[PD-dev] implementing pools of clocks?
Christof Ressi
info at christofressi.com
Sun Oct 25 12:20:25 CET 2020
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
> <mailto: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
> <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
>>>
>>> _______________________________________________
>>> Pd-dev mailing list
>>> Pd-dev at lists.iem.at <mailto:Pd-dev at lists.iem.at>
>>> https://lists.puredata.info/listinfo/pd-dev <https://lists.puredata.info/listinfo/pd-dev>
>>
>> _______________________________________________
>> Pd-dev mailing list
>> Pd-dev at lists.iem.at <mailto:Pd-dev at lists.iem.at>
>> https://lists.puredata.info/listinfo/pd-dev <https://lists.puredata.info/listinfo/pd-dev>
> _______________________________________________
> Pd-dev mailing list
> Pd-dev at lists.iem.at <mailto:Pd-dev at lists.iem.at>
> https://lists.puredata.info/listinfo/pd-dev
> <https://lists.puredata.info/listinfo/pd-dev>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.puredata.info/pipermail/pd-dev/attachments/20201025/21dbe8ba/attachment.html>
More information about the Pd-dev
mailing list