Subject: Re: [linux-audio-dev] realtime audio i/o with disk input
From: Tommi Ilmonen (tilmonen_AT_kosh.hut.fi)
Date: Wed Mar 20 2002 - 15:41:25 EET
Hi.
I do not know if you aware of my threaded-audio file C++ class that does
the things you are trying to do. It is fairly small and compact and has
been working well for more than six months with hardly and changes. So if
you want a proven solution, then give it a try. One year I'll do final
fixes to make it 5% faster, but right now I could not be bothered with
such optimization.
It is part of the DIVAIO library and distributed with Mustajuuri
under LPLG (don't worry, you do not need to compile or link all of
Mustajuuri to use DIVAIO).
URLS:
Doxygen documentation:
http://www.tml.hut.fi/~tilmonen/diva/doc/code/doxygen-di/class_di_audiofilethreaded.html
Code example:
http://www.tml.hut.fi/~tilmonen/diva/doc/code/doxygen-di/class_di_audiofilethreaded.html#_details
Download:
http://www.tml.hut.fi/~tilmonen/mustajuuri/
On Wed, 20 Mar 2002, Maarten de Boer wrote:
> Hello,
>
> I mainly write this mail to Paul Davis, but of course everybody is
> welcome to answer :-)
>
> I am trying to do realtime audio i/o in combination with disk input.
> I know that Paul has a lot of experience with this. Rather than
> looking through his code, I'd appreciate some description of the
> approach he uses. I will comment my approach here, to see if you
> consider it correct or not. .
>
> What I am doing:
>
> I have 3 threads:
>
> - standard priority GUI thread
>
> - audio I/O thread (SCHED_RR, max, using alsa polling)
>
> - disk input thread
>
> The disk-input thread writes to a circular buffer, reading blocks of
> data from the hard-disk. It actually pre-fills it before starting the
> thread.
>
> The audio i/o thread reads from the circular buffer (and mixes it
> with the soundcard input, and writes it to the soundcard output, but
> that is not really relevant)
>
> The disk-input thread looks if it has space to write the next block
> to the circular buffer (it checks if the write-index will not cross
> the read-index). If there is no space, it waits for the readindex to
> move. This is done with a pthread_cond_wait in the disk-input thread,
> and a pthread_cond_signal in the audio i/o thread.
>
> pseudo-code:
>
> disk-input thread:
> pthread_mutex_lock
> while writeindex+blocksize > readindex
> pthread_cond_wait
> pthread_mutex_lock
>
> audio i/o thread:
> pthread_mutex_lock
> readindex += fragmentsize
> pthread_cond_signal
> pthread_mutex_unlock
>
> In a similar way, the audio i/o thread also checks if the readindex
> will not pass the writeindex, which happens when the disk input
> thread did not fill the circular buffer in time. Also this is done
> with a pthread_cond_signal / pthread_cond_wait.
>
> Problems:
>
> - It tends to run well, but sometimes it chews up a lot of CPU or
> even all. I don't understand when this happens exactly. (this is the
> second time I write this mail: the first time I had the application
> running and it froze my machine. Hint: never write long e-mails while
> beta testing realtime priority applications ;-) )
>
> - I can cause internal underruns (not filling the circular buffer in
> time), by doing a cat /dev/hda > /dev/null
>
> - Also using X-windows can cause problems. I suppose this is VM related?
>
> Questions:
>
> - Is this approach correct?
>
> - Is the pthread_cond_wait the correct mechanism for the thread
> synchronisation?
>
> - With what priority should the disk input thread run?
>
> - Is it better to use SCHED_RR or SCHED_FIFO for the audio i/o
> thread?
>
> - Should I call mlockall? What would be the best moment to do that?
>
> - What would be a reasonable size for the circular buffer? And what
> about the blocksize to read the data from disk? Does that even matter?
>
> My system is an AMD k7 700 MHz, hdparm tuned,. 2.4.17 Andrew Morton
> LL kernel.
>
> Greatly appreciating your time and effort,
>
> Maarten
>
Tommi Ilmonen Researcher
>=> http://www.hut.fi/u/tilmonen/
Linux/IRIX audio: Mustajuuri
>=> http://www.tml.hut.fi/~tilmonen/mustajuuri/
3D audio/animation: DIVA
>=> http://www.tml.hut.fi/Research/DIVA/
This archive was generated by hypermail 2b28 : Wed Mar 20 2002 - 15:29:55 EET