Messages in this thread | | | Date | Fri, 26 Oct 2018 17:43:35 -0300 | From | Arnaldo Carvalho de Melo <> | Subject | Re: A concern about overflow ring buffer mode |
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Em Fri, Oct 26, 2018 at 04:11:51PM -0400, Liang, Kan escreveu: > > > On 10/26/2018 3:24 PM, Arnaldo Carvalho de Melo wrote: > > 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. > > <SMIP> > > > > 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. > > > > Ok. > > > > 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. > > It is mainly for performance reason to switch to overwrite mode. The impact > was very small when I did my test. But now the effect is easily noticeable > in other tests. Yes, I agree. We may change it back to non-overwrite mode > until the issue is addressed.
ok
> > 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.
> Could you please add this in the description of --overwrite? > It looks like the --overwrite is not default anymore. > +--overwrite:: > + This is the default, but for investigating problems with it or any other > strange > + behaviour like lots of unknown samples, we may want to disable this mode > by using > + --no-overwrite.
Ok, when I make that change, then I'll change the documentation for the option.
> > 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 :-) > > > > Supporting both overwrite and non-overwrite mode?
Not on the same ring buffer, two ring buffers, one overwrite, the other non-overwrite, get events from both and order, then consume, like perf_session does now when processing perf.data files.
> I think that needs some changes in kernel. May need to split the ring buffer > for different mode. I think it should be very complex. > But I don't have a better solution for now. :)
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