Added documentation and CHANGELOG item.

changelogs/unreleased/sidekiq-job-throttling.yml

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+---
+title: Added ability to throttle Sidekiq Jobs
+merge_request: 7292
+author: Patricio Cano

doc/administration/operations.md

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 # GitLab operations
 
 - [Sidekiq MemoryKiller](operations/sidekiq_memory_killer.md)
+- [Sidekiq Job throttling](operations/sidekiq_job_throttling.md)
 - [Cleaning up Redis sessions](operations/cleaning_up_redis_sessions.md)
 - [Understanding Unicorn and unicorn-worker-killer](operations/unicorn.md)
 - [Moving repositories to a new location](operations/moving_repositories.md)

doc/administration/operations/sidekiq_job_throttling.md

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+# Sidekiq Job throttling
+
+> Note: Introduced with GitLab 8.14
+
+When your GitLab installation needs to handle tens of thousands of background
+jobs, it can be convenient to prioritize queues that need to be executed
+immediately, e.g. user initiated actions like merging a Merge Request.
+
+In order to accomplish this, you can limit the amount of workers that certain
+slow running queues get can have available. This is what we call Sidekiq Job
+Throttling. Depending on your infrastructure, you might have different slow
+running queues, which is why you can choose which queues to throttle and by
+how much you want to throttle them.
+
+These settings are available in the Application Settings of your GitLab
+installation.
+
+![Sidekiq Job Throttling](img/sidekiq_job_throttling.png)
+
+The throttle factor determines the maximum number of workers a queue can run on.
+This value gets multiplied by `:concurrency` value set in the Sidekiq settings
+and rounded up to the closest full integer.
+
+So, for example, you set the `:concurrency` to 25 and the `Throttling factor` to
+0.1, the maximum workers assigned to the selected queues would be 3.
+
+```
+limit = (factor * Sidekiq.options[:concurrency]).ceil
+```
+
+After enabling the job throttling, you will need to restart your GitLab
+instance, in order for the changes to take effect.