OOMKilled container

This topic guides you on troubleshooting containers that are being "OOMKilled".

What is OOMKill?

Out of memory kill meaning a container or pod was terminated, because it was using more memory than it was allowed.

Symptom

The CustomerMagnoliaContainerOOMKilled alert is firing.

CustomerMagnoliaContainerOOMKilled alerts are sent to subscribers via email.

What’s using the memory?

Kubernetes restarts a pod if it exceeds its memory limit. The Magnolia JVM typically cannot exceed its memory limit - the JVM max heap setting - but the JVM also will consume off heap memory that can vary over time, depending on what Magnolia is doing. Other containers running in the Magnolia pod may also consume memory but they usually use very small amounts (10s of mb). Temporary filesystems may use memory as well.

Observations

Here are the details on the alert:

Alert: CustomerMagnoliaContainerOomKilled

Expression

magnolia:oomkill:interval > 0

Delay

0 minutes

Labels

team: customer

Annotations

  • summary

  • description

  • tenant

  • cluster_id

  • cluster_name

  • instance

  • namespace

The magnolia:oomkill:interval metric is defined as:

(kube_pod_container_status_restarts_total{pod=~".+-magnolia-helm-.+"} - kube_pod_container_status_restarts_total{pod=~".+-magnolia-helm-.+"} offset 10m >= 1) and ignoring (reason) min_over_time(kube_pod_container_status_last_terminated_reason{pod=~".+-magnolia-helm-.+",reason="OOMKilled"}[10m]) == 1

Determine the memory request and limit for Magnolia

The alert notes the affected Magnolia pod. You can view the memory request and limit for the Magnolia in Rancher or with kubectl.

kubectl -n <namespace from alert> describe pod <Magnolia pod from alert>
Actions

  • Look in the "Limits" section for the memory limit.

  • Look in the "Requests" section for the memory request.

  • The memory limit and the memory request are usually set to the same value.

Determine the JVM max heap setting used for Magnolia

The JVM max heap setting is usually defined as a property stored in a configmap: <namespace>-config-<author|public>. You can view the configmap in Rancher or display it with kubectl.

The JVM_RAM_MAX_PERCENTAGE property specifies the percentage of the pod memory request used by the Magnolia.

For example, if the memory request/limit is 10Gi and JVM_RAM_MAX_PERCENTAGE is 60:

  • The JVM max heap will be '6Gi'

  • 4Gi will be available to the pod and other containers running in the Magnolia pod

Considerations

JVM memory usage

The JVM uses more memory than just the heap. Constraining the max heap won’t necessarily stop the JVM from exceeding the memory limit set for the pod.

Non-heap memory usage

  • Direct memory allocated with the java.nio.ByteBuffer.allocateDirect.

  • Other classes in the java.nio library may allocate a direct memory buffer.

  • Classes dealing with compressed streams like DeflaterInputStream, DeflaterOutputStream, GZIPInputStream, GZIPOutputStream, ZipFile, ZipInputStream, and ZipOutputStream may allocate a direct memory buffer.

  • Magnolia imaging operations using compressed image formats like gif or webp may allocate direct memory buffers when resizing or modifying images.

Direct memory buffers may be deallocated when the Java object using it is garbage collected, but a large number requests, such as imaging requests, may cause non-heap memory to be used.

A Magnolia instance that is being OOMKilled frequently will:

  • Have high traffic - many requests per second

  • Have lots of imaging requests in that traffic

  • Have frequent cache flushes or be configured to not cache imaging requests

Memory usage metrics

We have many metrics that monitor the memory used by the JVM and Magnolia pod:

  • jvm_memory_bytes_used{area="heap"}: how much memory on the heap is being used

  • jvm_memory_bytes_used{area="nonheap"}: how much off heap memory is being used

  • jvm_memory_bytes_committed{area="heap"}: how much memory the JVM currently has allocated for the heap

  • jvm_memory_bytes_committed{area="nonheap"}: how much memory the JVM currently has allocated for off heap usage

  • jvm_memory_bytes_max{area="heap"}: the maximum size of the JVM heap

  • jvm_memory_bytes_max{area="nonheap"}: NOT the maximum size of the off heap memory, is always -1!

All JVM memory metrics above do not include direct memory usage (off heap) memory used by the JVM. They can’t be used to determine what is consuming heap and off heap memory in the JVM.

Usage metrics

There are some things to keep in mind about the above metrics.

  • The JVM memory metrics are collected every 60 seconds (1m). They may not reflect sudden spikes in memory usage.

  • The jvm_memory_bytes_used (heap and nonheap) metric is memory being used, but it’s not the memory actually being used by the JVM itself. jvm_memory_bytes_committed is the amount of memory used by the JVM for heap and off-heap memory.

    • Heap memory

    • Off heap memory

    jvm_memory_bytes_used < jvm_memory_bytes_committed ≤ jvm_memory_bytes_max (1)
    1 The jvm_memory_bytes_max{area="heap"} metric is the maximum size of the JVM heap (as controlled by the JVM options in the Helm chart values). 
    jvm_memory_bytes_used < jvm_memory_bytes_committed (1)

    <1>

Pod and container combined memory

There is another metric that determines the memory used the Magnolia pod and its containers:

  • container_memory_working_set_bytes

The same caveats for the JVM memory metrics apply to container_memory_working_set_bytes. The container_memory_working_set_bytes metric is collected every 60 seconds and may not reflect sudden spikes in memory usage.

The container_memory_working_set_bytes metric shows memory usage by container for a pod; it does not know what the memory is being used for (heap, off heap, etc).

container_memory_working_set_bytes seems to be a lagging indicator in that its value doesn’t actually exceed the memory limit for the pod.

Solutions

This section provides solutions that should help resolve the issue in most cases.

Limit the direct (off heap) memory

Limit the direct (off heap) memory available to the JVM as this is the best way to prevent OOMKills. The JVM command line option to set a limit for direct memory is:

-XX:MaxDirectMemorySize=<size> (1)
1 where <size> is 1[k|K|m|M|g|G] or greater. See IBM’s xxmaxdirectmemorysize for more details.

Limit memory

The memory limit should be set so that the max heap + max direct memory + a reasonable surplus (for other JVM off heap memory use) should equal less than the memory limit for the Magnolia pod specified in the Helm chart values.

A "reasonable surplus" for additional off heap memory for JVM can’t really be determined without turning on Java Native Memory Tracking and profiling the memory used while Magnolia is running.

We recommend a minimum surplus/reserve of at least 500MB.

Table 1. Example calculation
Memory type Amount

JVM Max Heap

7.2 GB

Direct memory

3.8 GB

Surplus

1 GB

Total

12 GB

Set via Helm chart

The Helm chart does not have a value for setting MaxDirectMemorySize but you can set it with the CATALINA_OPTS_EXTRA environment variable in the values.yml file used for Magnolia.

  • Public

  • Author

magnoliaPublic:
  env:
    - name: CATALINA_OPTS_EXTRA
      value: "-XX:MaxDirectMemorySize=300m"
magnoliaAuthor:
  env:
    - name: CATALINA_OPTS_EXTRA
      value: "-XX:MaxDirectMemorySize=300m"

Other tips for resolving

The default MaxDirectMemorySize is the max size of the JVM heap. You could prevent OOMKills by setting the JVM_RAM_MAX_PERCENTAGE to something less than 50% (remember to leave something for other off heap memory usage by the JVM).

If the non-JVM memory available to the Magnolia pod is less than 2Gi, adjust the memory request/limit and the JVM_RAM_MAX_PERCENTAGE value to allow at least 2Gi of memory not use by the Magnolia JVM.

Also make sure the Magnolia JVM also has sufficient memory:

  • 8gb or more max heap for a Magnolia public instance

  • 10gb or more max heap for a Magnolia author instance

Since the JVM_RAM_MAX_PERCENTAGE and memory request/limit settings are controlled by the Magnolia Helm chart, you must adjust the values and redeploy Magnolia.
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