Guided Exercise: Limit Compute Capacity for Applications

Configure an application with compute resource limits that allow and prevent successful execution of its pods.

Outcomes

You should be able to monitor the memory usage of an application, and set a memory limit for a pod.

As the student user on the workstation machine, use the lab command to prepare your system for this exercise.

This command ensures that all resources are available for this exercise. It also creates the reliability-limits project and the /home/student/DO180/labs/reliability-limits/resources.txt file. The resources.txt file contains some commands that you use during the exercise. You can use the file to copy and paste these commands.

[student@workstation ~]$ lab start reliability-limits

Procedure 6.4. Instructions

Log in to the OpenShift cluster as the developer user with the developer password. Use the reliability-limits project.

 [student@workstation ~]$ oc login -u developer -p developer \
   https://api.ocp4.example.com:6443
 Login successful.
 ...output omitted...

Set the reliability-limits project as the active project.

 [student@workstation ~]$ oc project reliability-limits
 ...output omitted...

Create the leakapp deployment from the ~/DO180/labs/reliability-limits/leakapp.yml file that the lab command prepared. The application has a bug, and leaks 1 MiB of memory every second.
1. Review the ~/DO180/labs/reliability-limits/leakapp.yml resource file. The memory limit is set to 35 MiB. Do not change the file.
```
 ...output omitted...
         resources:
           requests:
             memory: 20Mi
           limits:
             memory: 35Mi
```
2. Use the oc apply command to create the application. Ignore the warning message.
```
 [student@workstation ~]$ oc apply -f \
   ~/DO180/labs/reliability-limits/leakapp.yml
 Warning: would violate PodSecurity "restricted:v1.24":
 ...output omitted...
 deployment.apps/leakapp created
```
3. Wait for the pod to start. You might have to rerun the command several times for the pod to report a Running status. The name of the pod on your system probably differs.
```
 [student@workstation ~]$ oc get pods
 NAME                      READY   STATUS    RESTARTS   AGE
 leakapp-99bb64c8d-hk26k   1/1     Running   0          12s
```

Watch the pod. OpenShift restarts the pod after 30 seconds.

Use the watch command to monitor the oc get pods command. Wait for OpenShift to restart the pod, and then press Ctrl+C to quit the watch command.

 [student@workstation ~]$ watch oc get pods
 Every 2.0s: oc get pods                    workstation: Wed Mar  8 07:27:45 2023

 NAME                      READY   STATUS    RESTARTS      AGE
 leakapp-99bb64c8d-hk26k   1/1     Running   1 (15s ago)   48s

Retrieve the container status to verify that OpenShift restarted the pod due to an Out-Of-Memory (OOM) event.

 [student@workstation ~]$ oc get pods leakapp-99bb64c8d-hk26k \
   -o jsonpath='{.status.containerStatuses[0].lastState}' | jq .
 {
   "terminated": {
     "containerID": "cri-o://5800...1d04",
     "exitCode": 137,
     "finishedAt": "2023-03-08T12:29:24Z",
     "reason": "OOMKilled",
     "startedAt": "2023-03-08T12:28:53Z"
   }
 }

Observe the pod status for a few minutes, until the CrashLoopBackOff status is displayed. During this period, OpenShift restarts the pod several times because of the memory leak.

Between each restart, OpenShift sets the pod status to CrashLoopBackOff, waits an increasing amount of time between retries, and then restarts the pod. The delay between restarts gives the operator the opportunity to fix the issue.

After various retries, OpenShift finally sets the CrashLoopBackOff wait timer to five minutes. During this wait time, the application is not available to your customers.
```
 [student@workstation ~]$ watch oc get pods
 Every 2.0s: oc get pods                    workstation: Wed Mar  8 07:33:15 2023

 NAME                      READY   STATUS             RESTARTS      AGE
 leakapp-99bb64c8d-hk26k   0/1     CrashLoopBackOff   4 (82s ago)   5m25s
```
Press Ctrl+C to quit the watch command.

Fixing the memory leak would resolve the issue. However, it might take some time for the developers to fix the bug. In the meantime, set the memory limit to 600 MiB. With this setting, the pod can run for ten minutes before the application reaches the limit.

Use the oc set resources command to set the new limit. Ignore the warning message.

 [student@workstation ~]$ oc set resources deployment/leakapp \
   --limits memory=600Mi
 Warning: would violate PodSecurity "restricted:v1.24":
 ...output omitted...
 deployment.apps/leakapp resource requirements updated

Wait for the pod to start. You might have to rerun the command several times for the pod to report a Running status. The name of the pod on your system probably differs.
```
 [student@workstation ~]$ oc get pods
 NAME                      READY   STATUS    RESTARTS   AGE
 leakapp-6bc64dfcd-86fpc   1/1     Running   0          12s
```

Wait two minutes to verify that OpenShift no longer restarts the pod every 30 seconds.

 [student@workstation ~]$ watch oc get pods
 Every 2.0s: oc get pods                    workstation: Wed Mar  8 07:38:15 2023

 NAME                      READY   STATUS    RESTARTS   AGE
 leakapp-6bc64dfcd-86fpc   1/1     Running   0          3m12s

Press Ctrl+C to quit the watch command.

Review the memory that the pod consumes. You might have to rerun the command several times for the metrics to be available. The memory usage on your system probably differs.
```
 [student@workstation ~]$ oc adm top pods
 NAME                      CPU(cores)   MEMORY(bytes)
 leakapp-6bc64dfcd-86fpc   0m           174Mi
```
Optional. Wait seven more minutes. After this period, OpenShift restarts the pod, because it reached the 600 MiB memory limit.
1. Open a new terminal window, and then run the watch command to monitor the oc adm top pods command.
```
 [student@workstation ~]$ watch oc adm top pods
 Every 2.0s: oc adm top pods                workstation: Wed Mar  8 07:38:55 2023

 NAME                      CPU(cores)   MEMORY(bytes)
 leakapp-6bc64dfcd-86fpc   0m           176Mi
```
  Leave the command running and do not interrupt it.
  
  NOTE
  
  You might see a message that metrics are not yet available. If so, wait some time and try again.
2. In the first terminal, run the watch command to monitor the oc get pods command. Watch the output of the oc adm top pods command in the second terminal. When the memory usage reaches 600 MiB, the OOM subsystem kills the process inside the container, and OpenShift restarts the pod.
```
 [student@workstation ~]$ watch oc get pods
 Every 2.0s: oc get pods                    workstation: Wed Mar  8 07:46:35 2023

 NAME                      READY   STATUS    RESTARTS     AGE
 leakapp-6bc64dfcd-86fpc   1/1     Running   1 (3s ago)   9m58s
```
  Press Ctrl+C to quit the watch command.
3. Press Ctrl+C to quit the watch command in the second terminal. Close this second terminal when done.

Finish

On the workstation machine, use the lab command to complete this exercise. This step is important to ensure that resources from previous exercises do not impact upcoming exercises.

[student@workstation ~]$ lab finish reliability-limits

Syarif's Blog

Syarif's Blog

Guided Exercise: Limit Compute Capacity for Applications

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