Writing code that takes time to run

Joel Sherrill joel at rtems.org
Thu May 20 14:59:15 UTC 2021

On Thu, May 20, 2021 at 9:54 AM Richi Dubey <richidubey at gmail.com> wrote:

> Hi,
> We are thinking of writing a piece of code that takes some time to run (it
> would be amazing if it takes around 2 secs to run on hardware, but we would
> be happy with something that takes a few milliseconds as well).
> We tried writing this:
>   for(int i = 0; i<10000000; ++i){
>       fib2 = fib0 + fib1;
>       fib0 = fib1;
>       fib1 = fib2;
>   }
> which takes few milliseconds when tested on qemu, but only takes few
> microseconds on a real board. Do you have any suggestions of what else we
> can do?
> We want to write a code that is context switch safe (so, we can't simply
> check the time before a loop, run an infinite loop that keeps checking
> current time and stops after a few seconds - because this logic would fail
> if there happens a context switch inside the loop and the task gets the
> control back after a few seconds). We also don't want to do a wake_after()
> since we want the task to be running on the cpu during the entire time (it
> is okay if the task gets preempted due to a higher priority process), and
> not voluntarily giving the control to some other task.

The crypt test takes a long time to run. You could lift cases from there
until you hit the time you want.

> Any suggestions? The aim is to see the affect of a task getting removed
> from the cpu due to task shifting by the newly arrived task (in strong apa
> vs non task shifting scheduler).
> Thank you.
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