Re: [patch 0/4] perf_counter tools: support annotation of live kernel modules

Mike Galbraith
2009-07-03T07:36:32Z

On Fri, 2009-07-03 at 09:27 +0200, Ingo Molnar wrote:
> * Mike Galbraith <efault@gmx.de> wrote:
>
> > On Thu, 2009-07-02 at 09:42 +0200, Ingo Molnar wrote:
> >
> > (squirrels suggestions away for later reference)
> >
> > > Btw, another thing: we are thinking about making -F 1000 (1 KHz
> > > auto-freq sampling) the default for perf top and perf record. This
> > > way we'd always gather enough data (and never too much or too little
> > > data), regardless of the intensity of the workload. Have you played
> > > with -F before, what's your general experience about it? It's
> > > particularly useful for 'rare' and highly fluctuating events like
> > > cache-misses.
> > >
> > > Maybe 1KHz is a bit too low - Oprofile defaults to 100000 cycles
> > > interval by default which is about 10 KHz on a 1GHz box and 30
> > > KHz on a 3GHz box. Perhaps 10 KHz is a better default?
> >
> > My default usage is 1000Hz to keep overhead low. Works fine for
> > me.
>
> ah, so you use -F by default?

Yes.

> I still think 10 KHz would be better - especially for really short
> runs like 'perf record -f -g ./git gc'. Since Oprofile samples at 26
> KHz by default, we must not go to a too low frequency, otherwise
> people might get a 'hm, the perf profiles are worse than the
> Oprofile ones' first impression ...
>
> We could perhaps add a freq=1000 switch to .perfconfig - we have
> Git's util/config.c facility - it's just unused right now ;-)
>
> [ Look at the Git sources about how config.c is used. ]

Yeah, that's a good idea.. everyone can roll their own defaults.

For me, it doesn't matter what the default is, I always specify.

-Mike