prealloc/buffer time limit?

[Ron Cococcia <ron.cococcia@request.com>]

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


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