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

From: Paul Clarke
Date: Thu Sep 22 2016 - 14:23:20 EST

On 09/22/2016 12:50 PM, Vineet Gupta wrote:
On 09/22/2016 12:56 AM, Peter Zijlstra wrote:
On Wed, Sep 21, 2016 at 07:43:28PM -0500, Paul Clarke wrote:
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.)

Right, sorry for the poor wording.

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.


Each events get their own hardware counter. Grouped events are
co-scheduled on the hardware.

And if we don't group them, then they _may_ not be co-scheduled (active/counting
at the same time) ? But how can this be possible.
Say we have 2 counters, both the cmds below

perf -e cycles,instructions hackbench
perf -e {cycles,instructions} hackbench

would assign 2 counters to the 2 conditions which keep counting until perf asks
them to stop (because the profiled application ended)

I don't understand the "scheduling" of counter - once we set them to count, there
is no real intervention/scheduling form software in terms of disabling/enabling
(assuming no multiplexing etc)

If you assume no multiplexing, then this discussion on grouping is moot.

It depends on how many events you specify, how many counters there are, and which counters can count which events. If you specify a set of events for which every event can be counted simultaneously, they will be scheduled simultaneously and continuously. If you specify more events than counters, there's multiplexing. AND, if you specify a set of events, some of which cannot be counted simultaneously due to hardware limitations, they'll be multiplexed.