Merge branch 'docs-document-memory-heap-profiling' into 'master'

Document profiling Ruby heap for memory fragmentation

See merge request gitlab-org/gitlab!16947

doc/development/performance.md

@@ -251,6 +251,36 @@ These results can also be placed into a PostgreSQL database by setting the
 `RSPEC_PROFILING_POSTGRES_URL` variable. This is used to profile the test suite
 when running in the CI environment.
 
+## Memory profiling
+
+One of the reasons of the increased memory footprint could be Ruby memory fragmentation.
+
+To diagnose it, you can visualize Ruby heap as described in [this post by Aaron Patterson](https://tenderlovemaking.com/2017/09/27/visualizing-your-ruby-heap.html).
+
+To start, you want to dump the heap of the process you are investigating to a JSON file.  
+
+You need to run the command inside the process you are exploring, you may do that with `rbtrace`.  
+`rbtrace` is already present in GitLab `Gemfile`, you just need to require it.  
+It could be achieved running webserver or Sidekiq with the environment variable set to `ENABLE_RBTRACE=1`.
+
+To get the heap dump:
+
+```ruby
+bundle exec rbtrace -p <PID> -e 'File.open("heap.json", "wb") { |t| ObjectSpace.dump_all(output: t) }'
+```
+
+Having the JSON, you finally could render a picture using the script [provided by Aaron](https://gist.github.com/tenderlove/f28373d56fdd03d8b514af7191611b88) or similar:
+
+```sh
+ruby heapviz.rb heap.json
+```
+  
+Fragmented Ruby heap snapshot could look like this:
+
+![Ruby heap fragmentation](img/memory_ruby_heap_fragmentation.png)
+
+Memory fragmentation could be reduced by tuning GC parameters as described in [this post by Nate Berkopec](https://www.speedshop.co/2017/12/04/malloc-doubles-ruby-memory.html), which should be considered as a tradeoff, as it may affect overall performance of memory allocation and GC cycles.
+
 ## Importance of Changes
 
 When working on performance improvements, it's important to always ask yourself