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CPU profiling using perf

Alessio Podda 2024-12-02 07:40:24 +00:00
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CPU-profiling-using-perf.md Normal file

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To profile `named` using `perf`, the following steps:
1. Install the prerequisites. E.g. on Fedora:
```
sudo dnf install perf inferno
```
`inferno` is a Rust implementation of the flamegraph utility by Brendan Gregg. Since it is written in a native language, it is faster than the original perl script.
If your distribution does not package `inferno`, you can install the original `flamegraph.pl` script. E.g.
```
sudo dnf install perf flamegraph
```
Replace calls to `inferno` with `flamegraph.pl` if necessary.
1. Ensure you have compiled `named` with debug symbols. Optionally, set up a prefix
```sh
cd ~/Path/to/bind9/checkout
./configure CFLAGS='-O2 -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -ggdb3' --prefix=/path/to/parent
make clean && make
```
2. Install `named`. Normally `libtool` runs `named` through a wrapper shell script that can make perf results more difficult to read.
```sh
make install
```
3. Run `named` under `perf`.
```sh
perf record --compression-level=1 --user-callchains -g --call-graph=dwarf,65528 -e cycles:ppp --output=$OUTPUT_FILE -- $YOUR_NAMED_CMDLINE_GOES_HERE
```
Below is an explanation of what each of the command line flags do:
`perf record` Runs a command under a sampling profiler. See `perf-record(1)`
`--compression-level=1` *optional* Compresses the output file using zstd.
`--user-callchains` Do not record the kernel.
`-g --call-graph=dwarf,65528` Use dwarf symbols and a maximum stack size of 64k. If this option is omitted, inlined functions might not be visualized correctly.
`-e cycles:ppp` Record stack traces using precise events (:ppp). This option is needed only if you want to look at the assembly.
`--output=$OUTPUT_FILE` Name of the output file. Otherwise it defaults to perf.data.
4. Generate a flamegraph with `inferno`:
```sh
perf script --input=$INPUT_DATA | inferno-collapse-perf | inferno-flamegraph --width=1920 > $OUTPUT_SVG
```
You can also instruct `inferno` to output a "reversed" flamegraph. This is useful to see if there is any leaf function that is used through the codebase and can be optimized:
```sh
perf script --input=$INPUT_DATA | inferno-collapse-perf | inferno-flamegraph --reverse --width=1920 > $OUTPUT_SVG
```