Messages in this thread | | | Date | Mon, 22 Jun 2009 13:50:17 +0200 | From | Ingo Molnar <> | Subject | Re: I.2 - Grouping |
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> 2/ Grouping > > By design, an event can only be part of one group at a time. > Events in a group are guaranteed to be active on the PMU at the > same time. That means a group cannot have more events than there > are available counters on the PMU. Tools may want to know the > number of counters available in order to group their events > accordingly, such that reliable ratios could be computed. It seems > the only way to know this is by trial and error. This is not > practical.
Groups are there to support heavily constrained PMUs, and for them this is the only way, as there is no simple linear expression for how many counters one can load on the PMU.
The ideal model to tooling is relatively independent PMU registers (counters) with little constraints - most modern CPUs meet that model.
All the existing tooling (tools/perf/) operates on that model and this leads to easy programmability and flexible results. This model needs no grouping of counters.
Could you please cite specific examples in terms of tools/perf/? What feature do you think needs to know more about constraints? What is the specific win in precision we could achieve via that?
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