Messages in this thread | | | Date | Tue, 7 May 2013 09:01:27 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] perf: fix symbol processing bug and greatly improve performance |
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* Waiman Long <Waiman.Long@hp.com> wrote:
> When "perf record" was used on a large machine with a lot of CPUs, > the perf post-processing time could take a lot of minutes and even > hours depending on how large the resulting perf.data file was. > > While running AIM7 1500-user high_systime workload on a 80-core x86-64 > system with a 3.9 kernel, the workload itself took about 2 minutes > to run and the perf.data file had a size of 1108.746 MB. However, > the post-processing step took more than 10 minutes. > > With a gprof-profiled perf binary, the time spent by perf was as > follows: > > % cumulative self self total > time seconds seconds calls s/call s/call name > 96.90 822.10 822.10 192156 0.00 0.00 dsos__find > 0.81 828.96 6.86 172089958 0.00 0.00 rb_next > 0.41 832.44 3.48 48539289 0.00 0.00 rb_erase > > So 97% (822 seconds) of the time was spent in a single dsos_find() > function. After analyzing the call-graph data below: > > ----------------------------------------------- > 0.00 822.12 192156/192156 map__new [6] > [7] 96.9 0.00 822.12 192156 vdso__dso_findnew [7] > 822.10 0.00 192156/192156 dsos__find [8] > 0.01 0.00 192156/192156 dsos__add [62] > 0.01 0.00 192156/192366 dso__new [61] > 0.00 0.00 1/45282525 memdup [31] > 0.00 0.00 192156/192230 dso__set_long_name [91] > ----------------------------------------------- > 822.10 0.00 192156/192156 vdso__dso_findnew [7] > [8] 96.9 822.10 0.00 192156 dsos__find [8] > ----------------------------------------------- > > It was found that the vdso__dso_findnew() function failed to locate > VDSO__MAP_NAME ("[vdso]") in the dso list and have to insert a new > entry at the end for 192156 times. This problem is due to the fact that > there are 2 types of name in the dso entry - short name and long name. > The initial dso__new() adds "[vdso]" to both the short and long names. > After that, vdso__dso_findnew() modifies the long name to something > like /tmp/perf-vdso.so-NoXkDj. The dsos__find() function only compares > the long name. As a result, the same vdso entry is duplicated many > time in the dso list. This bug increases memory consumption as well > as slows the symbol processing time to a crawl. > > To resolve this problem, the dsos__find() function interface was > modified to enable searching either the long name or the short > name. The vdso__dso_findnew() will now search only the short name > while the other call sites search for the long name as before. > > With this change, the cpu time of perf was reduced from 848.38s to > 15.77s and dsos__find() only accounted for 0.06% of the total time. > > 0.06 15.73 0.01 192151 0.00 0.00 dsos__find
Very nice!
I noticed that you used gprof to instrument perf itself on a call graph level.
Does this method of profiling perf via perf:
perf record -g perf report perf report
... produce similarly useful call-graph instrumentation for you?
If not or not quite then could you describe the differences? We could use that to further improve perf call-graph profiling.
Thanks,
Ingo
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