substantial developer experience to support this view, and
>> these developers come from a wide range of backgrounds and experience
>> levels from complete novices playing with audio for the first time, all the
>> way to seasoned professional audio developers.
>>
>>
> If we look at it from this angle instead: Do you think that it would be a
> problem for Web developers if one browser neuters arrays (as the FF
> implementation currently does - which I think doesn't violate the spec),
> but another browser doesn't?
>

That seems like a very odd point of view.  There are an uncountable number
of things the spec doesn't say which an implementation *could* do, but
still wouldn't really be correct.  I would consider that if the spec
doesn't say something specifically about neutering buffers, then the
default assumption would be *not* to do it.

Chris



>
>
> /Marcus
>
>
>
>> Chris
>>
>>
>>
>>>
>>> -Kumar
>>>
>>> On 17 Jul, 2013, at 7:13 AM, "K. Gadd" <kg@luminance.org> wrote:
>>>
>>> Of course you can claim hypothetical performance benefits from any
>>> particular optimization, my point is that in this case we're considering
>>> whether or not to leave *race conditions* in a new Web API because we think
>>> it might make it faster. We *think* it *might*. Making that sort of
>>> sacrifice in favor of 'performance' without doing any reproducible,
>>> remotely scientific testing to see whether it's actually faster, let alone
>>> fast enough to justify the consequences, seems rash to me.
>>>
>>> It should be quite easy to test the performance benefits of the racy
>>> version of the API, as based on my understanding the Firefox implementation
>>> currently makes copies. You need only run your test cases in Firefox with
>>> SPS and see how much time is spent making calls to memcpy to get a rough
>>> picture of the actual overhead. And once you know that, you can look at how
>>> your test cases actually perform and see if the cost of that memcpy makes
>>> it impossible to ship an implementation that makes those copies.
>>>
>>> I am literally unable to imagine a use case where the cost of the copies
>>> would add up to the point where it would remotely be considered a
>>> bottleneck. It is the case that the copies probably have to be synchronous,
>>> so I could see this hurting the ability to trigger tons and tons of sounds
>>> in a single 'frame' from JS, or set tons and tons of curves, etc. But
>>> still, memcpy isn't that slow, especially for small numbers of bytes.
>>>
>>> Your hypothetical test case merely demonstrates the difference; my point
>>> is that it is silly to optimize for imaginary edge cases at the cost of
>>> real-world use cases where developers will get unexpected results due to
>>> leaving race conditions in this API. I should also note that it has come up
>>> in past discussions that we could always introduce new no-copy APIs that
>>> don't contain races, if the cost of memcpy is so severe.
>>>
>>>
>>> On Tue, Jul 16, 2013 at 6:27 PM, Jer Noble <jer.noble@apple.com> wrote:
>>>
>>>>
>>>> On Jul 16, 2013, at 1:18 PM, K. Gadd <kg@luminance.org> wrote:
>>>>
>>>> This claim has been made dozens of times now on the list and I've seen
>>>> multiple requests for even a single test case that demonstrates the
>>>> performance impact. Is there one? I haven't seen one, nor a comment to the
>>>> effect that one exists, or an explanation of why there isn't one.
>>>>
>>>>
>>>> Isn't this self-evident?  Any solution which involves additional
>>>> memcopy calls during the normal use of the API will have an inherant and
>>>> known performance cost at the point of the memcopy.  Additionally, there is
>>>> the ongoing performance cost of having duplicate, in-memory copies of audio
>>>> data, as well as the additional GC cost of those extra copies.
>>>>
>>>> That said, it would be very easy to demonstrate: in the hypothetical
>>>> test case, create a new ArrayBuffer from source data before passing it into
>>>> the API.  I.e.,
>>>>
>>>> sourceNode.buffer = buffer
>>>>
>>>>
>>>> becomes:
>>>>
>>>> sourceNode.buffer = buffer.slice(0)
>>>>
>>>>
>>>> -Jer
>>>>
>>>
>>>
>>>
>>
>
Received on Wednesday, 17 July 2013 21:41:25 UTC

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