Rugged/Puma config section in Puma doc

doc/administration/operations/puma.md

+# Switching to Puma
+
+## Puma
+
+GitLab plans to use [Puma](https://github.com/puma/puma) to replace
+[Unicorn](https://bogomips.org/unicorn/).
+
+## Why switch to Puma?
+
+Puma has a multi-thread architecture which uses less memory than a multi-process
+application server like Unicorn.
+
+Most Rails applications requests normally include a proportion of I/O wait time.
+During I/O wait time MRI Ruby will release the GVL (Global VM Lock) to other threads.
+Multi-threaded Puma can therefore still serve more requests than a single process.
+
+## Performance caveat when using Puma with Rugged
+
+For deployments where NFS is used to store Git repository, we allow GitLab to use
+[Direct Git Access](../gitaly/#direct-git-access-in-gitlab-rails) to improve performance via usage of [Rugged](https://github.com/libgit2/rugged).
+
+Rugged usage is automatically enabled if Direct Git Access is present, unless it
+is disabled by [feature flags](../../development/gitaly.md#legacy-rugged-code).
+
+MRI Ruby uses a GVL. This allows MRI Ruby to be multi-threaded, but running at
+most on a single core. Since Rugged can use a thread for long periods of
+time (due to intensive I/O operations of Git access), this can starve other threads
+that might be processing requests. This is not a case for Unicorn or Puma running
+in a single thread mode, as concurrently at most one request is being processed.
+
+We are actively working on removing Rugged usage. Even though performance without Rugged
+is acceptable today, in some cases it might be still beneficial to run with it.
+
+Given the caveat of running Rugged with multi-threaded Puma, and acceptable
+performance of Gitaly, we are disabling Rugged usage if Puma multi-threaded is
+used (when Puma is configured to run with more than one thread).
+
+This default behavior may not be the optimal configuration in some situations. If Rugged
+plays an important role in your deployment, we suggest you benchmark to find the
+optimal configuration:
+
+- The safest option is to start with single-threaded Puma. When working with
+Rugged, single-threaded Puma does work the same as Unicorn.
+
+- To force Rugged auto detect with multi-threaded Puma, you can use [feature
+flags](../../development/gitaly.md#legacy-rugged-code).