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

Ingo Molnar
2009-07-03T07:27:38Z

* 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?

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. ]

Ingo