Re: [Alsa-user] Master volume control

Re: [Alsa-user] Master volume control

[Paul D. DeRocco]

> From: Clemens Ladisch [mailto:cladisch@googlemail.com] 
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <alsa/asoundlib.h>
> 
> static void check(int err, const char *f)
> {
>         if (err < 0) {
>                 fprintf(stderr, "%s failed: %s\n", f, 
> snd_strerror(err));
>                 exit(EXIT_FAILURE);
>         }
> }
> #define CHECK(f) check(f, #f)
> 
> int main()
> {
>         snd_ctl_t *ctl;
>         snd_ctl_elem_value_t *value;
> 
>         CHECK(snd_ctl_open(&ctl, "default", 0));
> 
>         snd_ctl_elem_value_alloca(&value);
>         snd_ctl_elem_value_set_interface(value, 
> SND_CTL_ELEM_IFACE_MIXER);
>         snd_ctl_elem_value_set_name(value, "Master Playback Volume");
>         snd_ctl_elem_value_set_integer(value, 0, 42);
>         CHECK(snd_ctl_elem_write(ctl, value));
> 
>         snd_ctl_close(ctl);
>         return 0;
> }

Well, I finally got a chance to try this. It "works", but the
snd_ctl_elem_write call takes a couple of milliseconds (that's around a
million instructions on my Atom), and it apparently spends this time in
kernel state, because it causes my sound thread to underrun, so it's not
usable.

If I use alsamixer, I don't have this problem; I can manipulate the master
volume smoothly while I'm playing my synth app. So I went through the
source, and found that it's using snd_mixer_selem_set_playback_volume. This
is apparently part of something called the "Simple Mixer Interface", yet
another section of ALSA which is completely undocumented, other than the
Doxygen stuff (which is only meaningful as a reference for someone who
already knows it inside out, but can't remember e.g. whether playback_pcm
comes before or after capture_pcm in the snd_mixer_selem_regopt structure).

So what's the minimum I need to do in order to get at the Master Playback
Volume, via the Simple Mixer Interface?

-- 

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com 

Re: [Alsa-user] Master volume control

[Daniel Mack]

On 13.03.2013 17:47, Paul D. DeRocco wrote:
>> From: Clemens Ladisch [mailto:cladisch@googlemail.com] 
>>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <alsa/asoundlib.h>
>>
>> static void check(int err, const char *f)
>> {
>>         if (err < 0) {
>>                 fprintf(stderr, "%s failed: %s\n", f, 
>> snd_strerror(err));
>>                 exit(EXIT_FAILURE);
>>         }
>> }
>> #define CHECK(f) check(f, #f)
>>
>> int main()
>> {
>>         snd_ctl_t *ctl;
>>         snd_ctl_elem_value_t *value;
>>
>>         CHECK(snd_ctl_open(&ctl, "default", 0));
>>
>>         snd_ctl_elem_value_alloca(&value);
>>         snd_ctl_elem_value_set_interface(value, 
>> SND_CTL_ELEM_IFACE_MIXER);
>>         snd_ctl_elem_value_set_name(value, "Master Playback Volume");
>>         snd_ctl_elem_value_set_integer(value, 0, 42);
>>         CHECK(snd_ctl_elem_write(ctl, value));
>>
>>         snd_ctl_close(ctl);
>>         return 0;
>> }
> 
> Well, I finally got a chance to try this. It "works", but the
> snd_ctl_elem_write call takes a couple of milliseconds (that's around a
> million instructions on my Atom), and it apparently spends this time in
> kernel state, because it causes my sound thread to underrun, so it's not
> usable.

So why are you doing this from your sound thread then?


Daniel


> 
> If I use alsamixer, I don't have this problem; I can manipulate the master
> volume smoothly while I'm playing my synth app. So I went through the
> source, and found that it's using snd_mixer_selem_set_playback_volume. This
> is apparently part of something called the "Simple Mixer Interface", yet
> another section of ALSA which is completely undocumented, other than the
> Doxygen stuff (which is only meaningful as a reference for someone who
> already knows it inside out, but can't remember e.g. whether playback_pcm
> comes before or after capture_pcm in the snd_mixer_selem_regopt structure).
> 
> So what's the minimum I need to do in order to get at the Master Playback
> Volume, via the Simple Mixer Interface?
> 

Re: [Alsa-user] Master volume control

[Paul D. DeRocco]

> From: Daniel Mack [mailto:zonque@gmail.com] 
> 
> So why are you doing this from your sound thread then?

I'm not. I'm doing it from another thread, but somehow it's causing
underruns in my two sound threads (on a dual-core Atom). I'm speculating
that it's hogging one CPU by sitting in the kernel.

-- 

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com

Re: [Alsa-user] Master volume control

[Paul D. DeRocco]

More on this.

My synth app, running on a dual-core Atom, has two sound generation threads,
running at a "nice" value of -20, and a normal control thread running at
nice 0. The control thread is what calls snd_ctl_elem_write when I move the
volume slider. My sound threads normally consume less than 50% of their
respective CPUs, but when I move the volume slider, I get underruns.

If I run "top -H" in a terminal window, I see three other processes that
suddenly consume gobs of CPU time as I change the volume.

	name            nice    CPU%
	pulseaudio      -11      19
	threaded-ml       0      14
	indicator-sound   0      11

If instead of using my synth's volume slider, I open alsamixer in another
terminal window, I don't see the same CPU usage, but I now attribute that to
the fact that the keyboard is issuing volume changes at a much slower rate
than my slider, which is read in a 200Hz loop. In fact, when I use
alsamixer, and ramp the volume up and down, every now and then I do get an
underrun. So it apparently won't matter whether I use snd_ctl_elem_write or
snd_mixer_selem_set_playback_volume.

So what I'd like to know is why the hell does changing the volume every 5ms
suck up 44% of one CPU core? I get the feeling that the above processes
aren't integral to ALSA, and may go away when I move my app to a pure
embedded system, as opposed to Ubuntu. Is that the case? Am I chasing
phantoms? Since I'm not doing _anything_ related to sound on this machine,
other than developing my synth app (I don't even need a terminal beep), is
there any way I can uninstall the things that are hogging the CPU, like
pulseaudio?

-- 

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com 

Re: [Alsa-user] Master volume control

[Clemens Ladisch]

Paul D. DeRocco wrote:
>>         snd_ctl_elem_value_set_interface(value, SND_CTL_ELEM_IFACE_MIXER);
>>         snd_ctl_elem_value_set_name(value, "Master Playback Volume");
>>         snd_ctl_elem_value_set_integer(value, 0, 42);
>>         CHECK(snd_ctl_elem_write(ctl, value));
>
> Well, I finally got a chance to try this. It "works", but the
> snd_ctl_elem_write call takes a couple of milliseconds (that's around a
> million instructions on my Atom), and it apparently spends this time in
> kernel state, because it causes my sound thread to underrun, so it's not
> usable.

I'd guess that some driver uses a busy loop for writing the codec
registers.

Which drivers are you using for your sound hardware?

> If I use alsamixer, I don't have this problem; I can manipulate the master
> volume smoothly while I'm playing my synth app. So I went through the
> source, and found that it's using snd_mixer_selem_set_playback_volume.

Ultimately, this is a wrapper for the snd_ctl_* functions.


Regards,
Clemens

Re: [Alsa-user] Master volume control

[Paul D. DeRocco]

> From: Clemens Ladisch [mailto:clemens@ladisch.de] 
> 
> I'd guess that some driver uses a busy loop for writing the codec
> registers.
> 
> Which drivers are you using for your sound hardware?

It's Intel HD Audio.

But the point is that three other processes light up when I move the slider.
I think they're all monitoring the master volume changes, and not actually
involved in doing the volume change. I would expect lots of CPU usage if I
had alsamixer open at the time, because it monitors these changes, and
updates its display. But the numbers I got from "top -H" were with no
monitoring apps running that I know of.

What I'm wondering about these processes (pulseaudio, threaded-ml,
indicator-sound) is two things:

* Is there any way to disable or uninstall them in Ubuntu?

* If I create my own minimal embedded distro (I'm trying to use the Yocto
project for that), will they simply not be there, or will I have to go to
some lengths there, too, to disable or uninstall them?

If this is just an issue with Ubuntu, then it's not worth spending much time
on it, since this synth is targeted purely for an embedded environment.

-- 

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com 

Re: [Alsa-user] Master volume control

[Clemens Ladisch]

Paul D. DeRocco wrote:
>> I'd guess that some driver uses a busy loop for writing the codec
>> registers.
>>
>> Which drivers are you using for your sound hardware?
>
> It's Intel HD Audio.

That driver works fine.

> But the point is that three other processes light up when I move the slider.
> I think they're all monitoring the master volume changes, and not actually
> involved in doing the volume change.

I guess your CPU doesn't have too many cores ...

> What I'm wondering about these processes (pulseaudio, threaded-ml,
> indicator-sound)

PulseAudio has its own virtual volume sliders on top of the hardware
controls.  indicator-sound appears to be an OSD of your desktop
environment.  I don't know what threaded-ml is (maybe part of Chrome?).

> I would expect lots of CPU usage if I had alsamixer open at the time,
> because it monitors these changes, and updates its display.

Well, how much increas in CPU usage do you get wth alsamixer?

> * If I create my own minimal embedded distro (I'm trying to use the Yocto
> project for that), will they simply not be there

If you don't included the relevant packages.


Regards,
Clemens