Not able to disable concurrent jobs on a runner

Hello there,

I have a few machines with a single runner on each. I want each runner to handle one job at a time, because if two jobs that build the app execute on the same runner, they will run into resource sharing issues on that machine. So I want to configure my runners so that no matter where the job came from, it's never executing 2 jobs at the same time. Even though I've set limit = 1 and concurrent = 1 in my config.toml, I still observe that a runner may be picked twice in a job that has a parallel value > 1. I also have another job (without a parallel setting) failing occasionally due to another process having a lock on a file, which I suspect is because the runner is executing another job.

Is there more configuration required in order to completely disable concurrency for a runner? I've double checked that I don't have more than 1 runner on a single machine.

I've read about resource_group, but I'm not entirely sure if this is what I need. I don't want to prevent the same job from different pipelines from executing at the same time on different runners, I just want a single runner to take one job at a time.

Thanks in advance for any help/insight here.