Re: [alsa-devel] Alsa-lib: Dmix slowptr latency behaviour

20 Jun 2017

      Hi Clemens,
...
Matin Momin wrote:
...
...
arecord -D"dsp" -r48000  -c2 -fs32_le | aplay -D"dmix1"
...
To reduce the latency, reduce arecord's period size and/or aplay's
buffer size.
Actually, my aim is to understand if setting dmix slowptr to true
leads to increase in playback latency by buffer_time. Is it a known
issue or could it be because of the specific alsa device?
...
...
Now, in the same configuration if I disable slowptr in dmix1, the
delay comes down to 160 msecs, which corresponds exactly to buffer
time (85 msecs).
...
Did you do multuple measurements?
Yes, and in each iteration I get reproducible measurements.
Regards,
Matin
PS: Please keep me in CC, I am not subscribed to the mailing list.
On Fri, Jun 16, 2017 at 11:03 AM, Matin Momin matinmomin@gmail.com wrote:
...
Hi,
On my embedded device, I am using a push mode simple alsa pipeline
with following command
...
arecord -D"dsp" -r48000  -c2 -fs32_le | aplay -D"dmix1"
where the asound conf looks this
# This is the actual HW device
pcm.dsp {
        type hw
        card 1
        rate 48000
        channels 2
        format S32_LE
}
pcm.dmix1 {
        type dmix
        ipc_key 1024
        ipc_key_add_uid false
        ipc_perm 0666
        slave.pcm "dsp"
        slowptr true
}

I measured the pipeline delay by using Audacity on my PC, and
measuring the difference in spike of audio recorded on the PC in a
silent environment (1st spike will be from the actual sound and second
will be from the output of my embedded device).
Here I get a delay of around 245 msecs.
Now, in the same configuration if I disable slowptr in dmix1, the
delay comes down to 160 msecs, which corresponds exactly to buffer
time (85 msecs).
I want to know whether this is expected or is it an aberration?
I have measured the userland time for the data to travel from
alsa-lib/src/pcm/pcm_hw.c: snd_pcm_hw_readi() to
alsa-lib/src/pcm/pcm_dmix.c:mix_areas() (By adding incremental numbers
to each period recorded by arecord and seeing how much time they take
to reach dmix). I cannot find any difference in absolute time
difference with both slowptr true and false. Also, the critical hw_ptr
and appl_ptr variables in pcm_hw and pcm_dmix also reflect same values
for slowptr true and false.
Is the above mentioned method to measure delay correct? If I don't see
a delay here, where else should I look?
The alsa-lib version is 1.1.0 and kernel is 3.8.13.
Thanks in advance.
Regards,
Matin