5x increase in Git branch existence checks
This issue seems to be behind a production performance regression https://gitlab.com/gitlab-com/infrastructure/issues/3245
This seems to be a performance regression in GitLab 10.2. We were forced to turn off the
gitaly_ref_exists_branches feature flag in production because the hotter Gitaly servers were starting to buckle under the traffic.
Judging by this graph: https://prometheus.gitlab.com/graph?g0.range_input=2w&g0.end_input=2017-11-20%2015%3A38&g0.expr=gitaly%3Agrpc_server_handled_total%3Arate1m%7Bgrpc_method%3D%22RefExists%22%7D&g0.tab=0
The number of branch existence checks went up by a factor five when we deployed 10.2 RC on gitlab.com.
We need to find out what caused this increase.