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Chapter 5. Configuring PID limits

A process identifier (PID) is a unique identifier assigned by the Linux kernel to each process or thread currently running on a system. The number of processes that can run simultaneously on a system is limited to 4,194,304 by the Linux kernel. This number might also be affected by limited access to other system resources such as memory, CPU, and disk space.

In Red Hat OpenShift Service on AWS 4.11 and later, by default, a pod can have a maximum of 4,096 PIDs. If your workload requires more than that, you can increase the allowed maximum number of PIDs by configuring a KubeletConfig object.

Red Hat OpenShift Service on AWS clusters running versions earlier than 4.11 use a default PID limit of 1024.

5.1. Understanding process ID limits
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In Red Hat OpenShift Service on AWS, consider these two supported limits for process ID (PID) usage before you schedule work on your cluster:

  • Maximum number of PIDs per pod.

    The default value is 4,096 in Red Hat OpenShift Service on AWS 4.11 and later. This value is controlled by the podPidsLimit parameter set on the node.

  • Maximum number of PIDs per node.

    The default value depends on node resources. In Red Hat OpenShift Service on AWS, this value is controlled by the --system-reserved parameter, which reserves PIDs on each node based on the total resources of the node.

When a pod exceeds the allowed maximum number of PIDs per pod, the pod might stop functioning correctly and might be evicted from the node. See the Kubernetes documentation for eviction signals and thresholds for more information.

When a node exceeds the allowed maximum number of PIDs per node, the node can become unstable because new processes cannot have PIDs assigned. If existing processes cannot complete without creating additional processes, the entire node can become unusable and require reboot. This situation can result in data loss, depending on the processes and applications being run. Customer administrators and Red Hat Site Reliability Engineering are notified when this threshold is reached, and a Worker node is experiencing PIDPressure warning will appear in the cluster logs.

The podPidsLimit parameter for a pod controls the maximum number of processes and threads that can run simultaneously in that pod.

You can increase the value for podPidsLimit from the default of 4,096 to a maximum of 16,384. Changing this value might incur downtime for applications, because changing the podPidsLimit requires rebooting the affected node.

If you are running a large number of pods per node, and you have a high podPidsLimit value on your nodes, you risk exceeding the PID maximum for the node.

To find the maximum number of pods that you can run simultaneously on a single node without exceeding the PID maximum for the node, divide 3,650,000 by your podPidsLimit value. For example, if your podPidsLimit value is 16,384, and you expect the pods to use close to that number of process IDs, you can safely run 222 pods on a single node.

Note

Memory, CPU, and available storage can also limit the maximum number of pods that can run simultaneously, even when the podPidsLimit value is set appropriately. For more information, see "Planning your environment" and "Limits and scalability".

You can set a higher podPidsLimit for machine pools in an existing Red Hat OpenShift Service on AWS cluster by creating or editing a KubeletConfig object that changes the --pod-pids-limit parameter.

Important

Changing the podPidsLimit on an existing machine pool triggers nodes in the machine pool to reboot one at a time. Make this change outside of peak usage hours for workloads in your machine pool and avoid upgrading or hibernating your cluster until all nodes have rebooted.

Prerequisites

  • You have a Red Hat OpenShift Service on AWS cluster.
  • You have installed the ROSA command-line interface (CLI) (rosa).
  • You have logged in to your Red Hat account by using the ROSA CLI.

Procedure

  1. Create a new KubeletConfig object for your cluster that specifies a new --pod-pids-limit: 

    $ rosa create kubeletconfig -c <cluster_name> --name=<kubeletconfig_name> --pod-pids-limit=<value>
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    For example, the following command creates a set-high-pids KubeletConfig object for the my-cluster cluster that sets a maximum of 16,384 PIDs per pod: 

    $ rosa create kubeletconfig -c my-cluster --name=set-high-pids --pod-pids-limit=16384
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  2. Associate the new KubeletConfig object with a new or existing machine pool.

    • For a new machine pool: 

      $ rosa create machinepool -c <cluster_name> --name <machinepool_name> --kubelet-configs=<kubeletconfig_name>
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    • For an existing machine pool: 

      $ rosa edit machinepool -c <cluster_name> --kubelet-configs=<kubeletconfig_name> <machinepool_name>
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      Example output

      Editing the kubelet config will cause the Nodes for your Machine Pool to be recreated. This may cause outages to your applications. Do you wish to continue? (y/N)
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    For example, the following command associates the set-high-pids KubeletConfig object with the high-pid-pool machine pool in the my-cluster cluster: 

    $ rosa edit machinepool -c my-cluster --kubelet-configs=set-high-pids high-pid-pool
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    A rolling reboot of worker nodes is triggered when a new KubeletConfig object is attached to an existing machine pool. You can check the progress of the rollout in the machine pool description: 

    $ rosa describe machinepool --cluster <cluster_name> --machinepool <machinepool_name>
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Verification

  • Confirm that the new setting is in place on nodes in the machine pool: 

    $ rosa describe kubeletconfig --cluster=<cluster_name> --name <kubeletconfig_name>
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    The new PIDs limit appears in the output, as shown in the following example:

    Example output

    Pod Pids Limit:                       16384
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5.4. Removing custom configuration from a machine pool
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You can remove custom configuration on your machine pools by removing the KubeletConfig object that contains the configuration details.

Prerequisites

  • You have an existing Red Hat OpenShift Service on AWS cluster.
  • You have installed the ROSA CLI (rosa).
  • You have logged in to your Red Hat account by using the ROSA CLI.

Procedure

  • Edit the machine pool and set the --kubeletconfigs parameter so that the KubeletConfig object you want to remove is omitted.

    To remove all KubeletConfig objects from the machine pool, set an empty value for the --kubeletconfigs parameter, for example: 

    $ rosa edit machinepool -c <cluster_name> --kubelet-configs="" <machinepool_name>
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Verification steps

  • Confirm that the KubeletConfig object you removed is not visible in the machine pool description: 

    $ rosa describe machinepool --cluster <cluster_name> --machinepool=<machinepool_name>
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