Add docs on tuning Geo for parallelism

doc/gitlab-geo/README.md

@@ -202,6 +202,10 @@ example:
 
 This message shows that Geo detected that a repository update was needed for project 1.
 
+## Tuning Geo
+
+Read the [GitLab Geo tuning](tuning.md) documentation.
+
 ## Troubleshooting
 
 Read the [troubleshooting document](troubleshooting.md).

doc/gitlab-geo/tuning.md

+# Tuning GitLab Geo
+
+## Changing the sync capacity values
+
+In the Geo admin page (`/admin/geo_nodes`), there are several variables that
+can be tuned to improve performance of Geo:
+
+* Repository sync capacity
+* File sync capacity
+
+Increasing these values will increase the number of jobs that are scheduled,
+but this may not lead to a more downloads in parallel unless the number of
+available Sidekiq threads is also increased. For example, if repository sync
+capacity is increased from 25 to 50, you may also want to increase the number
+of Sidekiq threads from 25 to 50. See the [Sidekiq concurrency
+documentation](../administration/operations/extra_sidekiq_processes.html#concurrency)
+for more details.