add notes re. profiling

README.md

@@ -378,6 +378,30 @@ These are desired for the next DB format:
 - Compress strings with fsst.
 - Trim first 4 bytes from the cmph headers.
 
+Profiling
+---------
+
+Prepare `profile.data`:
+
+```
+zig build -Drelease-small=true
+perf record --call-graph=dwarf \
+    zig-out/bin/turbo-unix2db --passwd passwd2 --group group2
+```
+
+Perf interactive:
+
+```
+perf report -i perf.data
+```
+
+Flame graph:
+
+```
+perf script | inferno-collapse-perf > stacks.folded
+inferno-flamegraph stacks.folded > profile.svg
+```
+
 [git-subtrac]: https://apenwarr.ca/log/20191109
 [cmph]: http://cmph.sourceforge.net/
 [id]: https://linux.die.net/man/1/id