On 09/20/2016 03:56 PM, Vineet Gupta wrote:
On 09/01/2016 01:33 AM, Peter Zijlstra wrote:
- is that what perf event grouping is ?


Again, nope. Perf event groups are single counter (so no implicit
addition) that are co-scheduled on the PMU.

I'm not sure I understand - does this require specific PMU/arch support - as in
multiple conditions feeding to same counter.

My read is that is that what Peter meant was that each event in the perf event 
group is a single counter, so all the events in the group are counted 
simultaneously.  (No multiplexing.)

You can do it like:

        perf stat -e '{cycles,instructions}'

Which will place the cycles event and the instructions event in a group
and thereby guarantee they're co-scheduled.

Again when you say co-scheduled what do you mean - why would anyone use the 
event
grouping - is it when they only have 1 counter and they want to count 2
conditions/events at the same time - isn't this same as event multiplexing ?

I'd say it's the converse of multiplexing.  Instead of mapping multiple events 
to a single counter, perf event groups map a set of events each to their own 
counter, and they are active simultaneously.  I suppose it's possible for the 
_groups_ to be multiplexed with other events or groups, but the group as a 
whole will be scheduled together, as a group.

If you have a single counter, I don't believe you can support perf event 
groups, by definition.

Regards,
Paul Clarke, IBM


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