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 <[email protected]> > 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 <[email protected]> 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 > > > > > > > > > > > > > > > > > > On Monday, October 5, 2015 10:01 PM, Robert Esler > ><[email protected]> 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 > > > > > > > > _______________________________________________ > > [email protected] mailing list > > UNSUBSCRIBE and account-management -> > > http://lists.puredata.info/listinfo/pd-list > > > > > > > > > _______________________________________________ > > [email protected] mailing list > > UNSUBSCRIBE and account-management -> > > http://lists.puredata.info/listinfo/pd-list > > > _______________________________________________ > [email protected] mailing list > UNSUBSCRIBE and account-management -> > http://lists.puredata.info/listinfo/pd-list > > > > > _______________________________________________ [email protected] mailing list UNSUBSCRIBE and account-management -> http://lists.puredata.info/listinfo/pd-list
