Document how polling interval is used

app/views/admin/application_settings/_form.html.haml

@@ -571,6 +571,7 @@
           The multiplier can also have a decimal value.
           The default value (1) is a reasonable choice for the majority of GitLab
           installations. Set to 0 to completely disable polling.
+          = link_to icon('question-circle'), help_page_path('administration/polling')
   .form-actions
     = f.submit 'Save', class: 'btn btn-save'

doc/README.md

@@ -62,6 +62,7 @@ All technical content published by GitLab lives in the documentation, including:
 - [Migrate GitLab CI to CE/EE](migrate_ci_to_ce/README.md) Follow this guide to migrate your existing GitLab CI data to GitLab CE/EE.
 - [Monitoring uptime](user/admin_area/monitoring/health_check.md) Check the server status using the health check endpoint.
 - [Operations](administration/operations.md) Keeping GitLab up and running.
+- [Polling](administration/polling.md) Configure how often the GitLab UI polls for updates
 - [Raketasks](raketasks/README.md) Backups, maintenance, automatic webhook setup and the importing of projects.
 - [Reply by email](administration/reply_by_email.md) Allow users to comment on issues and merge requests by replying to notification emails.
 - [Repository checks](administration/repository_checks.md) Periodic Git repository checks.

doc/administration/polling.md

+# Polling configuration
+
+The GitLab UI polls for updates for different resources (issue notes, issue
+titles, pipeline statuses, etc.) on a schedule appropriate to the resource.
+
+In "Application settings -> Real-time features" you can configure "Polling
+interval multiplier". This multiplier is applied to all resources at once,
+and decimal values are supported. For the sake of the examples below, we will
+say that issue notes poll every 2 seconds, and issue titles poll every 5
+seconds; these are _not_ the actual values.
+
+- 1 is the default, and recommended for most installations. (Issue notes poll
+every 2 seconds, and issue titles poll every 5 seconds.)
+- 0 will disable UI polling completely. (On the next poll, clients will stop
+polling for updates.)
+- A value greater than 1 will slow polling down. If you see issues with
+database load from lots of clients polling for updates, increasing the
+multiplier from 1 can be a good compromise, rather than disabling polling
+completely. (For example: If this is set to 2, then issue notes poll every 4
+seconds, and issue titles poll every 10 seconds.)
+- A value between 0 and 1 will make the UI poll more frequently (so updates
+will show in other sessions faster), but is **not recommended**. 1 should be
+fast enough. (For example, if this is set to 0.5, then issue notes poll every
+1 second, and issue titles poll every 2.5 seconds.)

doc/development/fe_guide/performance.md

@@ -12,8 +12,8 @@ Thus, we must strike a balance between sending requests and the feeling of realt
 Use the following rules when creating realtime solutions.
 1. The server will tell you how much to poll by sending `Poll-Interval` in the header.
-Use that as your polling interval. This way it is easy for system administrators to change the
-polling rate.
+Use that as your polling interval. This way it is [easy for system administrators to change the
+polling rate](../../administration/polling.md).
 A `Poll-Interval: -1` means you should disable polling, and this must be implemented.
 1. A response with HTTP status `4XX` or `5XX` should disable polling as well.
 1. Use a common library for polling.

doc/development/polling.md

@@ -51,5 +51,6 @@ request path. By doing this we avoid query parameter ordering problems and make
 route matching easier.
 For more information see:
+- [`Poll-Interval` header](fe_guide/performance.md#realtime-components)
 - [RFC 7232](https://tools.ietf.org/html/rfc7232)
 - [ETag proposal](https://gitlab.com/gitlab-org/gitlab-ce/issues/26926)