Writing code that takes time to run

Richi Dubey richidubey at gmail.com
Thu May 20 14:53:22 UTC 2021


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.

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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