Re: A concern about overflow ring buffer mode

From: Arnaldo Carvalho de Melo
Date: Fri Oct 26 2018 - 15:24:31 EST

Em Fri, Oct 26, 2018 at 03:16:29PM -0400, Liang, Kan escreveu:
> On 10/26/2018 3:12 PM, Arnaldo Carvalho de Melo wrote:
> > Em Fri, Oct 26, 2018 at 03:07:40PM -0400, Liang, Kan escreveu:
> > > On 10/26/2018 3:02 PM, Arnaldo Carvalho de Melo wrote:
> > > > So, I'm adding the following to my tree to help in diagnosing problems
> > > > with this overwrite mode:
> > > Actually, you can use per-event overwrite term to disable overwrite mode for
> > > perf top.
> > I see, it will disable that opts->overwrite if it finds the no-overwrite
> > in the per-event definition, so the equivalent of the option I added
> > below:

> > perf top --no-overwrite

> > is:

> > perf top -e cycles/no-overwrite/

> > I checked and both have the same result. But I still think there is
> > value in having the shorter form, ok?

> Sure.


I think that we should default back to --no-overwrite till we get this
sorted out, as the effect is easily noticeable, as David reported and I
reproduced, when doing kernel builds.

On systems such as Knights Landing/Mill one can use --overwrite, knowing
about this current map resolving limitation, i.e. for workloads where
there are not that many short lived threads or mmap'ing, that could be
possibly tolerable.

Fixing this properly will probably involve using the ordered_events code
and two evlist, one for the PERF_RECORD_!SAMPLE in non-overwrite mode
and the other for PERF_RECORD_SAMPLE in overwrite mode, else someone
comes up with some better solution :-)


- Arnaldo