cachegrind counts do not reflect real performance

Question

Two versions of the same algorithm yield different total instruction fetch counts and cycle estimations under valgrind/cachegrind. The difference is about 25%. Process timing, however, is very similar (it is actually shorter for the cachegrind-slow version):

• version 1:

  Ir:     146,328,018,245
  CEst:   152,553,736,055
  timing: 17.93 s
  

• version 2:

  Ir:     185,221,836,610
  CEst:   197,531,381,950
  timing: 17.53 s
  

Is this behaviour expected? How can I learn more about why version 1 is slower?

Answer 1

I discovered that the inconsistency is due to the different compiler options used for the cachegrind runs and for the timing runs. In particular, I had disabled function inlining for the cachegrind runs (so that I could get meaningful per-function counts).