[pulseaudio-tickets] [Bug 49608] periodic audio skips with Intel HDA

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=49608

--- Comment #50 from rkfg <rkfg at rkfg.me> ---
So I've done some research. This article describes the timer model and how ALSA
works in regards of periods/fragments and software/hardware buffers:
http://0pointer.de/blog/projects/pulse-glitch-free.html

According to the article, PA works in two modes, with tsched=0 it's the old
IRQ-driven mode as ALSA does and it heavily relies to the fragments number and
size. Hence it gets ALSA underruns if the app is too slow to fill the buffer.
It happens to my work PC sometime because it isn't too fast (model name:
Intel(R) Pentium(R) CPU G620 @ 2.60GHz) and I mostly listen to music in Chrome
(HTML5 audio). Chrome has the latency of ~65 ms now. It's quite feasible that
some program would occupy the CPU for this much time so I'd have a
skip/underrun. In case of tsched=0 there's nothing we can do as period/buffer
size is set up on startup and basically can't be changed in runtime.

When tsched=1 or not specified (defaults to 1) PA sets up an ALSA buffer of 2
seconds long with two periods of 1 second each. This way it doesn't underrun in
ALSA but rather underruns in PA itself. This code is responsible for tweaking
the watermark and latency:
https://github.com/pulseaudio/pulseaudio/blob/master/src/modules/alsa/alsa-sink.c#L326

The problem here is that watermark can go in both directions, up and down,
while latency can only grow until the sink is reset (recreated). This actually
means that if some particular program has chosen too little latency and had
several underruns because of that, THE WHOLE LOT OF OTHER PROGRAMS now will
suffer bad latency as well! And there's no other way to lower it back other
than restarting PA itself together with all its clients or when PA gets
unsuspended (
https://github.com/pulseaudio/pulseaudio/blob/master/src/modules/alsa/alsa-sink.c#L1021
).

I haven't found a way to setup a default minimal latency so that if I know that
my system isn't capable to deliver 4 ms, I can set it to 16 instead of having 
0.5 ms and several underruns just to let PA get to that latency by itself. It
seems to me that this minimal latency is just hardcoded (
https://github.com/pulseaudio/pulseaudio/blob/master/src/pulsecore/sink.c#L54 )
and there are no tools to call pa_sink_set_latency_range (
https://github.com/pulseaudio/pulseaudio/blob/master/src/pulsecore/sink.c#L3091
)

And also, I repeat, launching pavucontrol for the first time and sometimes
pacmd list-sinks causes stuttering on my work system (works ok at home but
there I have i7-2600 and PCI SB Live 7.1). I don't know how and why but they
seem to block PA somewhere so that Chrome can't sink its audio to it in time
and hence we get an underrun, latency increase and the bad mood. I dunno, maybe
I have to file this pavucontrol bug separately.

-- 
You are receiving this mail because:
You are the QA Contact for the bug.
You are the assignee for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freedesktop.org/archives/pulseaudio-bugs/attachments/20150416/25f23753/attachment.html>