Re: prealloc/buffer time limit?

[James Courtier-Dutton <james.dutton@gmail.com>]

On 10/02/06, Ron Cococcia <ron.cococcia@request.com> wrote:
> Hello,
>
> Recently I've been playing around with buffering in ALSA to reduce the
> number of underruns I observe when playing audio under heavy system
> load.  The playback device has 64k allocated by default, and I upped it
> to 128k (echo 128 > /proc/asound/card0/pcm0p/sub0/prealloc).  I observed
> a significant reduction in underruns with this change.  The buffer time
> went from 371519 to 743038 (using snd_pcm_hw_params_set_buffer_time_near
> to 5000000 (not .5 sec, but 5 sec)).  I am assuming that the value is
> updated to give the actual available buffer time after snd_pcm_hw_params
> has complete (please inform me if I am wrong!!).
>
> I then tried to up it to 256, and it didn't work (prealloc remained at
> 128).  I went into the driver for the sound device (intel8x0), and
> noticed the buffer_max being set to 128.  I modified the max values to
> 512k (overkill, but I want to experiment a little), recompiled,
> installed, etc.
>
> When I tried to increase it to 256, prealloc reports 256 successfully
> (same for 512).  The buffer time did not increase, however.  I am
> looking to be able to increase the buffer time a little further.  I have
> been poking through code (at a snail's pace unfortunately) to see if I
> can find some other place where this value is limited (driver, lib,
> etc).  Either prealloc reports the value and doesn't actually have a
> larger buffer, or some other buffer time limit is in place that I
> haven't been able to find yet.
>
> Is there some other place I can check to increase the buffer time (in
> particular, if I increase the prealloc size).
>
> TIA,
> Ron
>

You are most likely running up against the hardware limitations of the
sound card.
If the hardware DMA pointer is only X bits long, it won't matter how
big you make the buffer, the sound card will not be able to use it.

James


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