[PD] readsf~ and writesf~, locking, blocking and real-time priority

Jonathan Wilkes jancsika at yahoo.com
Tue Oct 6 17:58:38 CEST 2015


If I were one of Matt's students, I'd ask why this "Pure Data readsf~ business" won't just tell me when it has actually opened the file.  Why does thecomputer get to know when it's ready, but we students have to guess bylistening for glitches?
-Jonathan



     On Tuesday, October 6, 2015 1:02 AM, Matt Barber <brbrofsvl at gmail.com> wrote:
   

 
4) Has anyone ever “broken” these objects or experienced glitching?

​Once in 2005 we were having awful trouble streaming through Pd but we were never sure whether it was [readsf~] per se, a very slow disk, or xruns in ALSA/JACK, and we had only one performance laptop available. My best guess is that it was an ALSA/JACK problem, since other software had a few issues with glitching just on realtime audio processing.
pthread_mutex_lock() ... This might be a good time for a PSA for interested newcomers to Pd, though, if any happen to be following this thread (ahem). Having taught Pd for some 10 years now, one bad habit I've seen nearly every student fall into is failing to preload the file before playing, trying to do the initial read and the playing at the same logical time. Usually there isn't a problem, but once in a while a taxed system that is already streaming several files can glitch hard on a new stream. I've attached a generic [readsf~] idiom that has been useful for first-year students when they want to jump in and get Pd to play some sound files with a GUI after they've fooled around with the control examples and oscillators. This is before we get into event triggering, so the clunky multiple play/stop buttons is edited out later on; the main thing is how to keep the file open at all times. This turns out to be even more important for rehearsal than for performance, when you need to be able to jump around at will. ... pthread_mutex_unlock()


On Tue, Oct 6, 2015 at 12:15 AM, Miller Puckette <msp at ucsd.edu> wrote:

I think you can get away with sharing a lock between two high-prioroty
processes as long as neither one holds the lock for more than a small
amount of time and if the OS can be counted on to give control to a real-time
process quickly once it becomes runnable (i.e., if it's blocked on a lock,
once that lock is released).

The situation I don't know about is this:  if Pd's main thread failed to get
the lock, so that control (presumably) passed back to the other thread that
had the lock, how much time can pass before the other thread blocks on
something so that control (again presumably) gets passed back to the main
thread?

But anyway, since neither thread holds onto the lock for more than a few
lines of C code (with no system calls) it's probably blue-moon rare that the
scheduler interrupts one thread right in the middle of a critical section and
passes control to the other one that then blocks.  So this is essentially
untested.

Threads can never be used confidently in a real-time situation.  But I don't
see any reasonable way without them to implement readsf~/writesf~, so there
we are...

cheers
Miller

P.S. one can issue non-blocking reads/writes, but there's also "open" which
is much more likely to hiccup than "read", and I don't know of any async open
call in any OS.


On Tue, Oct 06, 2015 at 03:10:31AM +0000, Jonathan Wilkes via Pd-list wrote:
> 1) One thing I noticed is that the article you cited seems to focus
> on tasks not critical to the computation/delivery of audio samples.For example, if your program were blocking or locking in order to do a GUIupdate.  But here, the data must arrive in time to compute the next block.  If ittakes too long to read the next portion of the sound file, then you're going to geta glitch.
> But I'm not sure I really grasp how locking works, nor really the whole file i/oprocess in general.
>
> Here's a naive question: why can't you just tell the OS to treat the file asif it were a non-blocking socket, add the fd to Pd's event loop with
> sys_addpollfn, and then receive the incoming data to the relevant function?(Warning: some or all of the above may technically be gibberish...)
>
> -Jonathan
>
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>      On Monday, October 5, 2015 10:01 PM, Robert Esler <robert at urbanstew.org> wrote:
>
>
>  I’m trying to understand why readsf~ and writesf~ work so well. 
>
> I’m particularly referencing Ross Bencina’s article: http://www.rossbencina.com/code/real-time-audio-programming-101-time-waits-for-nothing and his subsequent paper, http://www.rossbencina.com/static/writings/File_IO_ACMC2014_Bencina.pdf
>
> If you are not into asynchronous message passing and lock-free queueing then I’ll summarize the articles briefly:
>
> When engaging in file I/O (e.g reading from or writing to an audio file) do not use locks or blocking. He goes on to say that this can lead to priority inversion, unbound execution time and “scheduler paranoia”.
>
> This is all absolutely true in my experience in the audio jungle.
>
> Pd’s async file I/O objects (readsf~ and writesf~) use both locks and blocking via a mutex and the pthread_cond_signal and pthread_cond_init functions.  Look at the source code file d_soundfile.c for more details. The gist of it is that these objects have two threads.  One parent thread that sends the data to the dsp scheduler, and a child thread that grabs the data from the file, and subsequently the child signals the parent when it has more data.
>
> Based on Bencina’s paper, readsf~ and writesf~ could (should?) glitch and may not be real-time safe.
>
> My questions are:
>
> 1) Have I completely misunderstood d_soundfile.c and it is actually entirely safe. If so, why is it safe?
>
> 2) Why doesn’t Pd glitch more often when using these objects?
>
> 3) Does Pd need lock-free message queueing for such inter-thread communication?
>
> 4) Has anyone ever “broken” these objects or experienced glitching?
>
> Thanks for the extra brain power.
> -R
>
>
>
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