* Jiri Olsa <jo...@redhat.com> wrote:
> On Mon, Nov 18, 2013 at 09:59:45AM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Fri, Nov 15, 2013 at 06:46:09AM +0100, Ingo Molnar escreveu:
> > > btw., here's some 'perf top' call graph performance and profiling
> > > quality feedback, with the latest perf code:
> > >
> > > 'perf top --call-graph fp' now works very well, using just 0.2%
> > > of CPU time on a fast system:
> > >
> > > 4676 mingo 20 0 612m 56m 9948 S 1 0.2 0:00.68 perf
> > >
> > >
> > >
> > > 'perf top --call-graph dwarf' on the other hand is horrendously
> > > slow, using 20% of CPU time on a 4 GHz CPU:
> > >
> > > PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> > >
> > >
> > > 4646 mingo 20 0 658m 81m 12m R 19 0.3 0:18.17 perf
> > >
> > > On another system with a 2.4GHz CPU it's taking up 100% of CPU
> > > time (!):
> > >
> > > PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+
> > > COMMAND
> > >
> > > 8018 mingo 20 0 290320 45220 8520 R 99.5 0.3 0:58.81 perf
> > >
> > >
> > > Profiling 'perf top' shows all sorts of very high dwarf
> > > processing overhead:
> >
> > Yeah, top dwarf callchain has been so far a proof of concept, it
> > exacerbates problems that can be seen on 'report', but since its
> > live, we can see it more clearly.
> >
> > The work on improving callchain processing, (rb_tree'ing, new comm
> > infrastructure) alleviated the problem a bit.
> >
> > Tuning the stack size requested from the kernel and using
> > --max-stack can help when it is really needed, but yes, work on it
> > is *badly* needed.
>
> agreed ;-)
>
> also there's new remote unwind interface recently added into libdw,
> which seems to be faster than libunwind.
>
> I plan on adding this soon.
If the main source of overhead is libunwind (which needs independent
confirmation) then would it make sense to implement dwarf stack unwind
support ourselves?
I think SysProf does that and it appears to be faster - its unwind.c
is only 400 lines long as it only implements the small subset needed
to walk the stack - AFAICS.
Thanks,
Ingo
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