[mythtv-users] mythfrontend is always hungry for ALSA Hardware audio buffer

Josu Lazkano josu.lazkano at gmail.com
Thu Nov 8 22:26:12 UTC 2012


2012/11/8 Jean-Yves Avenard <jyavenard at gmail.com>:
> On 9 November 2012 07:24, Andrey Zhunev <a-j at a-j.ru> wrote:
>> Hello All,
>>
>>
>> I just noticed that mythfrontend is asking to allocate a larger buffer for alsa:
>> So I increased the value of /proc/asound/card0/pcm0p/sub0/prealloc to
>> '1024' - even more than requested. Strangely, mythfrontend asked for more:
>>
>>
>> 2012-11-09 00:10:18.681551 I [3892/3892] CoreContext audio/audiooutputalsa.cpp:139 (TryOpenDevice) - ALSA: OpenDevice default
>> 2012-11-09 00:10:18.683052 I [3892/3892] CoreContext audio/audiooutputalsa.cpp:678 (SetParameters) - ALSA: SetParameters(format=10, channels=2, rate=48000, buffer_time=500000, period_time=4)
>> 2012-11-09 00:10:18.683407 I [3892/3892] CoreContext audio/audiooutputalsa.cpp:736 (SetParameters) - ALSA: Buffer size range from 2048 to 16384
>> 2012-11-09 00:10:18.683421 I [3892/3892] CoreContext audio/audiooutputalsa.cpp:739 (SetParameters) - ALSA: Period size range from 1024 to 1024
>> 2012-11-09 00:10:18.683483 E [3892/3892] CoreContext audio/audiooutputalsa.cpp:783 (SetParameters) - ALSA: Requested 500000us got 341333 buffer time
>> 2012-11-09 00:10:18.683615 I [3892/3892] CoreContext audio/audiooutputalsa.cpp:224 (IncPreallocBufferSize) - ALSA: Hardware audio buffer cur: 1024 need: 1536 max allowed: 32768
>> 2012-11-09 00:10:18.683648 E [3892/3892] CoreContext audio/audiooutputalsa.cpp:242 (IncPreallocBufferSize) - ALSA: Try to manually increase audio buffer with: echo 1536 | sudo tee /proc/asound/card0/pcm0p/sub0/prealloc
>> 2012-11-09 00:10:18.683666 I [3892/3892] CoreContext audio/audiooutputalsa.cpp:791 (SetParameters) - ALSA: Buffer time = 341333 us
>> 2012-11-09 00:10:18.683677 I [3892/3892] CoreContext audio/audiooutputalsa.cpp:797 (SetParameters) - ALSA: Period time = 16 periods
>> 2012-11-09 00:10:18.683823 I [3892/3892] CoreContext audio/audiooutputalsa.cpp:806 (SetParameters) - ALSA: Buffer size = 16384 | Period size = 1024
>
> The way it works is this:
>
> We ask ALSA to allocate an audio buffer of a certain length. It
> allocates some, and return how much it allocated.
> If there's more than 10% difference between what was asked and what we
> got, we display a message.
> The hardware buffer sized suggested is calculated according to how
> much the hardware buffer size currently is, and how much we got.
>
> So, say the HW buffer size is 128kB. We asked for 500ms buffer, and we
> got 341ms instead. So myth assumes that we only got 341ms because the
> HW buffer size was set to 128kB and that should it be higher, we would
> have gotten more.
> so we need: 500 * 128 / 341 = 187kB, we round it to the next 64kB
> block, it will display that you need to increase the HW buffer to
> 192kB...
>
> This works and is accurate for 99.9% of the ALSA setup in my experience.
>
> Now you're the 2nd person to report the problem above. No matter what
> you ask ALSA, regardless of the HW memory buffer size, you always get
> the same maximum value, is 341333us.
> That throws off myth and the message displayed is incorrect.
>
> I would just ignore that message, it is not displayed unless you
> increase the verbosity (it used to be displayed all the time)
>
> JY
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> mythtv-users at mythtv.org
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Hello, I have same problem. I add this on the sudoer configuration file:

myuser ALL=(ALL) NOPASSWD:/usr/bin/tee /proc/asound/card0/pcm3p/sub0/prealloc

Now I can allocate the buffer size as mythfrontend user.

But it doesnt change nothing:

1. I check the initial buffer size:

$ cat /proc/asound/card0/pcm3p/sub0/prealloc
64

2. Change the buffer size:

$ echo 192 | sudo tee /proc/asound/card0/pcm3p/sub0/prealloc
192

3. Check the buffer size:

$ cat /proc/asound/card0/pcm3p/sub0/prealloc
64

As you can see, it does not change the buffer size. I am doing something wrong?

Thanks and best regards.

--
Josu Lazkano


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