[pulseaudio-discuss] [PATCH v4] Make module loopback honor requested latency

[Alexander E. Patrakov]

08.02.2015 02:14, Georg Chini wrote:
> Sorry, but I do not think the smoother is the problem here. I do get
> quite reliable latency results.
> The problem is really (if there is a problem at all) the execution time
> of the code. These are not
> asynchronously called functions, they wait until they are finished. And
> that exactly is the problem,
> sometimes the queue is quite full and so it takes a lot of time until
> the pa_asyncmsgq_send() returns.
> The smoother was one of the first things I suspected to be responsible
> for false reports, but I could
> not verify it. Since I measure the time for the second call to
> pa_asyncmsgq_send() the numbers
> reported look ok.
> I think there really is some "latency jitter" that cannot be avoided -
> interrupts that cannot be handled
> immediately, USB bus in use when the sound card wants to deliver data
> and you can probably come
> up with a lot more situations where it is possible that data cannot be
> delivered on time. You'll never get
> perfect results when you measure something and so you cannot apply a
> perfect regulator even
> if it would be nice in theory.
>
>> Unfortunately, this is easy to recommend, but I can't really see how
>> this can be done.
>>
>> The smoother is updated _after_ a successful write to the alsa device
>> (via traditional UNIX write or via mmap), while the pop callbacks are
>> executed just before that. So, they are called at the moment when the
>> influence from a bad rate estimation via the smoother is the greatest.
>>
>> Now the suggestions.
>>
>> I think that, ideally, for such use cases, it is important to have
>> timestamped latency snapshots for both sinks and sources in PulseAudio
>> core. This would mean introducing a new message that gets the latest
>> reliable latency snapshot (i.e., timestamp according to the wall
>> clock, send/receive counter, input/output buffer size, and the latency
>> itself), without any interpolation. If the sink does not implement
>> this, just fallback (in the generic sink code) to getting the current
>> latency. Also, because such snapshots for the sink and the source will
>> not happen at the same moment, you have to deal with it.
>
> You can actually try and get both snapshots at the same time. I did this
> and was quite astonished to
> find that the results were less reliable this way. I could not figure
> out why. (You can call
> get_latency_in_thread() for source and sink from both snapshot functions
> without crashing pulse,
> at least when you make sure they are ONLY called from the timer
> function. Something else seems to
> call one of the snapshots for whatever reason).
>
>>
>> Also, I have a very heretic thought. Namely, that the smoother in the
>> alsa sink and source may actually be a bad idea and is better removed.
>> I have not tested this. But it is used only in two places: for
>> reporting latency (where it confuses your module) and for calculating
>> the amount of time to sleep
>
> As I said, I think the latency deviations I see are real and not
> artifacts, so there is no confusion.
>
>> in the case of timer-based scheduling (where even module-alsa-sink
>> does not trust the result, i.e. discards it if it is greater than the
>> non-transformed time interval). And, if I recollect correctly, there
>> were complaints about it being fooled by batch cards, and they were
>> cited as one of the reasons not to enable timer-based scheduling on
>> batch cards. So - maybe, for the purposes of timer based-scheduling we
>> should just assume the worst case, i.e. the card that is, say, 0.75%
>> faster than nominal, and use the nominal rate together with the latest
>> snapshot time in {source,sink}_get_latency()? Basically, the fear is
>> that the smoother makes a greater mistake in the estimated rate than
>> just assuming the nominal one. Maybe you can try this suggestion?
>>
>
> For timer based scheduling the regulator works perfect, you would not
> even need a stop criterion,
> so why bother?
>
>> For Tanu's patch status page: please leave the status of this patch as
>> unreviewed. The general idea of the patch does not look brilliant, but
>> it's the best known-working idea that we currently have on the topic,
>> and I have not reviewed all the fine details.
>>
>
> Well from a practical point of view it does a pretty good job although
> the idea may not be brilliant.
> I'm willing to implement your better idea when you come up with it.
>   Did you ever test it? And compare it to what the current
> module-loopback does?

I did not test it, will do it now and add some logging in order to 
verify what you said above. And hopefully will try to implement an 
alternative latency-snapshotting implementation, just to compare.

-- 
Alexander E. Patrakov