Merge branch 'understanding-unicorn' into 'master'

doc/operations/unicorn.md

+# Understanding Unicorn and unicorn-worker-killer
+
+## Unicorn
+
+GitLab uses [Unicorn](http://unicorn.bogomips.org/), a pre-forking Ruby web
+server, to handle web requests (web browsers and Git HTTP clients). Unicorn is
+a daemon written in Ruby and C that can load and run a Ruby on Rails
+application; in our case the Rails application is GitLab Community Edition or
+GitLab Enterprise Edition.
+
+Unicorn has a multi-process architecture to make better use of available CPU
+cores (processes can run on different cores) and to have stronger fault
+tolerance (most failures stay isolated in only one process and cannot take down
+GitLab entirely). On startup, the Unicorn 'master' process loads a clean Ruby
+environment with the GitLab application code, and then spawns 'workers' which
+inherit this clean initial environment. The 'master' never handles any
+requests, that is left to the workers. The operating system network stack
+queues incoming requests and distributes them among the workers.
+
+In a perfect world, the master would spawn its pool of workers once, and then
+the workers handle incoming web requests one after another until the end of
+time. In reality, worker processes can crash or time out: if the master notices
+that a worker takes too long to handle a request it will terminate the worker
+process with SIGKILL ('kill -9'). No matter how the worker process ended, the
+master process will replace it with a new 'clean' process again. Unicorn is
+designed to be able to replace 'crashed' workers without dropping user
+requests.
+
+This is what a Unicorn worker timeout looks like in `unicorn_stderr.log`. The
+master process has PID 56227 below.
+
+```
+[2015-06-05T10:58:08.660325 #56227] ERROR -- : worker=10 PID:53009 timeout (61s > 60s), killing
+[2015-06-05T10:58:08.699360 #56227] ERROR -- : reaped #<Process::Status: pid 53009 SIGKILL (signal 9)> worker=10
+[2015-06-05T10:58:08.708141 #62538]  INFO -- : worker=10 spawned pid=62538
+[2015-06-05T10:58:08.708824 #62538]  INFO -- : worker=10 ready
+```
+
+### Tunables
+
+The main tunables for Unicorn are the number of worker processes and the
+request timeout after which the Unicorn master terminates a worker process.
+See the [omnibus-gitlab Unicorn settings
+documentation](https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/doc/settings/unicorn.md)
+if you want to adjust these settings.
+
+## unicorn-worker-killer
+
+GitLab has memory leaks. These memory leaks manifest themselves in long-running
+processes, such as Unicorn workers. (The Unicorn master process is not known to
+leak memory, probably because it does not handle user requests.)
+
+To make these memory leaks manageable, GitLab comes with the
+[unicorn-worker-killer gem](https://github.com/kzk/unicorn-worker-killer). This
+gem [monkey-patches](http://en.wikipedia.org/wiki/Monkey_patch) the Unicorn
+workers to do a memory self-check after every 16 requests. If the memory of the
+Unicorn worker exceeds a pre-set limit then the worker process exits. The
+Unicorn master then automatically replaces the worker process.
+
+This is a robust way to handle memory leaks: Unicorn is designed to handle
+workers that 'crash' so no user requests will be dropped. The
+unicorn-worker-killer gem is designed to only terminate a worker process _in
+between requests_, so no user requests are affected.
+
+This is what a Unicorn worker memory restart looks like in unicorn_stderr.log.
+You see that worker 4 (PID 125918) is inspecting itself and decides to exit.
+The threshold memory value was 254802235 bytes, about 250MB. With GitLab this
+threshold is a random value between 200 and 250 MB.  The master process (PID
+117565) then reaps the worker process and spawns a new 'worker 4' with PID
+127549.
+
+```
+[2015-06-05T12:07:41.828374 #125918]  WARN -- : #<Unicorn::HttpServer:0x00000002734770>: worker (pid: 125918) exceeds memory limit (256413696 bytes > 254802235 bytes)
+[2015-06-05T12:07:41.828472 #125918]  WARN -- : Unicorn::WorkerKiller send SIGQUIT (pid: 125918) alive: 23 sec (trial 1)
+[2015-06-05T12:07:42.025916 #117565]  INFO -- : reaped #<Process::Status: pid 125918 exit 0> worker=4
+[2015-06-05T12:07:42.034527 #127549]  INFO -- : worker=4 spawned pid=127549
+[2015-06-05T12:07:42.035217 #127549]  INFO -- : worker=4 ready
+```
+
+One other thing that stands out in the log snippet above, taken from
+Gitlab.com, is that 'worker 4' was serving requests for only 23 seconds. This
+is a normal value for our current GitLab.com setup and traffic.
+
+The high frequency of Unicorn memory restarts on some GitLab sites can be a
+source of confusion for administrators. Usually they are a [red
+herring](http://en.wikipedia.org/wiki/Red_herring).