[PD-dev] implementing pools of clocks?

Christof Ressi info at christofressi.com
Sun Oct 25 02:45:52 CEST 2020

> A) Am I right, both about being bad, and about clock pre-allocation 
> and pooling being a decent solution?
> B) Does anyone have tips on how one should implement and use said 
> clock pool?
ad A), basically yes, but in Pd you can get away with it. Pd's scheduler 
doesn't run in the actual audio callback (unless you run Pd in 
"callback" mode) and is more tolerant towards operations that are not 
exactly realtime friendly (e.g. memory allocation, file IO, firing lots 
of messages, etc.). The audio callback and scheduler thread exchange 
audio samples via a lockfree ringbuffer. The "delay" parameter actually 
sets the size of this ringbuffer, and a larger size allows for larger 
CPU spikes.

In practice, allocating a small struct is pretty fast even with the 
standard memory allocator, so in the case of Pd it's nothing to worry 
about. In Pd land, external authors don't really care too much about 
realtime safety, simply because Pd itself doesn't either.


Now, in SuperCollider things are different. Scsynth and Supernova are 
quite strict regarding realtime safety because DSP runs in the audio 
callback. In fact, they use a special realtime allocator in case a 
plugin needs to allocate memory in the audio thread. Supercollider also 
has a seperate non-realtime thread where you would execute asynchronous 
commands, like loading a soundfile into a buffer.

Finally, all sequencing and scheduling runs in a different program 
(sclang). Sclang sends OSC bundles to scsynth, with timestamps in the 
near future. Conceptually, this is a bit similar to Pd's ringbuffer 
scheduler, with the difference that DSP itself never blocks. If Sclang 
blocks, OSC messages will simply arrive late at the Server.


On 25.10.2020 02:10, Iain Duncan wrote:
> Hi folks, I'm working on an external for Max and PD embedding the S7 
> scheme interpreter. It's mostly intended to do things at event level, 
> (algo comp, etc) so I have been somewhat lazy around real time issues 
> so far. But I'd like to make sure it's as robust as it can be, and can 
> be used for as much as possible. Right now, I'm pretty sure I'm being 
> a bad real-time-coder. When the user wants to delay a function call, 
> ie  (delay 100 foo-fun), I'm doing the following:
> - callable foo-fun gets registered in a scheme hashtable with a gensym 
> unique handle
> - C function gets called with the handle
> - C code makes a clock, storing it in a hashtable (in C) by the 
> handle, and passing it a struct (I call it the "clock callback info 
> struct") with the references it needs for it's callback
> - when the clock callback fires, it gets passed a void pointer to the 
> clock-callback-info-struct, uses it to get the cb handle and the ref 
> to the external (because the callback only gets one arg), calls back 
> into Scheme with said handle
> - Scheme gets the callback out of it's registry and executes the 
> stashed function
> This is working well, but.... I am both allocating and deallocating 
> memory in those functions: for the clock, and for the info struct I 
> use to pass around the reference to the external and the handle. Given 
> that I want to be treating this code as high priority, and having it 
> execute as timing-accurate as possible, I assume I should not be 
> allocating and freeing in those functions, because I could get blocked 
> on the memory calls, correct? I think I should probably have a 
> pre-allocated pool of clocks and their associated info structs so that 
> when a delay call comes in, we get one from the pool, and only do 
> memory management if the pool is empty. (and allow the user to set 
> some reasonable config value of the clock pool). I'm thinking RAM is 
> cheap, clocks are small, people aren't likely to have more than 1000 
> delay functions running concurrently or something at once, and they 
> can be allocated from the init routine.
> My questions:
> A) Am I right, both about being bad, and about clock pre-allocation 
> and pooling being a decent solution?
> B) Does anyone have tips on how one should implement and use said 
> clock pool?
> I suppose I should probably also be ensuring the Scheme hash-table 
> doesn't do any unplanned allocation too, but I can bug folks on the S7 
> mailing list for that one...
> Thanks!
> iain
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