Re: [PATCH 00/15] reduce audio playback latency

[Christian Schoenebeck]

On Sonntag, 9. Januar 2022 18:06:44 CET Volker Rümelin wrote:
> > On Donnerstag, 6. Januar 2022 10:21:47 CET Volker Rümelin wrote:
> >> This patch series reduces the playback latency for audio backends,
> >> in some cases significantly. For PulseAudio, the audio buffer is
> >> also moved from the QEMU side to the PulseAudio server side. This
> >> improves the drop-out safety for PulseAudio.
> >> 
> >> I actually measured the latency reduction with the PulseAudio
> >> backend. For the test I used my Linux host configured to play
> >> audio with PulseAudio. The guest was a Linux guest, also
> >> configured to use PulseAudio.
> > 
> > I haven't reviewed all the patches yet, but from what I read so far, does
> > that mean the additional 3rd buffer is solely for PulseAudio, so for JACK
> > and other backends these changes would overall be a degradation, wouldn't
> > they?
> No, nothing changes for JACK and it's an improvement for all the other
> backends where I added a buffer_get_free function. The important changes
> are in [PATCH 10/15] audio: restore mixing-engine playback buffer size.
> That patch tries to keep the mixing-engine buffer empty at the end of
> audio_run_out().
> 
> I couldn't reduce the playback latency for JACK, because the JACK audio
> buffers are already very small and any further reduction introduces
> playback glitches on my system.

And that's actually my concern. A split 2 buffers -> 3 buffers while 
(approximately) retaining overall latency increases the chance of dropouts.

> For PulseAudio there is no additional buffer. I only increased the size
> of the server side buffer from 15ms to 46,4ms and added a
> buffer_get_free function. Before this patch series a few ten ms after
> playback started the mixing-engine buffer was full which added 2 *
> 46,4ms to the playback latency. With these patches the mixing-engine
> buffer is empty. This looks like the buffer in use was moved from the
> mixing-engine to the PulseAudio server side.