How to use linux perf tool for code comprehension

Question

I'm fascinated by the ability of 'perf' to record call graphs and am trying to understand how to use it to understand a new code base.

I compiled the code in debug mode, and ran unit tests using the following command:

perf record --call-graph dwarf make test

This creates a 230 meg perf.data. I then write out the call graph

perf report --call-graph --stdio > callgraph.txt

This creates a 50 meg file.

Ideally, I would only like to see code belonging to the project, not kernel code, system calls, c++ standard libraries, even boost and whatever other third party software. Currently I see items like __GI___dl_iterate_phdr, _Unwind_Find_FDE, etc.

I love the flamegraph project. However, that visualization isn't good for code comprehension. Are there any other projects, write-ups, ideas, which might be helpful?

Answer 1

perf report -g for huge application should not be dumped to external file as too verbose. Collected perf.data (with -g) will work without file redirection with interactive perf report TUI interface. You may disable callgraph reporting to find functions took most time with perf record without -g or perf report --no-children.

There is gprof2dot script (https://github.com/jrfonseca/gprof2dot) to visualize lagre perf report call-graphs as compact picture (graph).

There is also Brendan D. Gregg's interactive FlameGraphs in svg/js; and he often notes in presentations that perf report -g output shows many megabyte raw dumps of report as lot of A4 pages. There is usage instruction for the perf: http://www.brendangregg.com/FlameGraphs/cpuflamegraphs.html#perf:

# git clone https://github.com/brendangregg/FlameGraph  # or download it from github
# cd FlameGraph
# perf record -F 99 -g -- ../command
# perf script | ./stackcollapse-perf.pl > out.perf-folded
# ./flamegraph.pl out.perf-folded > perf-kernel.svg

PS: Why you are profiling make process? Try to select some test and profile only them. Use lower profile frequency to get smaller perf.data file. Also disable kernel-mode samples with :u suffix of default event "cycles": perf record -F 99 -g -e cycles:u -- ../command