[PD-dev] why must one never send a message from a perform routine ?

Tue Aug 22 15:08:20 CEST 2023

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 at 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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