Messages in this thread | | | Date | Tue, 23 Jun 2009 09:36:28 +0200 | Subject | Re: I.2 - Grouping | From | stephane eranian <> |
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On Tue, Jun 23, 2009 at 7:16 AM, Paul Mackerras<paulus@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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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