[alsa-devel] Timer instability

Lennart Poettering mznyfn at 0pointer.de
Mon Feb 23 03:42:34 CET 2009

On Sun, 22.02.09 04:14, Lennart Poettering (mznyfn at 0pointer.de) wrote:

> I wrote this little test to track down all kinds of timer issues: it
> opens and sets up a sound device. Then it will constantly query
> _avail(), _delay() and _htstamp() in a busy loop writing a single
> sample at a time. The three values are then printed along with a
> system timestamp. The data generated like this can be opened in
> gnumeric and a nice graph be drawn.

Hmm, did some more testing with this tool on other cards:

On es1969 snd_pcm_avail() sometimes returns values like
1073728596 samples. This smells a lot like an overflow given that this
times four (i.e. the sample size in bytes) is near to 2^32.

Here's an output of this tool for an emu10k1 card:


Check out line 4417: This is when the devcie starts playback. avail
goes rightfully to 0 (i.e column 4 -- don't get confused by the line
numbers the nopaste adds here), but the playback time suddenly jumps
from 100113 us down to 702 us (column 4 which is the written sample
counter minus the delay transformed to us). This is because in
contrast to most other drivers for this one writing a sample has no
direct effect on snd_pcm_delay while the device is stopped.

Also this shows that the data from _avail() and _status() together is
not atomic: the columns for avail delay change in
different rows when we don't get scheduled often enough.


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

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