These are some Frequently Asked Questions for Ray clusters. If you still have questions after reading this FAQ, reach out on the Ray Discourse forum.

Do Ray clusters support multi-tenancy?#

Yes, you can run multiple jobs from different users simultaneously in a Ray cluster but it’s not recommended in production. Some Ray features are still missing for multi-tenancy in production:

  • Ray doesn’t provide strong resource isolation: Ray resources are logical and they don’t limit the physical resources a task or actor can use while running. This means simultaneous jobs can interfere with each other and makes them less reliable to run in production.

  • Ray doesn’t support priorities: All jobs, tasks and actors have the same priority so there is no way to prioritize important jobs under load.

  • Ray doesn’t support access control: Jobs have full access to a Ray cluster and all of the resources within it.

On the other hand, you can run the same job multiple times using the same cluster to save the cluster startup time.


A Ray namespace is just a logical grouping of jobs and named actors. Unlike a Kubernetes namespace, it doesn’t provide any other multi-tenancy functions like resource quotas.

I have multiple Ray users. What’s the right way to deploy Ray for them?#

Start a Ray cluster for each user to isolate their workloads.

What’s the difference between --node-ip-address and --address?#

When starting a head node on a machine with more than one network address, you may need to specify the externally available address so worker nodes can connect. Use this command:

ray start --head --node-ip-address xx.xx.xx.xx --port nnnn``

Then when starting the worker node, use this command to connect to the head node:

ray start --address xx.xx.xx.xx:nnnn

What does a worker node failure to connect look like?#

If the worker node can’t connect to the head node, you should see this error:

Unable to connect to GCS at xx.xx.xx.xx:nnnn. Check that (1) Ray GCS with matching version started successfully at the specified address, and (2) there is no firewall setting preventing access.

The most likely cause is that the worker node can’t access the IP address given. You can use ip route get xx.xx.xx.xx on the worker node to start debugging routing issues.

You may also see failures in the log like:

This node has an IP address of xx.xx.xx.xx, while we can not found the matched Raylet address. This maybe come from when you connect the Ray cluster with a different IP address or connect a container.

The cause of this error may be the head node overloading with too many simultaneous connections. The solution for this problem is to start the worker nodes more slowly.

Problems getting a SLURM cluster to work#

A class of issues exist with starting Ray on SLURM clusters. While the exact causes aren’t understood, (as of June 2023), some Ray improvements mitigate some of the resource contention. Some of the issues reported are as follows:

  • Using a machine with a large number of CPUs, and starting one worker per CPU together with OpenBLAS (as used in NumPy) may allocate too many threads. This issue is a known OpenBLAS limitation. You can mitigate it by limiting OpenBLAS to one thread per process as explained in the link.

  • Resource allocation isn’t as expected: usually the configuration has too many CPUs allocated per node. The best practice is to verify the SLURM configuration without starting Ray to verify that the allocations are as expected. For more detailed information see Deploying on Slurm.

Where does my Ray Job entrypoint script run? On the head node or worker nodes?#

By default, jobs submitted using the Ray Job API run their entrypoint script on the head node. You can change this by specifying any of the options --entrypoint-num-cpus, --entrypoint-num-gpus, --entrypoint-resources or --entrypoint-memory to ray job submit, or the corresponding arguments if using the Python SDK. If these are specified, the job entrypoint will be scheduled on a node that has the requested resources available.