Re: Why the stack frame in perf.data isn't displayed in FlameGraph?

[Milian Wolff <milian.wolff@kdab.com>]

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On Donnerstag, 18. Mai 2017 06:09:55 CEST Nan Xiao wrote:
> Hi all,
> 
> I am a newbie of using perf tools and FlameGraph. I write a simple
> test program which is rafacimentoed from this
> NTL(http://www.shoup.net/ntl/doc/tour-ex3.html) program:
> 
> #include <NTL/ZZX.h>
> 
> using namespace std;
> using namespace NTL;
> 
> void inner(int i, ZZX& t, Vec<ZZX>& phi)
> {
>         for (long j = 1; j <= i-1; j++)
>          if (i % j == 0)
>             t *= phi(j);
> }
> 
> void outer(int i, Vec<ZZX>& phi)
> {
>         ZZX t;
>         t = 1;
>         inner(i, t, phi);
>         phi(i) = (ZZX(INIT_MONO, i) - 1)/t;
>         cout << phi(i) << "\n";
> }
> 
> int main()
> {
>    Vec<ZZX> phi(INIT_SIZE, 100);
> 
>    for (long i = 1; i <= phi.length(); i++) {
>       outer(i, phi);
>    }
> }
> 
> And compile it using following command:
> 
>     g++ -pthread test.cpp -lntl -lgmp

Frame pointers are missing, you probably want to use either

a) `g++ -fno-omit-frame-pointers` which may work if NTL is header-only
b) `g++ -g` to rely on dwarf debug information and then use `perf record --
call-graph dwarf`

In general, I suggest to always compile with `-O2 -g` when you want to 
profile, otherwise you are missing a lot of compiler optimizations, and/or 
cripple tools like perf that want to use debug information.

> I use "perf record -g ./a.out" to profile the program, but "perf
> report" can only show "main" and "outer", no "inner" function
> (https://github.com/NanXiao/images/blob/master/perf/perf.data):
> 
> ......
> +    7.10%     0.00%  a.out    a.out                [.] outer
> +    7.10%     0.00%  a.out    a.out                [.] main
> ......

I bet that's because the "inner" function got inlined and frame pointers are 
disabled (see above). Recompile with the command above, and then use `perf 
record --call-graph dwarf`.

Also, to see inline frames, try to build perf from git (acme's perf/core) and 
use `perf report --inline`.

> From the
> perf.svg(https://github.com/NanXiao/images/blob/master/perf/perf.svg),
> there is neither "outer" nor "inner" stack frames.
> 
> Since there is "outer" in perf.data, why it can't display in
> FlameGraph? Another doubt is why perf.data doesn't contain "inner"
> stack frame?

If the above fixes it for report, try to apply "[PATCH] perf script: Add --
inline option" by Namhyung Kim. Then you can generate the flamegraph via

perf script --inline | stackcollapse-perf.pl | flamegraph.pl > out.svg

Which should also show the inline frames.

Cheers, hope that helps

-- 
Milian Wolff | milian.wolff@kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts

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