Re: longest path through a task

[Simon Wright <simon@pushface.org>]

jan.de.kruyf@gmail.com writes:

> I dumped the multithread stuff and made a single-thread / multitask
> runtime (slow in Ada unfortunately, but not as slow as multi-threading
> and interrupts. The Ravenscar runtime tries to be everything for
> everybody).

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.

I wrote the C handling using weak symbols, so there's no reason (that I
can see) why an interrupt that needed better performance shouldn't hook
in directly, avoiding the Ada RTS; not so obvious how it would then
communicate with the rest of the code.

[1] http://stackoverflow.com/a/19124472
[2]
https://sourceforge.net/p/stm32f4-gnat-rts/code/ci/default/tree/stm32f429i-disco-rtos/src/stm32f4xx_it.c