On Wed, 23 Aug 2023 at 15:39, Joseph Larralde <joseph.larralde@gmail.com> 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 <info@christofressi.com> 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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