[alsa-devel] Timer instability

Lennart Poettering mznyfn at 0pointer.de
Fri Feb 20 02:22:20 CET 2009

On Thu, 19.02.09 07:52, Takashi Iwai (tiwai at suse.de) wrote:

> At Thu, 19 Feb 2009 03:46:11 +0100,
> Lennart Poettering wrote:
> > 
> > Heya!
> > 
> > As you might now PulseAudio schedules audio by system timers, not by
> > sound card interrupts. Unfortunately there are are couple of issues
> > with the reliability of the timing/buffering information we get from
> > ALSA. 
> > 
> > For example, on an ens1371 I have tracked down the following issue: I
> > use the full buffer that the sound card provides (64K, i.e. 371ms). I
> > sleep for about 350 ms, then fill up again, and sleep 350ms, and so
> > and so on. Everytime I wake up I check snd_pcm_avail() and write
> > exactly as much as this call tells me to. I also listen to POLLOUT
> > events on the fd, however I call snd_pcm_sw_params_set_period_event(,
> > 0) for it to make sure I actually don't get any real events on that.
> > 
> > After a couple of minutes of running this loop I can reliably
> > reproduce that _avail() tells me I should fill the buffer up, I do so,
> > query it again and it shows the buffer is filled up. I then go to
> > sleep but immediately get POLLIN (although I shouldnt, given that I
> > called snd_pcm_sw_params_set_period_event()), wake up again, call
> > snd_pcm_avail() and according to it the buffer ran completely
> > empty. The poll() took less than 1 ms time, and the last thing I did
> > before going to sleep was checking that the buffer was full. So
> > certainly ALSA is lying to me here... The buffer couldn't have run
> > empty in less than 1 ms!
> > 
> > The same with real numbers as one example:
> > 
> >   1. ... we are woken up
> >   2. _avail() returns 15496 samples (i.e. 350ms to fill up)
> >   3. we gather and write 15496 samples
> >   4. _avail() returns 48 samples, which we consider "filled up enough" to go to sleep
> >   5. we call poll()
> >   6. after < 1ms we are woken up by a POLLIN (which we actually thought we had explicitly disabled)
> >   7. _avail() returns 16448 samples (i.e. 372ms to fill up -- larger than the actual buffer of 16384 samples!)  
> >   
> > This smells a lot like some kind of overflow to me, given that in every
> > case where I could reproduce this it was possible to 'fix' the issue
> > by substracting the buffer size from the _avail() after the
> > poll(). After that substraction the value started to make sense
> > again. (i.e. in the example above it then becomes 64 samples, which is
> > reasonable after sleep less than 1ms I guess).
> > 
> > The stop threshold is set to the boundary here btw.
> > 
> > If necessary I could extract a test case for this, but my hope that
> > you guys might have an idea what goes on without a test case, given
> > that this smells so "overflowy" ;-)
> > 
> > And of course, how come ALSA triggers POLLIN if I asked it not to?
> Are you using *_period_event() stuff?

Yes, I am. But ignore that part for now. I have now commented the use
of that call. Now I certainly get more POLLOUTs as expected, but the
real problem stays: after a few minutes _avail() will suddenly jump
from next to zero to more then the hwbuf size in less than 1ms without
any further inteference and with a buffer size of 350ms! There is
something really wrong with the behaviour of _avail().

I can reproduce this only on ens1371 for now.

> Unless you use dmix, this is the only place where strange POLLIN may
> come in.  There are many new hacks there...

There is no plug in used but "hw".


Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net         ICQ# 11060553
http://0pointer.net/lennart/           GnuPG 0x1A015CC4

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