Re: [perfmon2] Multiplexing counters

[Drongowski, P. <Pau...@am...>]

Hello Leonardo --

Barcelona (Family 10h) has only four physical performance
counters. Thus, some other strategy such as multiplexing is
required when collecting 5 performance events during a run
on Barcelona.

As an aside, if people are using Family 15h, 15h has six
core performance counters and four Northbridge performance
counters. Even though there are six core counters, event
assignment is restricted on Family 15h. Thus, you will not
always be able to measure six events in a single run without
multiplexing. The Family 15h BKDG has the details about
event-to-counter assignment and restrictions.

-- pj


-----Original Message-----
From: Leonardo Piga [mailto:leo...@gm...] 
Sent: Thursday, April 26, 2012 11:52 PM
To: per...@li...
Subject: [perfmon2] Multiplexing counters

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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