[alsa-devel] Why does snd_seq_drain_output() need lots of time to execute?

[Clemens Ladisch]

Rafał Cieślak wrote:
> The main problem is that although the documentation states that
> snd_seq_drain_output() returns immediately,

| "The function returns immediately after the events are sent to the
|  queues ..."

> it turns out that it occasionally needs lots of time (even a second or
> two), and blocks my application, as I call it.

You could enable non-blocking mode to get an error instead of waiting.

Or you could to increase the size of the output buffer.

> And before you ask, I've set the output buffer to some huge amount,

A sequencer client has _two_ output buffers, one in alsa-lib, and one in
the kernel.  Events that are scheduled for later stay in the kernel
buffer until they are actually delivered; when this buffer would
overflow, functions that drain the userspace buffer to the kernel buffer
wait instead.

To increase the kernel buffer's size, use the snd_seq_client_pool* and
snd_seq_get/set_client_pool functions.  ("pool" is the buffer size, in
events; "room" is the number of free events that causes a blocked
function to wake up.)


Regards,
Clemens