On 05/07/2013 05:30 AM, Jiri Olsa wrote:
On Mon, May 06, 2013 at 09:43:53AM -0400, Waiman Long 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.
hi,
the issue is there and fix looks ok, thanks!
though I'm not able to get vdso callchains to pop out
even by investigating report with vdso heavy workload.
I'll have a closer look..
The test machine that I used have RHEL 6.4 installed in it with a
upstream 3.9 kernel layered on top. The kernel config is based on the
6.4 configuration file with modification to enable the X2APIC option
needed by the machine. Other than that, I didn't make too much
modification to the base configuration. I used the "-a -s" option when
running perf-record.
I don't think the vdso callchains were major part of the workload that I
tested. I think it is the high number of CPU cores plus the high number
of users (1500) that cause the performance bottleneck to surface. In a
smaller machine, those bottlenecks may be much less noticeable. The vdso
call-chain dominates the post-processsing time because of the need to
search through the while DSO list for the vdso library which can grow to
2M+ in my test case.
Regards,
Longman
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