Chapter 36. Application memory sizing
This page is intended to provide guidance to application developers using OpenShift Online on:
- Determining the memory and risk requirements of a containerized application component and configuring the container memory parameters to suit those requirements.
- Configuring containerized application runtimes (for example, OpenJDK) to adhere optimally to the configured container memory parameters.
- Diagnosing and resolving memory-related error conditions associated with running in a container.
It is recommended to read fully the overview of how OpenShift Online manages Compute Resources before proceeding.
For the purposes of sizing application memory, the key points are:
- For each kind of resource (memory, cpu, storage), OpenShift Online allows optional request and limit values to be placed on each container in a pod. For the purposes of this page, we are solely interested in memory requests and memory limits.
- The memory request value, if specified, influences the OpenShift Online scheduler. The scheduler considers the memory request when scheduling a container to a node, then fences off the requested memory on the chosen node for the use of the container.
- If a node’s memory is exhausted, OpenShift Online prioritizes evicting its containers whose memory usage most exceeds their memory request. In serious cases of memory exhaustion, the node OOM killer may select and kill a process in a container based on a similar metric.
- The memory limit value, if specified, provides a hard limit on the memory that can be allocated across all the processes in a container.
- If the memory allocated by all of the processes in a container exceeds the memory limit, the node OOM killer will immediately select and kill a process in the container.
- If both memory request and limit are specified, the memory limit value must be greater than or equal to the memory request.
- The cluster administrator may assign quota against the memory request value, limit value, both, or neither.
- The cluster administrator may assign default values for the memory request value, limit value, both, or neither.
- The cluster administrator may override the memory request values that a developer specifies, in order to manage cluster overcommit. This occurs on OpenShift Online, for example.
The steps for sizing application memory on OpenShift Online are as follows:
Determine expected container memory usage
Determine expected mean and peak container memory usage, empirically if necessary (for example, by separate load testing). Remember to consider all the processes that may potentially run in parallel in the container: for example, does the main application spawn any ancillary scripts?
Determine risk appetite
Determine risk appetite for eviction. If the risk appetite is low, the container should request memory according to the expected peak usage plus a percentage safety margin. If the risk appetite is higher, it may be more appropriate to request memory according to the expected mean usage.
Set container memory request
Set container memory request based on the above. The more accurately the request represents the application memory usage, the better. If the request is too high, cluster and quota usage will be inefficient. If the request is too low, the chances of application eviction increase.
Set container memory limit, if required
Set container memory limit, if required. Setting a limit has the effect of immediately killing a container process if the combined memory usage of all processes in the container exceeds the limit, and is therefore a mixed blessing. On the one hand, it may make unanticipated excess memory usage obvious early ("fail fast"); on the other hand it also terminates processes abruptly.
Note that some OpenShift Online clusters may require a limit value to be set; some may override the request based on the limit; and some application images rely on a limit value being set as this is easier to detect than a request value.
If the memory limit is set, it should not be set to less than the expected peak container memory usage plus a percentage safety margin.
Ensure application is tuned
Ensure application is tuned with respect to configured request and limit values, if appropriate. This step is particularly relevant to applications which pool memory, such as the JVM. The rest of this page discusses this.
36.4. Sizing OpenJDK on OpenShift Online
The default OpenJDK settings unfortunately do not work well with containerized environments, with the result that as a rule, some additional Java memory settings must always be provided whenever running the OpenJDK in a container.
The JVM memory layout is complex, version dependent, and describing it in detail is beyond the scope of this documentation. However, as a starting point for running OpenJDK in a container, at least the following three memory-related tasks are key:
- Overriding the JVM maximum heap size.
- Encouraging the JVM to release unused memory to the operating system, if appropriate.
- Ensuring all JVM processes within a container are appropriately configured.
Optimally tuning JVM workloads for running in a container is beyond the scope of this documentation, and may involve setting multiple additional JVM options.
36.4.1. Overriding the JVM Maximum Heap Size
For many Java workloads, the JVM heap is the largest single consumer of memory. Currently, the OpenJDK defaults to allowing up to 1/4 (1/
-XX:MaxRAMFraction) of the compute node’s memory to be used for the heap, regardless of whether the OpenJDK is running in a container or not. It is therefore essential to override this behaviour, especially if a container memory limit is also set.
There are at least two ways the above can be achieved:
If the container memory limit is set and the experimental options are supported by the JVM, set
-XX:MaxRAMto the container memory limit, and the maximum heap size (
-Xmx) to 1/
-XX:MaxRAMFraction(1/4 by default).
Directly override one of
This option involves hard-coding a value, but has the advantage of allowing a safety margin to be calculated.
36.4.2. Encouraging the JVM to Release Unused Memory to the Operating System
By default, the OpenJDK does not aggressively return unused memory to the operating system. This may be appropriate for many containerized Java workloads, but notable exceptions include workloads where additional active processes co-exist with a JVM within a container, whether those additional processes are native, additional JVMs, or a combination of the two.
The OpenShift Online Jenkins maven slave image uses the following JVM arguments to encourage the JVM to release unused memory to the operating system:
-XX:+UseParallelGC -XX:MinHeapFreeRatio=5 -XX:MaxHeapFreeRatio=10 -XX:GCTimeRatio=4 -XX:AdaptiveSizePolicyWeight=90. These arguments are intended to return heap memory to the operating system whenever allocated memory exceeds 110% of in-use memory (
-XX:MaxHeapFreeRatio), spending up to 20% of CPU time in the garbage collector (
-XX:GCTimeRatio). At no time will the application heap allocation be less than the initial heap allocation (overridden by
-Xms). Detailed additional information is available Tuning Java’s footprint in OpenShift (Part 1), Tuning Java’s footprint in OpenShift (Part 2), and at OpenJDK and Containers.
36.4.3. Ensuring All JVM Processes Within a Container Are Appropriately Configured
In the case that multiple JVMs run in the same container, it is essential to ensure that they are all configured appropriately. For many workloads it will be necessary to grant each JVM a percentage memory budget, leaving a perhaps substantial additional safety margin.
Many Java tools use different environment variables (
MAVEN_OPTS, and so on) to configure their JVMs and it can be challenging to ensure that the right settings are being passed to the right JVM.
JAVA_TOOL_OPTIONS environment variable is always respected by the OpenJDK, and values specified in
JAVA_TOOL_OPTIONS will be overridden by other options specified on the JVM command line. By default, the OpenShift Online Jenkins maven slave image sets
JAVA_TOOL_OPTIONS="-XX:+UnlockExperimentalVMOptions -XX:+UseCGroupMemoryLimitForHeap -Dsun.zip.disableMemoryMapping=true" to ensure that these options are used by default for all JVM workloads run in the slave image. This does not guarantee that additional options are not required, but is intended to be a helpful starting point.
36.5. Finding the Memory Request and Limit From Within a Pod
An application wishing to dynamically discover its memory request and limit from within a pod should use the Downward API. The following snippet shows how this is done.
apiVersion: v1 kind: Pod metadata: name: test spec: containers: - name: test image: fedora:latest command: - sleep - "3600" env: - name: MEMORY_REQUEST valueFrom: resourceFieldRef: containerName: test resource: requests.memory - name: MEMORY_LIMIT valueFrom: resourceFieldRef: containerName: test resource: limits.memory resources: requests: memory: 384Mi limits: memory: 512Mi
# oc rsh test $ env | grep MEMORY | sort MEMORY_LIMIT=536870912 MEMORY_REQUEST=402653184
The memory limit value can also be read from inside the container by the
36.6. Diagnosing an OOM Kill
OpenShift Online may kill a process in a container if the total memory usage of all the processes in the container exceeds the memory limit, or in serious cases of node memory exhaustion.
When a process is OOM killed, this may or may not result in the container exiting immediately. If the container PID 1 process receives the SIGKILL, the container will exit immediately. Otherwise, the container behavior is dependent on the behavior of the other processes.
If the container does not exit immediately, an OOM kill is detectable as follows:
- A container process exited with code 137, indicating it received a SIGKILL signal
The oom_kill counter in
$ grep '^oom_kill ' /sys/fs/cgroup/memory/memory.oom_control oom_kill 0 $ sed -e '' </dev/zero # provoke an OOM kill Killed $ echo $? 137 $ grep '^oom_kill ' /sys/fs/cgroup/memory/memory.oom_control oom_kill 1
If one or more processes in a pod are OOM killed, when the pod subsequently exits, whether immediately or not, it will have phase Failed and reason OOMKilled. An OOM killed pod may be restarted depending on the value of
restartPolicy. If not restarted, controllers such as the ReplicationController will notice the pod’s failed status and create a new pod to replace the old one.
If not restarted, the pod status is as follows:
$ oc get pod test NAME READY STATUS RESTARTS AGE test 0/1 OOMKilled 0 1m $ oc get pod test -o yaml ... status: containerStatuses: - name: test ready: false restartCount: 0 state: terminated: exitCode: 137 reason: OOMKilled phase: Failed
If restarted, its status is as follows:
$ oc get pod test NAME READY STATUS RESTARTS AGE test 1/1 Running 1 1m $ oc get pod test -o yaml ... status: containerStatuses: - name: test ready: true restartCount: 1 lastState: terminated: exitCode: 137 reason: OOMKilled state: running: phase: Running
36.7. Diagnosing an Evicted Pod
OpenShift Online may evict a pod from its node when the node’s memory is exhausted. Depending on the extent of memory exhaustion, the eviction may or may not be graceful. Graceful eviction implies the main process (PID 1) of each container receiving a SIGTERM signal, then some time later a SIGKILL signal if the process hasn’t exited already. Non-graceful eviction implies the main process of each container immediately receiving a SIGKILL signal.
An evicted pod will have phase Failed and reason Evicted. It will not be restarted, regardless of the value of
restartPolicy. However, controllers such as the ReplicationController will notice the pod’s failed status and create a new pod to replace the old one.
$ oc get pod test NAME READY STATUS RESTARTS AGE test 0/1 Evicted 0 1m $ oc get pod test -o yaml ... status: message: 'Pod The node was low on resource: [MemoryPressure].' phase: Failed reason: Evicted