performance counter

I 'm wanted to use the performance counter to know the speed of my DMA, but I don't understand what is "PERF_UNIT_BASE" in

------------------------

PERF_BEGIN (PERF_UNIT_BASE, INTERESTING_FUNCTION_SECTION);

..body of subroutine...

PERF_END (PERF_UNIT_BASE, INTERESTING_FUNCTION_SECTION);

---------------------------------

because when i put any value i have some results like that

Total Time: 3.68935E+11 seconds (-1 clock-cycles)

+---------------+-----+-----------+---------------+-----------+

| Section | % | Time (sec)| Time (clocks)|Occurrences|

+---------------+-----+-----------+---------------+-----------+

|interesting_fn | 100|368934881474.19104| -1| 4294967295|

+---------------+-----+-----------+---------------+-----------+

Thank you for sombody who had an answer.

Muffec, France