i am using PAPI liberary to tune and profile my application.
I want to know what (PAPI_REF_CYC : Reference clock cycles ) means actually?
Thanks in advance,
Some modern CPUs, including the Intel's and AMD's ones, are throttled.
This means that their clocks are not fixed but vary depending on the power management active - even if the CPU's brand frequency is X Ghz, more often than not, it is not running at that frequency.
For a couple of real example technology see the Intel Turbo boost technology/AMD Turbo core and Intel Enhanced Speedstep technology/AMD Quiet'n'Cool technology.
Since the core clock can slow down or speed up, comparing two different measures makes no sense.
Having a snippet A to run in 100 core clocks and a snippet B in 200 core clocks means that B is slower in general (it takes double the work), but not necessarily that B took more time than A since the units are different.
That's where the reference clock comes into play - it is uniform.
If snippet A runs in 100 ref clocks and snippet B runs in 200 ref clocks then B really took more time than A.
Converting ref clock ticks into time (e.g. seconds) is not that easy, each processor uses a difference reference frequency, even among processor with the same brand name.