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5.4.2. Using Perf
Using the basic PCL infrastructure for collecting statistics or samples of program execution is relatively straightforward. This section provides simple examples of overall statistics and sampling.
To collect statistics on
make and its children, use the following command:
# perf stat -- make all
The
perf command collects a number of different hardware and software counters. It then prints the following information:
Performance counter stats for 'make all':
244011.782059 task-clock-msecs # 0.925 CPUs
53328 context-switches # 0.000 M/sec
515 CPU-migrations # 0.000 M/sec
1843121 page-faults # 0.008 M/sec
789702529782 cycles # 3236.330 M/sec
1050912611378 instructions # 1.331 IPC
275538938708 branches # 1129.203 M/sec
2888756216 branch-misses # 1.048 %
4343060367 cache-references # 17.799 M/sec
428257037 cache-misses # 1.755 M/sec
263.779192511 seconds time elapsed
The
perf tool can also record samples. For example, to record data on the make command and its children, use:
# perf record -- make all
This prints out the file in which the samples are stored, along with the number of samples collected:
[ perf record: Woken up 42 times to write data ] [ perf record: Captured and wrote 9.753 MB perf.data (~426109 samples) ]
As of Red Hat Enterprise Linux 6.4, a new functionality to the
{} group syntax has been added that allows the creation of event groups based on the way they are specified on the command line.
The current
--group or -g options remain the same; if it is specified for record, stat, or top command, all the specified events become members of a single group with the first event as a group leader.
The new
{} group syntax allows the creation of a group like:
# perf record -e '{cycles, faults}' ls
The above results in a single event group containing cycles and faults events, with the cycles event as the group leader.
All groups are created with regards to threads and CPUs. As such, recording an event group within two threads on a server with four CPUs will create eight separate groups.
It is possible to use a standard event modifier for a group. This spans over all events in the group and updates each event modifier settings.
# perf record -r '{faults:k,cache-references}:p'
The above command results in the
:kp modifier being used for faults, and the :p modifier being used for the cache-references event.
Performance Counters for Linux (PCL) Tools conflict with OProfile
Both OProfile and Performance Counters for Linux (PCL) use the same hardware Performance Monitoring Unit (PMU). If OProfile is currently running while attempting to use the PCL perf command, an error message like the following occurs when starting OProfile:
Error: open_counter returned with 16 (Device or resource busy). /bin/dmesg may provide additional information. Fatal: Not all events could be opened.
To use the
perf command, first shut down OProfile:
# opcontrol --deinit
You can then analyze
perf.data to determine the relative frequency of samples. The report output includes the command, object, and function for the samples. Use perf report to output an analysis of perf.data. For example, the following command produces a report of the executable that consumes the most time:
# perf report --sort=comm
The resulting output:
# Samples: 1083783860000
#
# Overhead Command
# ........ ...............
#
48.19% xsltproc
44.48% pdfxmltex
6.01% make
0.95% perl
0.17% kernel-doc
0.05% xmllint
0.05% cc1
0.03% cp
0.01% xmlto
0.01% sh
0.01% docproc
0.01% ld
0.01% gcc
0.00% rm
0.00% sed
0.00% git-diff-files
0.00% bash
0.00% git-diff-index
The column on the left shows the relative frequency of the samples. This output shows that
make spends most of this time in xsltproc and the pdfxmltex. To reduce the time for the make to complete, focus on xsltproc and pdfxmltex. To list the functions executed by xsltproc, run:
# perf report -n --comm=xsltproc
This generates:
comm: xsltproc
# Samples: 472520675377
#
# Overhead Samples Shared Object Symbol
# ........ .......... ............................. ......
#
45.54%215179861044 libxml2.so.2.7.6 [.] xmlXPathCmpNodesExt
11.63%54959620202 libxml2.so.2.7.6 [.] xmlXPathNodeSetAdd__internal_alias
8.60%40634845107 libxml2.so.2.7.6 [.] xmlXPathCompOpEval
4.63%21864091080 libxml2.so.2.7.6 [.] xmlXPathReleaseObject
2.73%12919672281 libxml2.so.2.7.6 [.] xmlXPathNodeSetSort__internal_alias
2.60%12271959697 libxml2.so.2.7.6 [.] valuePop
2.41%11379910918 libxml2.so.2.7.6 [.] xmlXPathIsNaN__internal_alias
2.19%10340901937 libxml2.so.2.7.6 [.] valuePush__internal_alias
