Measuring performance with the perf kernel tool
top on Steroids
You can create a performance counter profile in real time with perf top
(Figure 1). The overview is similar to that of top
, but you can jump directly to the individual events.
![](/var/linux_magazin/storage/images/issues/2019/221/perf/figure-1/744537-1-eng-US/Figure-1_large.png)
perf stat
counts the events of the entire system until you abort by pressing Ctrl+C, or the specified command terminates. Listing 3 shows the subcommand in action; it lists two events for the factor
command.
Listing 3
Output from perf stat
01 $ sudo perf stat -e branches -e branch-misses factor 120808125801214124898080833 02 120808125801214124898080833: 13 29 911 7589 21089 77471 28369829 03 04 Performance counter stats for 'factor 120808125801214124898080833': 05 06 191,242 branches 07 10,332 branch-misses # 5.40% of all branches 08 09 0.000649438 seconds time elapsed 10 11 0.000699000 seconds user 12 0.000000000000 seconds sys
Data Recorder
The perf
record
command can generate a performance counter profile for a specific command. If you want an overview of the entire system, or if the process to be examined is unknown, simply add the sleep
command:
sudo perf record -ag sleep 5
This line creates the default perf.data
file in the current directory. The -a
switch ensures that perf
considers all CPUs, and -g
generates a call graph. -i
lets you feed a file to perf record
.
The perf report
command evaluates the stored data and uses the perf.data
file. As with perf top
, the command presents an overview. After pressing Enter, you also see details for the functions.
By the way, the --sort= comm,dso
switch (see Figure 1) provides a better overview. Figure 2 shows the output for an Intel Kaby Lake system with Gnome 3.30.1 playing a movie in Firefox 63.0.3. The figure shows that the CPU and not the GPU is used for decoding.
Event-Driven
If only certain events are of interest, the -e
switch can help. Listing 4 shows the output on a system with Rsync running.
Listing 4
Track Events
01 $ sudo perf report --stdio 02 [...] 03 # Samples: 1K of event 'block:block_rq_issue' 04 # Event count (approx.): 1741 05 # 06 # Children Self Command Shared Object Symbol 07 # ........ .... ........... ................ .................... 08 # 09 93.74% 93.74% rsync kernel.kallsyms] [k] blk_peek_request 10 | 11 |--89.20%-- __lxstat64 12 | blk_peek_request 13 | 14 |--3.45%-- __GI___mkdir 15 | blk_peek_request 16 | 17 |--0.63%-- 0x646c6975622f3930 18 | __GI___link 19 | blk_peek_request 20 | 21 -0.46%-- 0x6d492f636f642f65 22 __GI___link 23 blk_peek_request 24 25 89.20% 0.00% rsync libc-2.27.so [.] __lxstat64 26 | 27 ---__lxstat64 28 blk_peek_request 29 [...]
--stdio
tells perf
to display the overview in plain text directly on the console. The recorded event was a request to the kernel block layer. The command line was:
sudo perf record -e block:block_rq_issue -ag
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