On Mon, Nov 18, 2013 at 03:26:53PM +0100, Ingo Molnar wrote: > > * 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.
I think it's an option.. but it'll simpler to try the libdw interface first and see if it's good/fast enough.. also I recall discussing the speed with libdw developer Jan Kratochvil (CC-ed) and AFAICS they're open for suggestions/optimizations jirka -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
