Ingo Molnar writes: > > 2/ Grouping > > > > By design, an event can only be part of one group at a time.
To clarify this statement of Stephane's, a _counter_ can only be in one group. You can have multiple counters counting the same _event_ and those counters can (obviously) be in different groups. > > 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. That's not the only reason for having groups, or even the main reason IMO. The main reason for having groups is to provide a way to ask the scheduler to ensure that two or more counters are always scheduled together, so that you can meaningfully do arithmetic operations on the counter values that would be sensitive to the statistical noise introduced by the scheduling, such as ratios and differences. In other words, grouping is there because we don't guarantee to have all counters scheduled onto the PMU whenever possible. Heavily constrained PMUs increase the need for scheduling, but even if counters are completely orthogonal there are only a fixed number of them so we still need to schedule counters at some point. Paul. ------------------------------------------------------------------------------ Are you an open source citizen? Join us for the Open Source Bridge conference! Portland, OR, June 17-19. Two days of sessions, one day of unconference: $250. Need another reason to go? 24-hour hacker lounge. Register today! http://ad.doubleclick.net/clk;215844324;13503038;v?http://opensourcebridge.org _______________________________________________ perfmon2-devel mailing list perfmon2-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/perfmon2-devel
