GitLab Runner's Default concurrent=1 Setting Can Silently Stall Your CI Pipelines
A CI pipeline appearing stuck in a pending state for minutes can be caused by GitLab Runner's default configuration, not a system failure. GitLab Runner ships with a concurrent = 1 setting in its config.toml file, which limits all projects sharing that runner to a single execution lane at a time. This means one active build from any repository can block all others, while the runner still reports a healthy status. Engineers can identify the culprit by inspecting running Docker containers to read which project holds the active lane. The fix is straightforward: increasing the concurrent value to 2 or higher, then restarting the runner, typically unblocks queued pipelines within about 30 seconds.
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