Messages in this thread | | | Date | Tue, 22 Mar 2016 08:57:35 +0100 | From | Jiri Olsa <> | Subject | Re: [PATCH 1/2] perf, tools: Document event specifications better |
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On Mon, Mar 21, 2016 at 08:56:32AM -0700, Andi Kleen wrote:
SNIP
> +EVENT GROUPS > +------------ > + > +Perf supports time based multiplexing of events, when the number of events > +active exceeds the number of hardware performance counters. Multiplexing > +can cause measurement errors when the workload changes its execution > +profile. > + > +When metrics are computed using formulas from event counts, it is useful to > +ensure some events are always measured together as a group to minimize multiplexing > +errors. Event groups can be specified using { }. > + > + perf stat -e '{instructions,cycles}' ... > + > +When too many events are specified in the group perf the event will not ^^^^^^^^^^^^^^ maybe: 'none of them' will be meassured.
> +be measured. The number of available performance counters depend on the CPU. > +For example Intel Core CPUs have typically four generic performance counters > +for the core, plus a number of specialized fixed counters. > + > +Events from multiple different PMUs cannot be mixed in a group.
hum, but we allow that right?
only when there's mixture of SW and HW events we move it all silently under HW event context
jirka
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