Hello,
is it somehow possible to use perf based on some kernel timer? I'd like to get
an overview of where a userspace application is spending time, both on-CPU as
well as waiting off-CPU. E.g. something similar to using GDB as a poor-mans
profiler and regularly interrupting the process and investigating the
callgraphs. This is quite efficient for a high-level overview when you want to
figure out where time is spent, unrelated to how it was actually spent (cpu,
thread locks, io wait, ...).
E.g. what event would I use for a simple application like this:
~~~~~~~~~~~~~~
#include <unistd.h>
int main()
{
sleep(10);
return 0;
}
~~~~~~~~~~~~~~
Which perf event would show me that most of the time is spent sleeping? I
tried something like this to no avail:
$ perf record --call-graph dwarf -e cpu-clock -F 100 ./a.out
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.007 MB perf.data (~304 samples) ]
perf report --stdio
Error:
The perf.data file has no samples!
# To display the perf.data header info, please use --header/--header-only
options.
I read https://perf.wiki.kernel.org/index.php/Tutorial#Profiling_sleep_times
and tried it out. The result is odd, as I get the "same" backtrace multiple
times, all with 100% cost:
~~~~~~~~~~~~~~~~~~~~~~~~~~
100.00% 0.00% 0 a.out libc-2.19.so [.]
__GI___libc_nanosleep
|
--- __GI___libc_nanosleep
100.00% 0.00% 0 a.out [kernel.kallsyms] [k]
system_call_fastpath
|
--- system_call_fastpath
__GI___libc_nanosleep
100.00% 0.00% 0 a.out [kernel.kallsyms] [k]
sys_nanosleep
|
--- sys_nanosleep
system_call_fastpath
__GI___libc_nanosleep
100.00% 0.00% 0 a.out [kernel.kallsyms] [k]
hrtimer_nanosleep
|
--- hrtimer_nanosleep
sys_nanosleep
system_call_fastpath
__GI___libc_nanosleep
~~~~~~~~~~~~~~~~~~~~~~~~~~
And generally, this would *only* profile sleep time and would ignore the on-
CPU time (and maybe thread waits) and so forth.
Is there a technical reason on why it is not possible to use a plain timer as
a sampling event? If I'm not mistaken, then Intel VTune actually uses a
similar technique for its simple profiling modes which can already give
extremely useful data - both to find CPU hotspots as well as locks&waits.
Bye
--
Milian Wolff
[email protected]
http://milianw.de
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