Re: [fltk.coredev] Re: RFC: unify timer callback handling on all platforms

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Re: Re: RFC: unify timer callback handling on all platforms Albrecht Schlosser 07:04 Jul 06 top right image
On 7/6/21 7:22 AM Manolo wrote:

Le lundi 5 juillet 2021 à 22:02:45 UTC+2, Albrecht Schlosser a écrit :

@Manolo: Before you dive too deep into a specific implementation for Wayland I'd like to share some thoughts I'm having since some time now to unify the timer handling on all platforms. I believe that the Linux timer implementation is superior to the Windows and maybe also the macOS implementation. The Linux timer implementation works like this (maybe over-simplified):
I believe this means the timer implementation for the X11 FLTK platform (which covers Linux but also Unix and Darwin).

(1) Every call to Fl::add_timeout() or Fl::repeat_timeout() adds a timer entry to the internal timer queue. This queue is sorted by the timer's due time.

(2) There's only one system timer, using the smallest delta value, i.e. the time of the first timer in the queue.
In my view, there's no system timer at all. FLTK sets the max length of the next select/poll call to the smallest delta value,
which has the effect of breaking the event loop at the desired time. This setup is possible because with X11 (and with Wayland too)
the event loop is built using a select/poll call that returns when data arrive on a fd or when the waiting delay expires.

Yes, that's true. What I wanted to say is that FLTK does not schedule a different (system) timer for each timer entry of the timer queue as opposed to the Windows implementation (IIRC).

(3) Whenever the timer triggers (or maybe more often) the event handling decrements the delta time of all timers.
I find this procedure awkward, even though it's correct.

Awkward or not, it's more precise than that of the Windows timers. My intention is to simplify and unify the timer event handling on all platforms. We have the documented "delta adjustment" of Fl::repeat_timeout() which definitely doesn't work well (or not at all) on Windows which is why timers on Windows tend to drift away. One reason of inaccuracy is the Windows timer granularity which has limits (this affects each single timer). The overall timer accuracy of Fl::repeat_timer() could, however, be improved (less or no overall timer drift). Let's say: if you n repeated timers then the average timer interval should be as accurate as possible.

(4) All timers callbacks of expired timers are called.

(5) A new timer with the shortest delay (which is always the first timer in the queue) is scheduled.

(6) Wait for timer events...
In my view, there're no real timer events: the poll/select call expires.

Yep, I agree. As said above...

This is AFAICT done because the standard Unix timers can be interrupted and need to [be] re-scheduled whenever such interrupts occur.

The benefit of this approach is as described in Fl::repeat_timer() docs: if the call to Fl::repeat_timer() is done "late" it can be corrected by the delay and the overall timer sequence of repeated timers is more accurate than on other platforms.

On the Windows platform we're (AFAICT) using one system timer per Fl::add/repeat_timer() call. The Windows timer events are less accurate anyway, but a change as designed for Unix/Linux could probably contribute to more accuracy of repeated timer events because the correction of timer delay as on Unix/Linux could work better (it does not currently on Windows).

I know less about the macOS platform, but I know for sure that the timer handling is different. There are inconsistencies WRT Unix/Linux/Windows on the user visible level (which I intend to demonstrate with a test program anyway) but which are too difficult (and OT now).

The macOS FLTK platform uses a system timer: the event loop is made by calling a function that does "wait until an event arrives",
and Fl::add_timeout creates a system object that makes the waiting function run the timer cb when the delay has expired.

Is it correct that this is one distinct system timer per timer queue entry?

My idea was to also use a true system timer for the Wayland platform (but that could be for all Linux). Posix timers do that.
They trigger either a signal or a thread after a specified delay. With the thread approach, having the child thread call Fl::awake(cb, data)
allows the main thread to stop waiting and process the timeout cb.

Hmm, is this a necessary change for the Wayland platform, or do you want to do it because you find the current implementation "awkward"?

Did you consider the possibly new requirements, platform dependencies, and such? If we extended this to the normal Unix platform ("all Linux", as you wrote), would this affect compatibility?

Note: I don't consider the current Unix/Linux implementation awkward (it has its advantages), and if you want to change it to POSIX timers we should discuss this here in general. I would rather change all other implementations to the "manage our own timer event queue" approach like it is in our current Unix/Linux implementation. But this is not a contradiction because the timer event scheduling and the timer event processing can be independent - see my description below.

That all said: I hope that the Wayland implementation would be basically like the Unix/Linux timer queue handling so we can easily unify all platforms.

More about the unification: I'm thinking of a platform independent timer queue where Fl::add_timeout() and friends would be platform independent. They would add an Fl_Timeout_XX object to the timer queue which may contain platform specific timer data (or not?). Triggering the timeout would then, as always, be done by the system, the timer queue handling would still be platform independent, as well as calling the callbacks etc.. The more I think about it, the more I believe that only the scheduling of this single timer event would be a platform dependent (i.e. system driver) function.

As written above,  the X11 approach uses the fd through which all X11 data arrives and the poll/select call on this fd to simulate
timeout events: it reduces the max waiting time of the poll/select call. Is your idea to change the organization of the event loop
of other platforms (namely macOS) and have it wait for GUI events for a time determined by the next scheduled timeout?

I can't answer this question with yes or no.

My basic idea is to unify (and therefore simplify) the timer event handling on all platforms. I've seen IMHO too much platform specific code to handle timer events. The more platforms we add, the more platform specific code we need to maintain.

My goal is to do as much as possible in platform independent code. This platform independent code would schedule timer events by adding them to the timer event queue - in my model the Unix/Linux code would be a valid implementation. This could be done on all current and future platforms in a platform independent way. There would also be only one platform independent timer event processing function. This could be (like) the current  Unix/Linux implementation which decrements the timer delay of all timer events in the queue.

The only platform dependent code should be the scheduling of the timer event. This could be as it is now on Unix/Linux to reduce the select/poll timer to the next timer delay or to schedule exactly one system timer on each platform which would be the delta time to the next (first) timer event. The only requirement is that the resulting timer event would call the platform independent process_timer_event() function.

For further clarification: my model would allow to use the fd approach we're using now as well as POSIX timers and Windows or macOS timers as long as we're only scheduling one timer for all systems and we're doing the timer event processing in only one system independent function. This is basically what I want to achieve.

This would allow us to get the same behavior on all current and future platforms including the optimal "repeated timer delay correction" with the minimum of platform specific code. A nice side effect would be that porting to another platform would be simplified.

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