Glad that I could help! Very little of this is really documented
(accurately). Personally, I figured this out by reading the source code.
Ideally, we should improve the official documentation in
http://msp.ucsd.edu/Pd_documentation/x3.htm#s1.0. Some things are
outdated, misleading or just plain wrong (in particular the section
"audio buffer size and block size"). I just put this on my
(ever-growing) TODO list.
Christof
On 23.08.2023 16:38, Joseph Larralde wrote:
Wow, thanks again Christof, this greatly improves my understanding of
Pd's engine.
Indeed, I never use callback mode because everytime I did in the past
I got some xruns, but had no clue about what was happening behind the
scene.
I feel a bit ashamed, I'm pretty sure I could have figured this out by
reading more posts / docs / code ...
I'm very grateful to both of you for taking the time to explain this
in detail, and I hope this will benefit other users on the list.
Best regards,
Joseph
Le 22/08/2023 à 17:22, Christof Ressi a écrit :
I've always been puzzled by the fact that everything runs on a
single thread in Pd.
By default, Pd operates in "polling mode", i.e. the scheduler runs in
its own thread (the main thread) and communicates with the audio
callback via two lockfree ringbuffers (one for input, one for
output). The size of the ringbuffers is set by the ominous "delay"
parameter in the audio settings. The actual audio thread only
reads/writes samples from/to the ringbuffers.
If Pd operates in "callback mode" (= "callbacks" is ticked in the
audio settings), the scheduler runs directly in the audio callback.
You can save a little bit of latency, but it is less forgiving to CPU
spikes or non-realtime-safe operations.
Christof
On 22.08.2023 15:08, Joseph Larralde wrote:
Thanks Christof for the additional insight.
I've always been puzzled by the fact that everything runs on a
single thread in Pd.
I guess this single thread IS the audio thread because it processes
audio, and I've always heard that one must never perform too many
non-audio operations during an audio callback.
But as you say, Pd runs fine for the general user base which I am
part of.
I'll probably give your version a try if I hit the limits with my
current (rapidly growing) project running on a Pi 3 B+, but can't
make any promises with my current schedule.
Thanks for your work anyways.
Best,
Joseph Larralde
--
freelance developer
www.josephlarralde.fr
Le 22/08/2023 à 11:55, Christof Ressi a écrit :
How well does it work?
It seems to work quite well. With synthetic benchmarks I can get a
6x speedup on my 8 core machine, but I need to do some more
practical testing and benchmarking.
It looks like the repo is based off of 0.52?
I think it's based on 0.53. I want to rebase it on 0.54, but there
are lots of conflicts I need to resolve. It's definitely on my TODO
list. That's also why I haven't really made a formal announcement yet.
Multithreaded DSP would have been much higher on my list than
multi-channel,
Priorities are very subjective. Personally, I don't really think
that multithreaded DSP has high priority for the general user base,
as many patches seem to run fine on a single CPU. However, I do
have projects that reach or exceed the limits of a single CPU -
even on a beefy machine -, that's why I started working on this.
so I'm wondering if I could get away with using your tree as my
basis for a while :)
Actually, it would be great to have some testers apart from myself!
Christof
On 22.08.2023 10:32, Day Rush wrote:
How well does it work? It looks like the repo is based off of
0.52? Multithreaded DSP would have been much higher on my list
than multi-channel, so I'm wondering if I could get away with
using your tree as my basis for a while :)
- d
On Tue, 22 Aug 2023 at 01:17, Christof Ressi
<[email protected]> wrote:
To expand on Miller's reply:
Conceptually, messaging and DSP are two separate domains.
Sending a
message from a perform routine violates this separation.
Instead you
should use a clock with delay 0 to defer the message to the
begin of the
next scheduler tick.
Miller already mentioned the greatest danger, but there are
other, more
subtle issues. DSP objects typically operate on the premise
that the
object's state won't change from the outside during the
perform routine.
For example, imagine a delay object with a buffer that can be
resized
with a message; by sending a Pd message from the perform
routine, it
might accidentally feed back into the object and reallocate
the buffer
while still in progress.
Unfortunately, very little of this is documented. Ideally,
this should
be covered in the externals-how-to
(https://github.com/pure-data/externals-howto); I just added
an item on
my (long) TODO list.
Finally, although Pd is currently single-threaded, this could
change in
the future. FWIW, here is a PoC for multi-threaded DSP:
https://github.com/spacechild1/pure-data/tree/multi-threading.
This is
only possible because perform routines may only use a
restricted set of
API functions - which, in my fork, are annoted with the (empty)
THREADSAFE macro (and made thread-safe, if necessary).
Christof
On 21.08.2023 20:55, Joseph Larralde wrote:
> Hmm, I see ... unfortunately my random bug is totally
unrelated to
> this weakness of my code.
> Thanks Miller for the explanation and pointers to examples !
> And thanks Claude for the extra example.
> I'll check all my objects to see if there are other ones I can
> consolidate.
>
> Cheers !
>
> Joseph
>
> Le 21/08/2023 à 19:08, Claude Heiland-Allen a écrit :
>> See bang~ in pure-data/src/d_misc.c for an example that
uses a clock
>> to send a message from DSP.
>>
>> On 21/08/2023 18:02, Miller Puckette wrote:
>>> The built-in objects "delay", "metro" and "pipe" use
clocks in
>>> various ways.
>>>
>>> On 8/21/23 18:02, Joseph Larralde wrote:
>>>> I just read in an answer from Christof to Alexandre :
"never ever
>>>> send a Pd message directly from a perform routine !
Always use a
>>>> clock !"
>>
>>
>>
>>
>> _______________________________________________
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>> [email protected]
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>
>
>
>
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