How to remove the time taken by the clock from the total measured execution time?

Question

I am trying to measure the time taken by a set of statements. The following is a pseudocode. The code is implemented in C++ on a Xilinx chipset, with a custom RTOS, so the traditional c++ clock functions do not work here.

I do not need help on the actual time measurement, but more on the math on how to calculate the actual execution time.

one = clock.getTime();
/*statement 
*     1      *
*     to     *
*     10     */
two = clock.getTime();
fTime = two - one;

Now I know the time taken by the statements. This time is also includes the time taken by getTime() too right?

one = clock.getTime();
clock.getTime();
two = clock.getTime();
cTime = two - one;     //Just measure and the min value i get is 300 microseconds.

Now this block gives me the time taken by getTime().

Finally, my question is:

What is the actual time taken by the statements?

1. fTime - cTime
2. fTime - (2* cTime)
3. Other equation ?

Answer 1

Your time measurement shifts the time

if it is on stable enough platform then the shift is the same for all times so

one = t1 + dt
two = t2 + dt

after substraction the shift eliminates itself so

two-one = (t2+dt)-(t1+dt) = t2-t1

so there is no need to make corrections for time measurement shift in this case.

Problems starts on multi-scalar/vector architectures where the code execution is variable

• due to different cache miss-es
• different prefetch invalidation

and so on then you have to play with cache invalidation. Also if your getTime() waits for interrupt or HW event that can also add few error T's

In that case measure many times and get the avg or smallest valid result something like:

• Negative clock cycle measurements with back-to-back rdtsc?