On 03/01/2012 11:08 AM, Victor Jimenez wrote:
I am trying to sample instruction pointer along time on a Power7 system. I know
that there are accurate mechanisms to do so in Intel processors (e.g., PEBS and
Branch Trace Store).
Is it possible to do something similar in Power7? Will the samples be accurate?
I am worried that significant delays (skids) may appear.
Victor,
There's a subset of events that have a prefix of "PM_MRK_". These are referred
to as "marked events". When profiling with such events, the PMU will randomly
select (mark) an instruction in a group and follow it through the pipeline. By
profiling on marked events, it's possible to uniquely identify which instruction
caused a particular event.
This technique results in fewer samples being recorded than for non-marked
events, but those that are recorded are almost always precise (no skid).
Because you will get fewer than the normal number of samples, you should either
profile at a higher sampling rate than normal or make sure your application runs
long enough to give you statistically valid results. My rule of thumb is to use
a sampling rate of 100x what I would normally use for the non-marked event.
-Maynard
Thank you,
Victor
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