Re: Sampling instruction pointer on PPC

[Maynard Johnson <maynardj@us.ibm.com>]

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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