[alsa-devel] [PATCH] ALSA: pcm_lib: avoid timing jitter in snd_pcm_read/write()

Jaroslav Kysela perex at perex.cz
Mon Jun 28 16:11:21 CEST 2010

On Mon, 28 Jun 2010, David Dillow wrote:

> On Mon, 2010-06-28 at 09:47 +0200, Jaroslav Kysela wrote:
>> On Sat, 26 Jun 2010, David Dillow wrote:
>>> When using poll() to wait for the next period -- or avail_min samples --
>>> one gets a consistent delay for each system call that is usually just a
>>> little short of the selected period time. However, When using
>>> snd_pcm_read/write(), one gets a jittery delay that alternates between
>>> less than a millisecond and approximately two period times. This is
>>> caused by snd_pcm_lib_{read,write}1() transferring any available samples
>>> to the user's buffer and adjusting the application pointer prior to
>>> sleeping to the end of the current period. When the next period
>>> interrupt occurs, there is then less than avail_min samples remaining to
>>> be transferred in the period, so we end up sleeping until a second
>>> period occurs.
>>> This is solved by using runtime->twake as the number of samples needed
>>> for a wakeup in addition to selecting the proper wait queue to wake in
>>> snd_pcm_update_state(). This requires twake to be non-zero when used
>>> by snd_pcm_lib_{read,write}1() even if avail_min is zero.
>>> Signed-off-by: Dave Dillow <dave at thedillows.org>
>> Thanks. It looks like another way to get things right. I applied your
>> patch to my tree.
> I think we should probably better define the semantics of read/write for
> sizes < avail_min, == avail_min, and > avail_min. As it is, I fear that
> this patch makes things worse as I hadn't thought through all of the
> cases when I posted it. If you've not pushed, please revert. If you
> have, then it improves one case, but let's makes sure we get the others
> correct before it hits the mainline.

I don't see any drawbacks in your solution. When I/O size is greater than 
avail_min, the behaviour is same as in previous code (avail_min is used 
for wake_up). For smaller I/O sizes (requests are not aligned to period 
size), each period interrupt will cause a wake up for the application 
task. In this case, the application may do own optimization of I/O 
transfers (sizes).


Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.

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