[PD-dev] how to not cause audio drops with threads

Thomas Grill gr at grrrr.org
Fri Sep 10 10:50:46 CEST 2004


Hi Schouten,
the ring buffer idea is really great and if you don't mind i'll take this
over to flext which uses dynamic allocation at the moment.
The flext message queue has a worker thread which is sleeping until being
triggered by a thread signal whenever a new message needs to be delivered.
It then takes the pd global thread lock (sys_lock) and sends the messages to
pd. I consider this to be quite clean since message delivery will take
little time.
I have chosen this way to work with any pd version, but there a sure more
efficient ways which probably only Miller can tell....

best greetings,
Thomas

----- Original Message ----- 
From: "Tom Schouten" <doelie at zzz.kotnet.org>
To: <pd-dev at iem.at>
Sent: Friday, September 10, 2004 4:18 AM
Subject: [PD-dev] how to not cause audio drops with threads


> hi all,
>
> i was thinking about adding some non-blocking message
> queue mechanism to pd, to simplify writing externals
> with their own worker threads that cannot, by design,
> block the audio thread.
>
> i've found the need for this in almost all of my pd
> projects that use threads, and the only one that has
> solved it somewhat properly is wvvw.. adding support
> to pd would prevent a lot of hassle. it could serve as
> an 'external design pattern' actually.
>
> this could be implemented in the form of a pd-message
> queue. even a simple fixed size ringbuffer containing fixed
> size pd atoms would be a very nice thing to have as a standard
> part of pd. it doesn't require platform-specific code,
> and doesn't use dynamic memory allocation for each message.
>
> this can catch most of the communication needed between the
> main pd thread and a bunch of isolated worker threads, and do
> it in a way that forces you to not cause drops.
>
> something like:
>
> typedef struct {
> t_atom *buffer;
> int length;  // single message length in atoms
> int size;    // total number of messages
> int read;    // read atom index
> int write;   // write atom index
> } t_queue;
>
> t_atom *queue_read(t_queue *q);         // returns NULL if queue empty
> int queue_write(t_queue *q, t_atom *a); // returns error code if full
> void queue_free(t_queue *q);
> t_queue *queue_new(int length, int size);
>
>
>
> another thing that i've been looking for, is a standard
> way to hook into the main scheduler loop to add a poll
> hook. you can do this with clocks, but an explicit hook at
> the point where the gui is polled for updates would be much
> cleaner. maybe 2 hooks: one that runs at the dsp tick, and
> one that runs at gui tick.
>
> so the pattern is:
> thread->pd: a message queue with poller in pd scheduler
> pd->thread: another message queue with poller in worker thread
>
> the pollers are application-specific, but some could be included
> in the pd api, like a poller that sends messages to a certain object,
> or a receiving symbol. this is not really necessary though.
>
> my apologies if i missed out one some discussion on this subject,
> but i think this would be trivial to add and very useful.
>
> tom
>
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