.. _cluster-FAQ:

===
FAQ
===

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 <https://discuss.ray.io/>`__.

Do Ray clusters support multi-tenancy?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Yes, you can run multiple :ref:`jobs <jobs-overview>` 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 :ref:`resources <core-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.

.. note::
    A Ray :ref:`namespace <namespaces-guide>` 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:

.. code:: bash

    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:

.. code:: bash

    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 cannot find the
    matched Raylet address. This may 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 :ref:`ray-slurm-deploy`.

.. _`known OpenBLAS limitation`: http://www.openmathlib.org/OpenBLAS/docs/faq/#how-can-i-use-openblas-in-multi-threaded-applications

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

By default, jobs submitted using the :ref:`Ray Job API <jobs-quickstart>` 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.