CI configuration performance

Interruptible pipelines

By default, all jobs are interruptible, except the dont-interrupt-me job which runs automatically on main, and is manual otherwise.

If you want a running pipeline to finish even if you push new commits to a merge request, be sure to start the dont-interrupt-me job before pushing.

Git fetch caching

Because GitLab.com uses the pack-objects cache, concurrent Git fetches of the same pipeline ref are deduplicated on the Gitaly server (always) and served from cache (when available).

This works well for the following reasons:

  • The pack-objects cache is enabled on all Gitaly servers on GitLab.com.
  • The CI/CD Git strategy setting for gitlab-org/gitlab is Git clone, causing all jobs to fetch the same data, which maximizes the cache hit ratio.
  • We use shallow clone to avoid downloading the full Git history for every job.

Fetch repository via artifacts instead of cloning/fetching from Gitaly

Lately we see errors from Gitaly look like this: (see the issue)

fatal: remote error: GitLab is currently unable to handle this request due to load.

While GitLab.com uses pack-objects cache, sometimes the load is still too heavy for Gitaly to handle, and thundering herds can also be a concern that we have a lot of jobs cloning the repository around the same time.

To mitigate and reduce loads for Gitaly, we changed some jobs to fetch the repository from artifacts in a job instead of all cloning from Gitaly at once.

For now this applies to most of the RSpec jobs, which has the most concurrent jobs in most pipelines. This also slightly improved the speed because fetching from the artifacts is also slightly faster than cloning, at the cost of saving more artifacts for each pipeline.

Based on the numbers on 2023-12-20 at Fetch repo from artifacts for RSpec jobs, the extra storage cost was about 280M for each pipeline, and we save 15 seconds for each RSpec jobs.

We do not apply this to jobs having no other job dependencies because we don’t want to delay any jobs from starting.

This behavior can be controlled by variable CI_FETCH_REPO_GIT_STRATEGY:

  • Set to none means jobs using .repo-from-artifacts fetch repository from artifacts in job clone-gitlab-repo rather than cloning.
  • Set to clone means jobs using .repo-from-artifacts clone repository as usual. Job clone-gitlab-repo does not run in this case.

To disable it, set CI_FETCH_REPO_GIT_STRATEGY to clone. To enable it, set CI_FETCH_REPO_GIT_STRATEGY to none.

Caching strategy

Caches in GitLab CI/CD pipelines must follow these criteria:

  • Pull-push vs pull-only configuration: Jobs in the sync stage (which runs at the start of pipelines) responsible for populating cache use pull-push configuration, while jobs that consume cache use pull-only configuration to be branch agnostic and populate cache for specific branch pipeline runs.
  • File-specific checksums: All cache keys should be coupled to their file-specific checksums to ensure caches are invalidated correctly when dependencies change.
  • Language version coupling: Cache keys should include respective language-specific versions where applicable to ensure language version upgrades invalidate cache appropriately.
  • Sync stage placement: Cache update jobs should be defined in the sync stage to run at the very start of the pipeline.
  • Reusable definitions: Reusable cache key definitions are placed in .gitlab/ci/global.gitlab-ci.yml file for consistency across the project.

Complex checksum calculation

In cases when cache keys require complex multiple file checksum calculation, a specific job should be added in the sync stage which calculates the cache checksum and makes it available via environment variable by saving it as a dotenv type report artifact. This ensures that all subsequent jobs can use the pre-calculated checksum for consistent cache key generation across the pipeline.

Artifacts strategy

We limit the artifacts that are saved and retrieved by jobs to the minimum to reduce the upload/download time and costs, as well as the artifacts storage.

Stripped binaries

By default, setup-test-env creates an artifact which contains stripped binaries to save storage and speed-up artifact downloads of subsequent CI jobs.

To make debugging a crash from stripped binaries easier comment line with strip_executable_binaries in the setup_test_env function in scripts/gitlab_component_helpers.sh shell script and start a new pipeline.