[PD-dev] implementing pools of clocks?

Iain Duncan iainduncanlists at gmail.com
Sun Oct 25 16:16:19 CET 2020

Thanks Christof, that is helpful again, and also encouraging as it
describes pretty well what I've done so far in the Max version. :-)

I've enabled both one shot clocks (storing them in a hashtable owned by the
external) and periodic timers. The one shot clocks exist in both a
transport aware and transport ignorant format for working with and
quantizing off the Max global transport, and there are periodic timers for
both too. (The transport time stops when the transport is stopped, the
other is just a general X ms timer). I have also ensured the callback
handle gets passed back so that any timer or clock can be cancelled from
Scheme user-space. (is there such a thing as a global transport in PD?)

I was actually planning on something like you described in Scheme space: a
user defined scheduler running off the timers. I will look into priority
queues. I had one thought, which I have not seen much, and was part of the
reason I was asking on here about schedulers. I would like to ensure the
user can run multiple transports at once, and hop around in time without
glitches. I was thinking that instead of using just a priority queue, I
would do something like a two stage structure, perhaps with a hashmap or
some other fast-read-anywhere structure with entries representing a time
period, and holding priority queues for each period. This would be to
enable the system to seek instantly to the bar (say) and iterate through
the queue/list for that bar. Wondering if anyone has used or seen this type
of pattern or has suggestions? Basically I want to make sure random access
in time will work ok even if the number of events in the schedule is very
high, thus allowing us to blur the lines between a scheduler and a full
blown sequencer engine. Thoughts, suggestions, and warnings are all welcome.


On Sun, Oct 25, 2020 at 4:21 AM Christof Ressi <info at christofressi.com>

> Actually, there is no need to use a clock for every scheduled LISP
> function. You can also maintain a seperate scheduler, which is just a
> priority queue for callback functions. In C++, you could use a
> std::map<double, callback_type>. "double" is the desired (future) system
> time, which you can get with "clock_getsystimeafter".
> Then you create a *single* clock in the setup function *) with a tick
> method that reschedules itself periodically (e.g. clock_delay(x, 1) ). In
> the tick method, you get the current logical time with
> "clock_getlogicaltime", walk over the priority queue and dispatch + remove
> all items which have a time equal or lower. You have to be careful about
> possible recursion, though, because calling a scheduled LISP function might
> itself schedule another function. In the case of std::map, however, it is
> safe, because insertion doesn't invalidate iterators.
> Some more ideas:
> Personally, I like to have both one-shot functions and repeated functions,
> being able to change the time/interval and also cancel them. For this, it
> is useful that the API returns some kind of identifier for each callback
> (e.g. an integer ID). This is what Javascript does with
> "setTimeout"/"clearTimeout" and "setInterval"/"clearInterval". I use a very
> similar system for the Lua scripting API of my 2D game engine, but I also
> have "resetTimeout" and "resetInterval" functions.
> On the other hand, you could also have a look at the scheduling API of the
> Supercollider, which is a bit different: if a routine yields a number N, it
> means that the routine will be scheduled again after N seconds.
> Generally, having periodic timers is very convenient in a musical
> environment :-)
> Christof
> *) Don't just store the clock in a global variable, because Pd can have
> several instances. Instead, put the clock in a struct which you allocate in
> the setup function. The clock gets this struct as the owner.
> typedef struct _myscheduler { t_clock *clock; } t_myscheduler; // this
> would also be a good place to store the priority queue
> t_scheduler *x = getbytes(sizeof(t_myscheduler));
> t_clock *clock = clock_new(x, (t_method)myscheduler_tick);
> x->clock = clock;
> On 25.10.2020 02:02, Iain Duncan wrote:
> Thanks Christof, that's very helpful.
> iain
> On Sat, Oct 24, 2020 at 5:53 PM Christof Ressi <info at christofressi.com>
> wrote:
>> But if you're still worried, creating a pool of objects of the same size
>> is actually quite easy, just use a
>> https://en.wikipedia.org/wiki/Free_list.
>> Christof
>> On 25.10.2020 02:45, Christof Ressi wrote:
>> 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.
>> Christof
>> 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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