Re: [RFC PATCH 0/7] Share events between metrics

[Ian Rogers]

On Thu, May 7, 2020 at 2:47 PM Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
>
> > > - without this change events within a metric may get scheduled
> > >   together, after they may appear as part of a larger group and be
> > >   multiplexed at different times, lowering accuracy - however, less
> > >   multiplexing may compensate for this.
> >
> > I agree the heuristic in this patch set is naive and would welcome to
> > improve it from your toplev experience. I think this change is
> > progress on TopDownL1 - would you agree?
>
> TopdownL1 in non SMT mode should always fit. Inside a group
> deduping always makes sense.
>
> The problem is SMT mode where it doesn't fit. toplev tries
> to group each node and each level together.

Thanks Andi, I've provided some examples of TopDownL3_SMT in the cover
letter of the v3 patch set:
https://lore.kernel.org/lkml/20200508053629.210324-1-irogers@xxxxxxxxxx/
I tested sandybridge and cascadelake and the results look similar to
the non-SMT version. Let me know if there's a different variant to
test.

> >
> > I'm wondering if what is needed are flags to control behavior. For
> > example, avoiding the use of groups altogether. For TopDownL1 I see.
>
> Yes the current situation isn't great.
>
> For Topdown your patch clearly is an improvement, I'm not sure
> it's for everything though.
>
> Probably the advanced heuristics are only useful for a few
> formulas, most are very simple. So maybe it's ok. I guess
> would need some testing over the existing formulas.

Agreed, do you have a pointer on a metric group where things would
obviously be worse? I started off with a cache miss and hit rate
metric and similar to topdown this approach is a benefit.

In v3 I've added a --metric-no-merge option to retain existing
grouping behavior, I've also added a --metric-no-group that avoids
groups for all metrics. This may be useful if the NMI watchdog can't
be disabled.

Thanks for the input!
Ian

> -Andi