Out of curiosity, can you give a test case where my idiom above (which keeps the file loaded as long as you need it) would be too onerous or just impossible to use? I think I can think of a few, like some kind of patch that follows an improvisation and chooses files to stream immediately in response to something the performer did. In those cases, though, I'd be very reluctant to trust the disk drive to serve multiple files immediately. SSDs might perform better and a ramdisk better still, but in the general case I think I would do everything I could do design my composition/patch to be consistent with preloading any files I need immediately.
On Tue, Oct 6, 2015 at 4:41 PM, Jonathan Wilkes via Pd-list < pd-list@lists.iem.at> wrote:
I'm not asking when you would implement this. We were talking about determinism, after all. :)
But I guess I'm confusing a program's deterministic behavior-- e.g., output at predictable times-- with a message that has a value derived from machine- or OS-specific behavior.
-Jonathan
On Tuesday, October 6, 2015 3:32 PM, Miller Puckette msp@ucsd.edu wrote:
I don't think the design constrains it out - I just haven't done it, since it's rather more than a minor project.
cheers M
On Tue, Oct 06, 2015 at 07:27:51PM +0000, Jonathan Wilkes wrote:
Hm... still, I can't get that value from within a patch because of
your design constraints, right?
On Tuesday, October 6, 2015 2:20 PM, Miller Puckette <msp@ucsd.edu>wrote:
So in that case you'd really want a different metric, perhaps 'what was
the
minimum fill count while the file was playing'.
cheers M
On Tue, Oct 06, 2015 at 05:40:27PM +0000, Jonathan Wilkes via Pd-list
wrote:
But as Matt's student I want to be able to measure the time it takes
to open the file, to make a more informed decision when I design my patch. I wantthe pure data when I'm only opening a few soundfiles, and then Iwant the pure data when I try to open lots of files. Pd already gives me[realtime] which I could use to create non-deterministic patches. Obviouslyits author realized that the ability to measure time outweighs the risk of doingthat. One would assume that same tradeoff to be equally important, if notmoreso, for the few instances of object behavior which [realtime] cannotmeasure.
The other students are now rolling their eyes. I think they're on to
me.
-Jonathan
On Tuesday, October 6, 2015 12:11 PM, Miller Puckette <msp@ucsd.edu> wrote:
A worthy question.
If you want the soundfile to start exactly when you specify it (I
think this
should normally be the case :) then it's beside the point exactly when
the
computer could have coughed it up - it only matters that it be there
by the
desired time.
If you want the soundfile to play "whenever the computer can manage
it" - and
the sooner the better - well, then a "ready" message would be useful.
I
imagine one could shave off 1/5 second or so, but it would be
inconsistent.
Perhaps this is useful in some cases but I don't think it would be
often -
and the downside is that it wouldn't be deterministic (a fundamental
design
principle of Pd).
cheers Miller
On Tue, Oct 06, 2015 at 03:58:38PM +0000, Jonathan Wilkes wrote:
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@gmail.com> wrote:
- 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@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:
- 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
On Monday, October 5, 2015 10:01 PM, Robert Esler <robert@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-f...
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:
- Have I completely misunderstood d_soundfile.c and it is
actually entirely safe. If so, why is it safe?
Why doesn’t Pd glitch more often when using these objects?
Does Pd need lock-free message queueing for such inter-thread
communication?
- Has anyone ever “broken” these objects or experienced glitching?
Thanks for the extra brain power. -R
Pd-list@lists.iem.at mailing list UNSUBSCRIBE and account-management ->
http://lists.puredata.info/listinfo/pd-list
Pd-list@lists.iem.at mailing list UNSUBSCRIBE and account-management ->
http://lists.puredata.info/listinfo/pd-list
Pd-list@lists.iem.at mailing list UNSUBSCRIBE and account-management ->
http://lists.puredata.info/listinfo/pd-list
Pd-list@lists.iem.at mailing list UNSUBSCRIBE and account-management ->
http://lists.puredata.info/listinfo/pd-list
Pd-list@lists.iem.at mailing list UNSUBSCRIBE and account-management -> http://lists.puredata.info/listinfo/pd-list