Re: longest path through a task

[jan.de.kruyf@gmail.com]

On Friday, May 29, 2015 at 6:31:27 PM UTC+2, Simon Wright wrote:

> 
> > Just did some measurements (clumsily) using the Cortex-M4 counter as
> > in [1], and from entering the Ada handler to the triggered task
> > starting to execute averaged at 1200 cycles (6.7 us) with -Og, 1070
> > cycles (5.9 us) with -O2. There is some code in the C handler to
> > redirect to the Ada handler: see [2], starting at line 236.
> 
> The code in the C handler takes 150 cycles.
> 
> So, for my FreeRTOS-based RTS, interrupt to Ada handler is 150 cycles,
> Ada handler to task is 1200 cycles, grand total 1350 cycles or 7.5
> microseconds (core clock 180 MHz).
> 
> I understand that a 1-millisecond tick is inadequate for some control
> applications (though the Crazyflie seems to do OK with FreeRTOS); maybe
> one could use one of the other timers on the STM32F4 to act as the basis
> for the hard control loop, while retaining the Ada tasking for less
> demanding aspects?

Yes a person can, You can also modify the runtime.

it is in bbbosu.adb:

   -----------------------
   -- Sys_Tick Handling --
   -----------------------

   --  We use the Sys_Tick timer as a periodic timer with 1 kHz rate. This
   --  is a trade-off between accurate delays, limited overhead and maximum
   --  time that interrupts may be disabled.

   Tick_Period   : constant Timer_Interval := Clock_Frequency / 1000;

-----------------

But from too much looking at that code I got squint eyes ;)

In any case based on that comment and my experience I thought I will construct a single thread multitask environment, which is easy. The programming for a novice will be a bit more complex especially when you have hard deadlines.
It is basically just a choice.

I did find a write-up from Brega on the Xoberon runtime, I am still looking for the code. In any case they did have multi-threading but mixed it with hard task dead times, and some more tricks, quite a feat.

Now back to my kernel module, I am making good progress, but it is hairy, at least if you want to have any runtime at all. And I cant use gnat for linking either. So I do every step by hand. 
cheers,

j.