On Tue, Jun 23, 2009 at 7:16 AM, Paul Mackerras<pau...@samba.org> wrote: > 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. > Okay.
What happens if I do: fd0 = perf_counter_open(&hwc1, getpid(), -1, -1, 0); fd1 = perf_counter_open(&hwc2, getpid(), -1, fd0, 0); And then: fd2 = perf_counter_open(&hwc2, getpid(), -1, fd1, 0); >> > 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. > I agree completely. ------------------------------------------------------------------------------ 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
