[PD] Multiprocessing in Pd (was DSP and Gem in the same instance of Pd)

Fri Apr 1 08:13:16 CEST 2016

> duration in which the durations must finish
I meant "duration to specify the maximum time for the loop to finish". 

    On Friday, April 1, 2016 2:11 AM, Jonathan Wilkes <jancsika at yahoo.com> wrote:

 This might be too esoteric to be practical, but:

There's also the possibility of spreading spikey control operations over 
a longer duration of time.
For example, let's say you analyze some input and want to see if it matches 
anything in a list of 500 elements.  If you have one second before you need 
to act on the result of that test and your sample rate is 44100, you can trigger each iteration of the loop with [bang~] and be guaranteed that the test will 
finish on time.  That way each dsp tick only requires a single iteration through 
the loop, which probably gets rid of your spike.
Furthermore, you could just wrap this in an abstraction that takes two 
floats-- one for the number of iterations, and the other for a duration in which 
the durations must finish.  Then it queries the sample rate and figures 
out how many iterations must happen each dsp tick without exceeding 
the goal duration.
-Jonathan

    On Friday, April 1, 2016 1:09 AM, Matt Barber <brbrofsvl at gmail.com> wrote:

 Pd's constraints make the automatic allocation unlikely.
Read this:http://puredata.info/community/conventions/convention09/puckette.pdf

'Since at least 1990, users and critics of Max/FTS haveobserved that it would be desirable for objects to be automaticallyallocated to processors in a way that would minimizethe bandwidth of interconnections between the objects. Thiswould free the user from the cumbersome task of understandingthe actual flow of data between objects in the patch; thesoftware would automatically assess that.This didn’t prove practical, for two reasons. First, as haslong been well known, one can’t compute the quantity of datathat will flow between any given pair of objects in a patch (atleast, not if the patching language is able to solve arbitrarycomputing problems). Predicting how much data will flowwhere is hopeless.The second problem is that nobody has been able to makean expressive patching language that doesn’t depend on objectssharing data. In Max/FTS (and in Pd as well) this takesthe form of “named” objects such as arrays. Any automaticdistribution of patches that allows accessing arrays would haveto place every object that accesses any particular array on thesame processor, or else use some kind of locking mechanismthat would be unlikely to work in real time. Also, any situationin which there is of recombination of message fanoutwould require that both message paths be synchronized, i.e.,that both message paths go through the same itinerary of processorsor be otherwise delay-equalized. In combination, theseconstraints would require that, for complete transparency, almostany interesting patch would have to reside on a singleprocessor. It appears to be an inescapable fact that multiprocessinghas non-hideable effects on the execution of “patches”and can’t effectively be carried out without the user’s activeparticipation.'

It might be easier if Pd used a system of buses for routing rather than arbitrarily drawn patch connections, or if a graphical patching environment had a good way of implementing something like SuperCollider's Synth, which works with a flexible node order but has relatively limited means of input and output. Here's what I see as the bare minimum of what we'd need to address to make your wish a reality:
1) Unit generators instantiated in Pd have to exist somewhere in a running patch to output. This is in distinction to SC3 and csound, where instances of Synths or instrument templates are instantiated and destroyed. In the latter two, the order of creation and destruction of instruments (and in csound, the order they're defined in the orchestra) matters a lot in the DSP graph, which makes it more predictable. SC3 also has user methods for ordering nodes.
2) Connections in Pd are flexible and arbitrarily complex, which makes the DSP graph a lot more ramified than a mixer model with buses, inserts, aux sends, etc., and therefore much less predictable in the abstract.
3) Pd is deterministic, which means that (as noted in the quote above), any memory sharing across threads would need to involve locks, which can be killer in real-time, not to mention difficult to scale and guarantee thread safety. [pd~] communicates via FIFO because it needs to be able to keep messages and audio in sync by block.
I'm sure there's more.

On Thu, Mar 31, 2016 at 11:07 PM, William Huston <williamahuston at gmail.com> wrote:

Slightly OT-- but related: 
I'm going to throw out a wish-list item 
which is probably impossible or very hard to implement,

Find a way for the DSP Graph compiler to naturally
break up the task into small chunks, which all use shared
memory in a thread-safe way, so that the PD job automagically
spreads itself out over available cores without any special 
work by the programmer. 

Now that I've got my big, fast, lots of ram, 6-core AMD
box running again, I notice that I can run MUCH larger
graphs there than on my Raspberry Pi. 

But I think it's just a raw CPU speed, or Cache Speed, or
speed to RAM which matters, and not the number of cores.

I notice that on my Pi there seems to be two processes, one for
the GUI and one for the DSP. 2 cores are wasted unless I use
the [pd~] object, and I have to basically guess how to split up the job. 

I know, I'm dreaming here....

On Thu, Mar 31, 2016 at 10:57 PM, Lucas Cordiviola <lucarda27 at hotmail.com> wrote:

And more,
A single thread calculation divided between 2 cores in its 1 core time is more stable.
1 00 11 00 1
transistors have more idle time.
Mensaje telepatico asistido por maquinas.

From: brbrofsvl at gmail.com
Date: Thu, 31 Mar 2016 22:45:57 -0400
Subject: Re: [PD] DSP and Gem in the same instance of Pd
To: jancsika at yahoo.com
CC: lucarda27 at hotmail.com; pd-list at lists.iem.at

Right, so the point of [pd~] is that the OS can now throw whatever is going on in the subprocess onto another core. The idea from what I've heard for Gem is that you can leave the DSP off in the [pd~] instance, run Gem from there (on another core, possibly). Then if together they would have maxed out one core they could split the work among two and proceed in time.
But if the problem is that Gem has to wait for something to happen elsewhere before it can proceed, it won't help. Kind of in the same way that running an infinite [until] loop on the subprocess will halt the main process, too.
On Thu, Mar 31, 2016 at 9:24 PM, Jonathan Wilkes via Pd-list <pd-list at lists.iem.at> wrote:

But [cpu_hungry_hippo~] needs input from [pd~] in order to 
compute its output.  So [pd~] must send output before [cpu_hungry_hippo~] 
can execute its perform routine. 

    On Thursday, March 31, 2016 9:17 PM, Lucas Cordiviola <lucarda27 at hotmail.com> wrote:

 Isn`t 
[pd~] <-- some dsp stuff going on in here 
To take advantage of multi-core CPUs?
Mensaje telepatico asistido por maquinas.

Date: Fri, 1 Apr 2016 00:37:26 +0000
To: brbrofsvl at gmail.com; reduzent at gmail.com
CC: pd-list at lists.iem.at
Subject: Re: [PD] DSP and Gem in the same instance of Pd
From: pd-list at lists.iem.at

I'm not sure I understand [pd~].  Consider:
[foo~]|[pd~] <-- some dsp stuff going on in here
|[cpu_hungy_hippo~]
How does [pd~] help me in this case, as opposed to just putting the 
"dsp stuff" directly in the patch?

And in general, how is the super-process able to anything 
other than block when waiting for output from [pd~]?

-Jonathan

 On Thursday, March 31, 2016 5:17 PM, Matt Barber <brbrofsvl at gmail.com> wrote:

 One other thing that's helped in an emergency is increasing Pd's audio buffer in the preferences.
One thing I've heard of but never tried is running Gem from a slave instance in [pd~]. I don't know enough about it to know whether this could work or why; it might just be a rain dance.
On Thu, Mar 31, 2016 at 7:16 AM, Roman Haefeli <reduzent at gmail.com> wrote:

On Thu, 2016-03-31 at 11:35 +0200, cyrille henry wrote:
>
> Le 31/03/2016 11:19, Roman Haefeli a écrit :
> >
> > BTW: Why does the graphics rendering|clock have precedence over the
> > audio rendering (at least, it seems to be like that in Pure Data/Gem)? I
> > guess most softwares do it the other way around, since clicks are much
> > more noticeable than a frame being a few milliseconds late.
>
> Gem have no precedence over audio : they both have the same priority.
> when having priorities on audio, the openGL rendering did not have
> fixed frame rate, and it's not possible any-more to have smooth hight
> speed movement.
>
> So, i like the way it is, even if it cause implementation problem.

Oh, now since I understand, I like the way it is, too ;-)

> one possible explanation of your problem is that you are rendering a
> 60 fps, and that openGL is sync on the 60fps screen.
> You can have jitter between the 2 different 60fps clock. If Gem is
> waiting for the screen, then everything (including audio) is on pause.

That is exactly what I was doing.

> if this is the cause of your problem, then reduce Gem fps to 59, or
> remove openGL syncro (sync to vblank).

This is exactly what helped (reducing fps to 59). Thanks for your sharp
thinking.

Roman

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