Em Thu, Sep 18, 2014 at 02:32:10PM +0200, Milian Wolff escreveu:
> Hello,
>
> is it somehow possible to use perf based on some kernel timer? I'd like to
> get
Try with tracepoints or with probe points combined with callchains
instead of using a hardware counter.
- Arnaldo
> 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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