linux-kernel - Re: I.2

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Date:	Tue, 23 Jun 2009 09:36:28 +0200
From:	stephane eranian <eranian@...glemail.com>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Robert Richter <robert.richter@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andi Kleen <andi@...stfloor.org>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Carl Love <cel@...ibm.com>,
	Corey J Ashford <cjashfor@...ibm.com>,
	Philip Mucci <mucci@...s.utk.edu>,
	Dan Terpstra <terpstra@...s.utk.edu>,
	perfmon2-devel <perfmon2-devel@...ts.sourceforge.net>
Subject: Re: I.2 - Grouping

On Tue, Jun 23, 2009 at 7:16 AM, Paul Mackerras<paulus@...ba.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.
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