Re: [PATCH v1] perf top: Merge hybrid common events

[Andi Kleen <ak@linux.intel.com>]

> With frequency scaling, doesn't IPC remain relatively constant with
> frequency when ignoring external factors like memory? The IPC on an
> e-core is expected to be lower than on a p-core.

For most workloads you cannot ignore memory.

Also the e-core IPC is often not lower than p-core, it's 
actually quite competitive in many things. It just has
a much lower frequency ceiling.

> So I was in a presentation today where someone claimed that larger
> cycle counts translated to memory system overhead, but they could be
> unique :-) In my two loop example the lower IPC core's loop would show
> a greater number of cycles than the higher-IPC core's loop. This would
> make it appear higher in `perf top`, leading the user to drill into it
> assuming more time was spent there due to issues with the code. As I'm
> hypothesizing the code is identical then the higher `perf top` is down
> to being scheduled on the e-core and its lower IPC. Having identical
> code where by default in perf one appears worse than the other doesn't
> strike me as desirable as the same behavior wouldn't happen on a
> non-hybrid system.

Yes performance analysis is hard and a lot of people get it wrong.

> 
> > > How can we merge non-legacy events? On ARM there is no PMU with a type
> > > number file with TYPE_HARDWARE.
> >
> > Very few people use top with anything other than cycles, so it's
> > probably
> > not a very urgent problem.  I suppose you could push the problem to
> > the user with some configuration file.
> 
> So I think users (and me) don't like the choose your event thing at
> the beginning of `perf top`. I think the work in the histogram code to

I'm not sure what you're proposing here. You want to make 
merging default to off?


-Andi