Restart gitlab

# Restart all GitLab components
sudo gitlab-ctl restart

From the documentation:

Note that on a single-core server it may take up to a minute to restart Unicorn and Sidekiq. Your GitLab instance will give a 502 error until Unicorn is up again.

Unicorn supports zero-downtime reloads. These can be triggered as follows:

sudo gitlab-ctl hup unicorn

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