Re: [PATCH] perf: fix symbol processing bug and greatly improveperformance

[Ingo Molnar]

* Waiman Long <Waiman.Long@xxxxxx> wrote:

> On 05/07/2013 03:01 AM, Ingo Molnar wrote:
> >* Waiman Long<Waiman.Long@xxxxxx>  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.
> 
> Thank for the comment.
> 
> The slowdown that I was trying to fix was in the "perf record" part of 
> the profiling process, not the "perf report" part. I didn't try 
> perf-record on perf-record as the performance counters are limited 
> resources and I don't want resource conflicts to affect the results.

ah, ok. In general two instances of cycles-profiling should work just fine 
without any resource conflicts.

But yeah, with "perf record" that's a valid worry and I can see how you 
wanted to not worry about that.

Thanks,

	Ingo
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