Re: [PATCH v2] perf record: fix symbol processing bug and greatly improve performance

Ingo Molnar
2013-05-10T08:12:22Z

* 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 (the time after the workload was done
> until the perf command itself exited) 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 (with only the -s -a options used),
> 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
>
> Signed-off-by: Waiman Long <Waiman.Long@hp.com>
> ---
> tools/perf/util/dso.c | 10 ++++++++--
> tools/perf/util/dso.h | 3 ++-
> tools/perf/util/vdso.c | 2 +-
> 3 files changed, 11 insertions(+), 4 deletions(-)

Acked-by: Ingo Molnar <mingo@kernel.org>

Thanks,

Ingo