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

[Pavel Hofman]

On Sun, Sep 8, 2019 at 5:23 PM Pavel Hofman <pavhofman at gmail.com> wrote:

> Hi,
> Gathering data for USB async gadget feedback (
> https://mailman.alsa-project.org/pipermail/alsa-devel/2019-August/154819.html
> ) requires measuring average data rate to/from a device. For testing I
> added measuring/averaging code to methods snd_pcm_hw_writei, snd_pcm_hw_readi,
> and snd_pcm_hw_mmap_commit in
> https://github.com/alsa-project/alsa-lib/blob/master/src/pcm/pcm_hw.c .
> In all cases I take the parameter "size" of all the three methods as the
> number of samples written/read.
>
> MMAP: When mmap is used (e.g. plughw:X + sample format conversion), I get
> very stable results for MMAP playback on all soundcards I have tested. But
> for capture I get identical results to playback for one soundcard (PCI
> Envy24), while the averaged rate calculated from "size" in
> snd_pcm_hw_mmap_commit fluctuates for capture on Intel HDA.
>
>
> I suspect the size parameter of snd_pcm_hw_mmap_commit is not the actual
> amount of samples read during the MMAP capture. Please how to calculate the
> number of samples read from the variables available in that method?
>

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.

I see the hw.ptr gets zeroed at XRUN. If I do not check for xruns (
snd_pcm_state(snd_pcm_t *pcm)?), can I just monitor continuity of hw.ptr
and restart the averaging if the new hw.ptr is lower than the previous one,
or are there any quirks to watch out?

Thanks a lot,

Pavel.