Hello,
I am using libpfm 4.2 to measure about 15 performance counters on a
AMD Barcelona CPU. I am able to collect 5 performance counter without
multiplexing.
However, when multiplexing I am getting "negative" delta values using
the syst from perf_examples.
Here is an example:
Sample n
"core" : 0,
"name" : "perf::PERF_COUNT_HW_CPU_CYCLES",
"val" : 131956093,
"raw" : 62161538,
"ena" : 1000618391,
"run" : 471368743,
"ratio" : 0.47,
"delta" : 131956093
Sample n+1
"core" : 0,
"name" : "perf::PERF_COUNT_HW_CPU_CYCLES",
"val" : 118435137,
"raw" : 63447168,
"ena" : 2002214687,
"run" : 1072611170,
"ratio" : 0.54,
"delta" : 18446744073696030660
As you can see Sample_n$val is greater than Sample_n+1$val that is why
delta is so big (negative actually). (The raw value is growing
though).
So, I think that I am not doing the measurements in the best way.
My questions are:
1)How does libpfm do the multiplexation?
2) What is the best way to multiplex 15 performance counters in time
window of 1 second? Is there any example available with the library to
do this?
3) What is the purpose of group (-g option on syst tool)? Can it help
on my issue?
--
Leonardo
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