[PD-dev] per-thread storage in Pd in support of pdlib - discussion?

[Charles Henry]

On Wed, Jan 25, 2012 at 11:46 AM, Peter Brinkmann
<peter.brinkmann at googlemail.com> wrote:
>
> Hi Chuck,
> Check out the early bits of this thread --- various use cases already came
> up along the way:
> http://lists.puredata.info/pipermail/pd-dev/2012-01/017992.html.  The short
> version is that libpd is being used in such a wide range of settings that
> you can come up with legitimate use cases for pretty much anything (single
> Pd instance shared between several threads, multiple Pd instances in one
> thread, and anything in between).  At the level of the audio library, it's
> impossible to make good assumptions about threading.

Hi Peter

That's the part I really don't understand, and I don't really have a
clear picture of how you want to be able to control/choose between
those cases.
I can also see how there could be more capabilities tied to having
multiple threads generally.    But specifically, I can't say.  I have
no clue.

>> I remember a conversation with IOhannes in August about
>> multi-threading audio via sub-canvas user interface object (propose
>> thread~ akin to block~).  If all you're after is audio
>> multi-threading--there's no need for multiple instances of Pd.
>> Threads could be used to start a portion of the dsp chain, running
>> asynchronously, and then join/synchronize with Pd when finished.
>
>
> I don't think a patch is the place where decisions about threading should be
> made.  Threading is an implementation detail that users shouldn't have to
> worry about, and besides, whether you have anything to gain from threading
> will depend on a number of factors that users won't necessarily be able to
> control or even know about.

I have a different view.  Every sort of use for Pd is like writing a
program--you should assume Pd users are writing programs with every
sort of tool you give them--the flipside to having to control
threading explicitly is that you get to control how finely grained the
threading is.  Putting it on the patching level is just the user
interface--and it can work out nicely for grouping.  Even if you have
some automatic tools, you may still want to have explicit control
through another available interface (e.g. for debugging).

>> What this would look like:  Add a thread_prolog, thread_epilog, and
>> thread_sync function. The thread_prolog function that occurs before
>> block_prolog, starts a thread running the portion of dsp chain
>> cointained within, and returns the pointer to the function following
>> the thread_epilog.  The thread_epilog function that occurs after
>> block_epilog--waits for synchronization and returns.
>>
>>
>> What's the difficult part: You would need to have a good ordering of
>> the dsp chain to take advantage of concurrency--each subcanvas having
>> a thread~ object needs to kick off as early as possible, followed by
>> objects that have no dependence on its output.  Secondly, you'd need
>> to put thread_sync on the dsp chain immediately before you will
>> encounter functions with data dependencies.
>
>
> I believe it's much simpler than that.  It should be enough to just do a
> topological sort of the signal processing graph; that'll tell you which
> objects are ready to run at any given time, and then you can parallelize the
> invocation of their perform functions (or not, depending on how many
> processors are available).  I don't think there's any need to explicitly
> synchronize much; tools like OpenMP should be able to handle this
> implicitly.
> Cheers,
>      Peter

For that--the dspchain (an array of int*) makes a very bad structure.
So, you'll want to re-write a handful of functions and data structures
around having multiple concurrent branches of computations.  I
actually really like this problem :D  I can picture a linked list of
dspchains to do this.  But... the description of the sort algorithm
really will determine what the data structure ought to be.

Re-writing dsp_tick() is nearly sacrilege to me... beautiful bit of
code there, but that would have to be done according to whatever you
do to dspchain.

Chuck