Provisioning real-time and low latency workloads | Scalability and performance

Scheduling a low latency workload onto a compute node
Creating a pod with a guaranteed QoS class
Disabling CPU load balancing in a Pod
Disabling power saving mode for high priority pods
Disabling CPU CFS quota
Configuring interrupt processing for individual pods

If your organization needs high performance computing and low, predictable latency, especially in the financial and telecommunications industries, you can use the Node Tuning Operator to implement automatic tuning to achieve low latency performance and consistent response time for OKD applications.

You use the performance profile configuration to make these changes.

You can update the kernel to kernel-rt, reserve CPUs for cluster and operating system housekeeping duties, including pod infra containers, isolate CPUs for application containers to run the workloads, and disable unused CPUs to reduce power consumption.

When writing your applications, follow the general recommendations described in RHEL for Real Time processes and threads.

Additional resources

Creating a performance profile

Scheduling a low latency workload onto a compute node

You can schedule low latency workloads onto a compute node where a performance profile that configures real-time capabilities is applied.

To schedule a workload on specific nodes, use label selectors in the Pod custom resource (CR). The label selectors must match the nodes that are attached to the machine config pool that was configured for low latency by the Node Tuning Operator.

Prerequisites

You have installed the OpenShift CLI (oc).
You have logged in as a user with cluster-admin privileges.
You have applied a performance profile in the cluster that tunes compute nodes for low latency workloads.

Procedure

Create a Pod CR for the low latency workload and apply it in the cluster, for example:

Example Pod spec configured to use real-time processing

apiVersion: v1
kind: Pod
metadata:
  name: dynamic-low-latency-pod
  annotations:
    cpu-quota.crio.io: "disable"
    cpu-load-balancing.crio.io: "disable"
    irq-load-balancing.crio.io: "disable"
spec:
  securityContext:
    runAsNonRoot: true
    seccompProfile:
      type: RuntimeDefault
  containers:
  - name: dynamic-low-latency-pod
    image: "registry.redhat.io/openshift4/cnf-tests-rhel8:v4.21"
    command: ["sleep", "10h"]
    resources:
      requests:
        cpu: 2
        memory: "200M"
      limits:
        cpu: 2
        memory: "200M"
    securityContext:
      allowPrivilegeEscalation: false
      capabilities:
        drop: [ALL]
  nodeSelector:
    node-role.kubernetes.io/worker-cnf: ""
  runtimeClassName: performance-dynamic-low-latency-profile (5)
# ...

where

metadata.annotations.cpu-quota.crio.io: Disables the CPU completely fair scheduler (CFS) quota at the pod run time.
metadata.annotations.cpu-load-balancing.crio.io: Disables CPU load balancing.
metadata.annotations.irq-load-balancing.crio.io: Opts the pod out of interrupt handling on the node.
spec.nodeSelector.node-role.kubernetes.io/worker-cnf: The nodeSelector label must match the label that you specify in the Node CR.
spec.runtimeClassName: runtimeClassName must match the name of the performance profile configured in the cluster.

Enter the pod runtimeClassName in the form performance-<profile_name>, where <profile_name> is the name from the PerformanceProfile YAML. In the previous YAML example, the name is performance-dynamic-low-latency-profile.

Ensure the pod is running correctly. Status should be running, and the correct cnf-worker node should be set.

$ oc get pod -o wide

Expected output

NAME                     READY   STATUS    RESTARTS   AGE     IP           NODE
dynamic-low-latency-pod  1/1     Running   0          5h33m   10.131.0.10  cnf-worker.example.com

Get the CPUs that the pod configured for IRQ dynamic load balancing runs on:

$ oc exec -it dynamic-low-latency-pod -- /bin/bash -c "grep Cpus_allowed_list /proc/self/status | awk '{print $2}'"

Expected output

Cpus_allowed_list:  2-3

Verification

Log in to the node to verify the configuration.
```
$ oc debug node/<node-name>
```
Verify that you can use the node file system:
```
sh-4.4# chroot /host
```
Expected output
```
sh-4.4#
```
Ensure the default system CPU affinity mask does not include the dynamic-low-latency-pod CPUs, for example, CPUs 2 and 3.
```
sh-4.4# cat /proc/irq/default_smp_affinity
```
Example output
```
33
```

Ensure the system IRQs are not configured to run on the dynamic-low-latency-pod CPUs:

sh-4.4# find /proc/irq/ -name smp_affinity_list -exec sh -c 'i="$1"; mask=$(cat $i); file=$(echo $i); echo $file: $mask' _ {} \;

Example output

/proc/irq/0/smp_affinity_list: 0-5
/proc/irq/1/smp_affinity_list: 5
/proc/irq/2/smp_affinity_list: 0-5
/proc/irq/3/smp_affinity_list: 0-5
/proc/irq/4/smp_affinity_list: 0
/proc/irq/5/smp_affinity_list: 0-5
/proc/irq/6/smp_affinity_list: 0-5
/proc/irq/7/smp_affinity_list: 0-5
/proc/irq/8/smp_affinity_list: 4
/proc/irq/9/smp_affinity_list: 4
/proc/irq/10/smp_affinity_list: 0-5
/proc/irq/11/smp_affinity_list: 0
/proc/irq/12/smp_affinity_list: 1
/proc/irq/13/smp_affinity_list: 0-5
/proc/irq/14/smp_affinity_list: 1
/proc/irq/15/smp_affinity_list: 0
/proc/irq/24/smp_affinity_list: 1
/proc/irq/25/smp_affinity_list: 1
/proc/irq/26/smp_affinity_list: 1
/proc/irq/27/smp_affinity_list: 5
/proc/irq/28/smp_affinity_list: 1
/proc/irq/29/smp_affinity_list: 0
/proc/irq/30/smp_affinity_list: 0-5

When you tune nodes for low latency, the usage of execution probes in conjunction with applications that require guaranteed CPUs can cause latency spikes. Use other probes, such as a properly configured set of network probes, as an alternative.

Additional resources

Creating a pod with a guaranteed QoS class

You can create a pod with a quality of service (QoS) class of Guaranteed for high-performance workloads. Configuring a pod with a QoS class of Guaranteed ensures that the pod has priority access to the specified CPU and memory resources.

To create a pod with a QoS class of Guaranteed, you must apply the following specifications:

Set identical values for the memory limit and memory request fields for each container in the pod.
Set identical values for CPU limit and CPU request fields for each container in the pod.

In general, a pod with a QoS class of Guaranteed will not be evicted from a node. One exception is during resource contention caused by system daemons exceeding reserved resources. In this scenario, the kubelet might evict pods to preserve node stability, starting with the lowest priority pods.

Prerequisites

Access to the cluster as a user with the cluster-admin role.
The OpenShift CLI (oc).

Procedure

Create a namespace for the pod by running the following command:
```
$ oc create namespace qos-example
```
- qos-example: Specifies a qos-example example namespace.
  Example output
  namespace/qos-example created

Create the Pod resource:

Create a YAML file that defines the Pod resource:

Example qos-example.yaml file

apiVersion: v1
kind: Pod
metadata:
  name: qos-demo
  namespace: qos-example
spec:
  securityContext:
    runAsNonRoot: true
    seccompProfile:
      type: RuntimeDefault
  containers:
  - name: qos-demo-ctr
    image: quay.io/openshifttest/hello-openshift:openshift
    resources:
      limits:
        memory: "200Mi"
        cpu: "1"
      requests:
        memory: "200Mi"
        cpu: "1"
    securityContext:
      allowPrivilegeEscalation: false
      capabilities:
        drop: [ALL]

where:

spec.containers.image

Specifies public image, such as the hello-openshift image.

spec.containers.resources.limits.memory

Specifies a memory limit of 200 MB.

spec.containers.resources.limits.cpu

Specifies a CPU limit of 1 CPU.

spec.containers.resources.requests.memory

Specifies a memory request of 200 MB.

spec.containers.resources.requests.cpu

Specifies a CPU request of 1 CPU.

If you specify a memory limit for a container, but do not specify a memory request, OKD automatically assigns a memory request that matches the limit. Similarly, if you specify a CPU limit for a container, but do not specify a CPU request, OKD automatically assigns a CPU request that matches the limit.

Create the Pod resource by running the following command:

$ oc apply -f qos-example.yaml --namespace=qos-example

Example output

pod/qos-demo created

Verification

View the qosClass value for the pod by running the following command:

$ oc get pod qos-demo --namespace=qos-example --output=yaml | grep qosClass

Example output

    qosClass: Guaranteed

Disabling CPU load balancing in a Pod

Functionality to disable or enable CPU load balancing is implemented on the CRI-O level. Before CRI-O disables or enables CPU load balancing, you must ensure certain prerequisites are met.

The pod must use the performance-<profile-name> runtime class. You can get the proper name by looking at the status of the performance profile, as shown here:

apiVersion: performance.openshift.io/v2
kind: PerformanceProfile
...
status:
  ...
  runtimeClass: performance-manual

The Node Tuning Operator is responsible for the creation of the high-performance runtime handler config snippet under relevant nodes and for creation of the high-performance runtime class under the cluster. It will have the same content as the default runtime handler except that it enables the CPU load balancing configuration functionality.

To disable the CPU load balancing for the pod, the Pod specification must include the following fields:

apiVersion: v1
kind: Pod
metadata:
  #...
  annotations:
    #...
    cpu-load-balancing.crio.io: "disable"
    #...
  #...
spec:
  #...
  runtimeClassName: performance-<profile_name>
  #...

Only disable CPU load balancing when the CPU manager static policy is enabled and for pods with guaranteed QoS that use whole CPUs. Otherwise, disabling CPU load balancing can affect the performance of other containers in the cluster.

Disabling power saving mode for high priority pods

To protect high priority workloads when using power saving configurations on a node, apply performance settings at the pod level. This ensures that the configuration applies to all cores used by the pod, maintaining performance stability.

By disabling P-states and C-states at the pod level, you can configure high priority workloads for best performance and lowest latency.

Table 1. Configuration for high priority workloads
Annotation	Possible Values	Description
`cpu-c-states.crio.io:`	`"enable"` `"disable"` `"max_latency:microseconds"`	This annotation allows you to enable or disable C-states for each CPU. Alternatively, you can also specify a maximum latency in microseconds for the C-states. For example, enable C-states with a maximum latency of 10 microseconds with the setting `cpu-c-states.crio.io`: `"max_latency:10"`. Set the value to `"disable"` to provide the best performance for a pod.
`cpu-freq-governor.crio.io:`	Any supported `cpufreq governor`.	Sets the `cpufreq` governor for each CPU. The `"performance"` governor is recommended for high priority workloads.

Prerequisites

You have configured power saving in the performance profile for the node where the high priority workload pods are scheduled.

Procedure

Add the required annotations to your high priority workload pods. The annotations override the default settings.

Example high priority workload annotation

apiVersion: v1
kind: Pod
metadata:
  #...
  annotations:
    #...
    cpu-c-states.crio.io: "disable"
    cpu-freq-governor.crio.io: "performance"
    #...
  #...
spec:
  #...
  runtimeClassName: performance-<profile_name>
  #...

Restart the pods to apply the annotation.

Additional resources

Configuring power saving for nodes that run colocated high and low priority workloads

Disabling CPU CFS quota

To eliminate CPU throttling for pinned pods, create a pod with the cpu-quota.crio.io: "disable" annotation. This annotation disables the CPU completely fair scheduler (CFS) quota when the pod runs.

Procedure

To eliminate CPU throttling for pinned pods, create a pod with the cpu-quota.crio.io: "disable" annotation. This annotation disables the CPU completely fair scheduler (CFS) quota when the pod runs.

Example pod specification with cpu-quota.crio.io disabled

apiVersion: v1
kind: Pod
metadata:
  annotations:
      cpu-quota.crio.io: "disable"
spec:
    runtimeClassName: performance-<profile_name>
#...

Only disable CPU CFS quota when the CPU manager static policy is enabled and for pods with guaranteed QoS that use whole CPUs. For example, pods that contain CPU-pinned containers. Otherwise, disabling CPU CFS quota can affect the performance of other containers in the cluster.

Additional resources

Recommended firmware configuration for vDU cluster hosts

Configuring interrupt processing for individual pods

To achieve low latency for workloads, some containers require that the CPUs they are pinned to do not process device interrupts. You can use the irq-load-balancing.crio.io pod annotation to control whether device interrupts are processed on CPUs where the pinned containers are running.

The annotation supports the following values:

disable: Disables IRQ load balancing for all CPUs allocated to the container. Use this value for latency-sensitive workloads when you want to exclude container CPUs from interrupt handling.
housekeeping: Preserves IRQ handling on the first CPU that is allocated to the container, including that CPU’s thread siblings. The subsequent CPUs allocated to the container are excluded from interrupt processing. This configuration also injects the OPENSHIFT_HOUSEKEEPING_CPUS environment variable into the container. Use this variable to see which CPUs are designated for housekeeping tasks.

You can use the housekeeping value to reduce the overall CPU footprint by allowing a small subset of container CPUs to handle both application housekeeping work and system interrupts.

When using the housekeeping value, the CPUs designated for housekeeping handle interrupts for the entire system.

Prerequisites

You configured a performance profile for the node.
You set the globallyDisableIrqLoadBalancing field to false in the performance profile.

Procedure

Create the Pod resource and configure the irq-load-balancing.crio.io annotation:
Example pod specification
```
apiVersion: v1
kind: Pod
metadata:
  name: dpdk-workload
  annotations:
    irq-load-balancing.crio.io: "disable"
spec:
  runtimeClassName: performance-<profile_name>
  containers:
  - name: app
    image: example-image
    resources:
      requests:
        cpu: "8"
        memory: "4Gi"
      limits:
        cpu: "8"
        memory: "4Gi"
```
- metadata.annotations.irq-load-balancing.crio.io: Specifies if device interrupts are processed on the container CPUs. Set to disable to prevent all container CPUs from handling IRQs, or set to housekeeping to allow the first allocated CPU and its thread siblings to handle IRQs while excluding the remaining CPUs from IRQ handling.
- spec.runtimeClassName: Specifies the runtime class for the performance profile. Replace <profile_name> with the name of your performance profile.
Apply the Pod resource by running the following command:
```
$ oc apply -f pod.yaml
```

Verification

Verify the CPUs assigned to the pod:

$ oc exec <pod_name> -- cat /sys/fs/cgroup/cpuset.cpus

For pods using the housekeeping annotation, verify the housekeeping CPU environment variable:
```
$ oc exec <pod_name> -- printenv OPENSHIFT_HOUSEKEEPING_CPUS
```
Replace <pod_name> with the name of the pod.
On the worker node, verify the CPUs excluded from IRQ handling:
```
$ grep IRQBALANCE_BANNED_CPUS /etc/sysconfig/irqbalance
```
The output is a hexadecimal bitmask representing the CPUs excluded from IRQ handling.

Additional resources

Managing device interrupt processing for guaranteed pod isolated CPUs