[PD] Understanding the mechanics of rebuilding Pd's DSP graph

[Roman Haefeli]

Hi all

First, I'm not even sure if 'DSP graph' is the correct term. Pd's
documentation[1] states that all DSP objects are internally arranged
into a linear order which I believe is often called 'DSP graph'. There
are apparently some actions that cause this DSP graph to be rebuilt.
Rebuilding takes time and is often the cause of audio drop-outs. I would
like to have a better understanding of the mechanics going on behind the
scenes with the hope to be able to optimize my Pd programming. 

One thing I'd like to know: Is there one graph for all patches in a
certain instance of Pd? It seems that adding a tilde-object to a patch
causes the DSP graph to be recalculated. Now, if _everything_ is in the
same graph, this would mean the whole graph needs to be recalculated
when adding objects (or abstractions containing tilde-objects, for that
matter), no matter where I put them. It would make no difference whether
I have one big patch with 1000 tilde-objects loaded or 100 smaller
patches with 10 tilde-objects each, when adding new objects, would it?
Is the time it takes to recalculate the graph only dependent on the
number of tilde-objects running in the current instance of Pd? If so, is
that a linear correlation? 10 times more tilde-objects means it takes 10
times as long to recalculate the graph? Or is it even exponential? There
is no way to partition the graph and update only one partition, is
there?

On a related note, I made the following observation and I'm wondering
if/how that is related to the DSP graph: I create a minimalist patch
with a small [table foo 100] and I measure the time it takes to 'resize'
it to 99 with [realtime]. On my box, this takes 0.01 ms. I expected it
to be fast, since memory access is very quick. Now, I additionally load
a much more complex patch with many tilde-objects. I 'resize' the table
again and it still takes only 0.01ms. Now I put a [tabread~ foo]
somewhere in the patch. Now,'resize'-ing the table foo to 100 takes
20ms. Even if I remove the [tabread~ foo] again, resizing the table
still takes at least 20ms. There is no way to make it fast again except
restarting Pd. I also figured out that when only a non-tilde [tabread
foo] is refencing the table I'm resizing, the resizing keeps being fast.
Only when tilde-objects are referencing the table, resizing that very
table becomes slow. The actual time seems dependent on the complexity of
the loaded patch(es). And it also corresponds with the time it takes to
send 'dsp 1' to pd (when dsp is switched off). 

Why is resizing tables so much slower, when tilde-objects are
referencing it? I noticed that even resizing very small tables can be a
cause for audio drop-outs. I wonder whether 'live-resizing' should be
avoided altogether.

Yeah, that's a bunch of questions... Even when knowing the answer to
only some them, it might clear things up quite a bit.

Roman



[1] "Pd sorts all the tilde objects into a linear order for
running." ( http://msp.ucsd.edu/Pd_documentation/x2.htm#s4.2 )
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