Improve memory performance by reducing dirty pages before fork

The nakayoshi_fork gem works around the lack of a Ruby compacting garbage collecting by calling GC.start a few times to "promote" the age of young objects to the maximum age 3. This reduces the number of dirty pages and consequently improves copy-on-write behavior with unicorn and other forking Web application servers.

See Aaron Patterson's 2017 RubyConf talk on this in https://www.youtube.com/watch?v=8Q7M513vewk&feature=youtu.be&t=960.

See source code: https://github.com/ko1/nakayoshi_fork/blob/master/lib/nakayoshi_fork.rb

https://github.com/discourse/discourse/blob/master/script/memstats.rb is a good script to aggregate PSS instead of RSS per process. I've noticed some encouraging results with these changes. For example, on my test instance PSS is about half of RSS:

$ sudo ruby memstats.rb 7025  
Process:             7025
Command Line:        unicorn worker[1] -D -E production -c /var/opt/gitlab/gitlab-rails/etc/unicorn.rb /opt/gitlab/embedded/service/gitlab-rails/config.ru
Memory Summary:
  private_clean                   0 kB
  private_dirty             120,096 kB
  pss                       219,844 kB
  rss                       426,040 kB
  shared_clean               10,268 kB
  shared_dirty              295,676 kB
  size                      771,432 kB
  swap                            0 kB

shared_dirty tends to decrease as the OS copies more dirty pages to the forked process. Compare this to a unicorn worker on dev, which shows PSS approaching RSS:

$ sudo ruby memstats.rb 7680
Process:             7680
Command Line:        unicorn worker[1] -D -E production -c /var/opt/gitlab/gitlab-rails/etc/unicorn.rb /opt/gitlab/embedded/service/gitlab-rails/config.ru
Memory Summary:
  private_clean                 424 kB
  private_dirty             476,896 kB
  pss                       488,507 kB
  rss                       592,648 kB
  shared_clean               17,592 kB
  shared_dirty               97,736 kB
  size                    1,111,464 kB
  swap                       37,392 kB