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

Waiman Long
2013-07-05T17:09:30Z

On 05/10/2013 04:12 AM, Ingo Molnar wrote:
> * 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

Thank for the Ack. Will that patch go into v3.11?

Regards,
Longman