Add documentation for the Sidekiq MemoryKiller

doc/operations/README.md

+# GitLab operations
+
+- [Sidekiq MemoryKiller](sidekiq_memory_killer.md)

doc/operations/sidekiq_memory_killer.md

+# Sidekiq MemoryKiller
+
+The GitLab Rails application code suffers from memory leaks. For web requests
+this problem is made manageable using
+[unicorn-worker-killer](https://github.com/kzk/unicorn-worker-killer) which
+restarts Unicorn worker processes in between requests when needed. The Sidekiq
+MemoryKiller applies the same approach to the Sidekiq processes used by GitLab
+to process background jobs.
+
+Unlike unicorn-worker-killer, which is enabled by default for all GitLab
+installations since GitLab 6.4, the Sidekiq MemoryKiller is enabled by default
+_only_ for Omnibus packages. The reason for this is that the MemoryKiller
+relies on Runit to restart Sidekiq after a memory-induced shutdown and GitLab
+installations from source do not all use Runit or an equivalent.
+
+With the default settings, the MemoryKiller will cause a Sidekiq restart no
+more often than once every 15 minutes, with the restart causing about one
+minute of delay for incoming background jobs.
+
+## Configuring the MemoryKiller
+
+The MemoryKiller is controlled using environment variables.
+
+- `SIDEKIQ_MEMORY_KILLER_MAX_RSS`: if this variable is set, and its value is
+  greater than 0, then after each Sidekiq job, the MemoryKiller will check the
+  RSS of the Sidekiq process that executed the job. If the RSS of the Sidekiq
+  process (expressed in kilobytes) exceeds SIDEKIQ_MEMORY_KILLER_MAX_RSS, a
+  delayed shutdown is triggered. The default value for Omnibus packages is set
+  [in the omnibus-gitlab
+  repository](https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/files/gitlab-cookbooks/gitlab/attributes/default.rb).
+- `SIDEKIQ_MEMORY_KILLER_GRACE_TIME`: defaults 900 seconds (15 minutes). When
+  a shutdown is triggered, the Sidekiq process will keep working normally for
+  another 15 minutes.
+- `SIDEKIQ_MEMORY_KILLER_SHUTDOWN_WAIT`: defaults to 30 seconds. When the grace
+  time has expired, the MemoryKiller tells Sidekiq to stop accepting new jobs.
+  Existing jobs get 30 seconds to finish. After that, the MemoryKiller tells
+  Sidekiq to shut down, and an external supervision mechanism (e.g. Runit) must
+  restart Sidekiq.

doc/operations/sidekiq_restarter.md

+# Sidekiq MemoryKiller
+
+The GitLab Rails application code suffers from memory leaks. For web requests
+this problem is made manageable using
+[unicorn-worker-killer](https://github.com/kzk/unicorn-worker-killer) which
+restarts Unicorn worker processes in between requests when needed. The Sidekiq
+MemoryKiller applies the same approach to the Sidekiq processes used by GitLab
+to process background jobs.
+
+