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SubjectRe: I.2 - Grouping
From
Corey,

On Tue, Jun 23, 2009 at 12:04 AM, Corey
Ashford<cjashfor@linux.vnet.ibm.com> wrote:
> stephane eranian wrote:
>>
>> On Mon, Jun 22, 2009 at 1:50 PM, Ingo Molnar<mingo@elte.hu> wrote:
>>>>
>>>> 2/ Grouping
>>>>
>>>> By design, an event can only be part of one group at a time.
>>
>> As I read this again, another question came up. Is the statement
>> above also true for the group leader?
>>
>>
>>>> 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.
>>>
>> But then, does that mean that users need to be aware of constraints
>> to form groups. Group are formed by users. I thought, the whole point
>> of the API was to hide that kind of hardware complexity.
>>
>> Groups are needed when sampling, e.g., PERF_SAMPLE_GROUP.
>> For instance, I sample the IP and the values of the other events in
>> my group.  Grouping looks like the only way to sampling on Itanium
>> branch buffers (BTB), for instance. You program an event in a generic
>> counter which counts the number of entries recorded in the buffer.
>> Thus, to sample the BTB, you sample on this event, when it overflows
>> you grab the content of the BTB. Here, the event and the BTB are tied
>> together. You cannot count the event in one group, and have the BTB
>> in another one (BTB = 1 config reg + 32 data registers + 1 position reg).
>>
>
> Stephane, if you were to just place into groups those events which must be
> correlated, and just leave all of the others not grouped, wouldn't this
> solve the problem?  The kernel would be free to schedule the other events
> how and when it can, but would guarantee that your correlated events are on
> the PMU simultaneously.
>
It would work.
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