Re: perf event grouping for dummies (was Re: [PATCH] arc: perf: Enable generic "cache-references" and "cache-misses" events)

From: Paul Clarke
Date: Wed Sep 21 2016 - 20:43:45 EST

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.

Paul Clarke, IBM