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
