Docs: Profiling

Author: glebm

docs/dev-profiling.md

@@ -0,0 +1,73 @@
+# Profiling LibSass
+
+## Linux perf and pprof
+
+On Linux, you can record the profile with `perf` and inspect it with `pprof`.
+
+### Install required tools
+
+Pre-requisites:
+
+1. Linux `perf`, commonly found in the `linux-tools-generic` package.
+2. [go], for installing `pprof`.
+3. [bazel], for installing `perf_to_profile`.
+
+[go]: https://golang.org
+[bazel]: https://bazel.build
+
+First, install `pprof` with:
+
+```bash
+go get -u github.com/google/pprof
+```
+
+Then, build and install `perf_to_profile`:
+
+```bash
+git clone https://github.com/google/perf_data_converter
+cd perf_data_converter
+bazel build -c opt src:perf_to_profile
+sudo cp bazel-bin/src/perf_to_profile /usr/local/bin/
+```
+
+Finally, in your libsass repository, clone and build `sassc`:
+
+```bash
+git clone https://github.com/sass/sassc.git
+make sassc
+```
+
+### Record perf data
+
+```bash
+sudo perf record sassc/bin/sassc input.scss > /dev/null && sudo chown $USER:$USER perf.data
+```
+
+This will create a `perf.data` file that you can vizualize with `pprof`.
+
+### Inspect perf data
+
+A web server with various visualization options:
+
+```bash
+pprof -http=localhost:3232 sassc/bin/sassc perf.data
+```
+
+Simple text output:
+
+```bash
+pprof -text sassc/bin/sassc perf.data
+```
+
+Example output:
+
+```
+      flat  flat%   sum%        cum   cum%
+  24651348  6.97%  6.97%   24651348  6.97%  [[kernel.kallsyms]]
+  20746241  5.87% 12.84%   20746241  5.87%  Sass::SharedPtr::decRefCount
+  18401663  5.20% 18.04%   20420896  5.78%  __libc_malloc
+  15205959  4.30% 22.34%   15205959  4.30%  [libc-2.27.so]
+  12974307  3.67% 26.01%   14070189  3.98%  _int_malloc
+  10958857  3.10% 29.11%   10958857  3.10%  Sass::SharedPtr::incRefCount
+   9837672  2.78% 31.89%   18433250  5.21%  cfree
+```

docs/setup-environment.md

@@ -66,3 +66,6 @@ bundle install
 
 Voila! Now you are testing against Sass too!
 
+## Profiling
+
+Is libsass being slow? See the [Profiling guide](dev-profiling.md).