[alsa-devel] Measuring Device Throughput - MMAP vs. RW

[Takashi Iwai]

On Mon, 09 Sep 2019 21:31:49 +0200,
Pavel Hofman wrote:
> 
> On Mon, Sep 9, 2019 at 7:44 PM Takashi Iwai <tiwai at suse.de> wrote:
> 
>     > The mmap fluctuations seem to be fixed by using increases in hardware
>     > pointer pcm->hw.ptr values instead of frames variable. Please do these
>     > values wrap up or do they grow  to max long? Still contiguous after 10
>     > minutes of running.
>    
>     Could you elaborate the requirement?  The description isn't clear to
>     me.
> 
> I need to slave EP OUT of asynchronous USB-audio gadget  (i.e. device) to
> another (output) soundcard in the system so that adaptive resampling in the
> chain "USB gadget -> processing -> output soundcard" is avoided.  For that I
> need to measure average throughput to/from alsa hw:X devices, to gather data
> for the feedback endpoint implemented by the gadget driver.  I do not have any
> control over configuration of the devices, the feedback must run independently
> of the rest of the chain, of the applications used.
> 
> In snd_pcm_hw_mmap_commit I subtract the advancement of pcm->hw.ptr from value
> approx. 10 seconds ago, divided by passed nanosecs measured by clock_gettime
> (CLOCK_MONOTONIC). For that I need the pcm->hw.ptr either to be monotonic, or
> I just reset the averaging if a non-monotony is detected (hw pointer in
> previous (n-1) call of snd_pcm_hw_mmap_commit was higher than  current value).
> 
> For better precision, would it make sense to update the pcm->hw.ptr with 
> snd_pcm_avail call in that method? It may not be even possible in that method
> due to some locking, I really do not know. But my precision requirements are
> not very strict since I can average over long time (minutes) for the feedback
> (the clock difference is usually very small).

The value read from snd_pcm_status() should give the accurate pair of
the position and the timestamp.  You need to check the delay count as
well, which compensates the in-flight samples.


Takashi

> 
>     You should be able to skip XRUN check by changing the stop_threshold.
>     Setting it to the boundary size essentially means skipping XRUN
>     checks.
> 
> Actually if  xruns could be detected by discontinuity of the hw pointer, I
> would not need any other method.
>  
> 
>     In anyway, I put Pierre to Cc, as he's done some works in the past for
>     the accurate position reporting on HD-audio and USB-audio devices.
>     He can give a better hint in this regard.
> 
> Thanks a lot, I very much appreciate your help. I do not know these low-level
> alsa details. But I will gladly learn.
> 
> Pavel. 
> 
>