Node-pressure Eviction

Node-pressure eviction is the process by which the kubelet proactively terminates pods to reclaim resource on nodes.

The kubelet monitors resources like memory, disk space, and filesystem inodes on your cluster's nodes. When one or more of these resources reach specific consumption levels, the kubelet can proactively fail one or more pods on the node to reclaim resources and prevent starvation.

During a node-pressure eviction, the kubelet sets the phase for the selected pods to Failed, and terminates the Pod.

Node-pressure eviction is not the same as API-initiated eviction.

The kubelet does not respect your configured PodDisruptionBudget or the pod's terminationGracePeriodSeconds. If you use soft eviction thresholds, the kubelet respects your configured eviction-max-pod-grace-period. If you use hard eviction thresholds, the kubelet uses a 0s grace period (immediate shutdown) for termination.

Self healing behavior

The kubelet attempts to reclaim node-level resources before it terminates end-user pods. For example, it removes unused container images when disk resources are starved.

If the pods are managed by a workload management object (such as StatefulSet or Deployment) that replaces failed pods, the control plane (kube-controller-manager) creates new pods in place of the evicted pods.

Self healing for static pods

If you are running a static pod on a node that is under resource pressure, the kubelet may evict that static Pod. The kubelet then tries to create a replacement, because static Pods always represent an intent to run a Pod on that node.

The kubelet takes the priority of the static pod into account when creating a replacement. If the static pod manifest specifies a low priority, and there are higher-priority Pods defined within the cluster's control plane, and the node is under resource pressure, the kubelet may not be able to make room for that static pod. The kubelet continues to attempt to run all static pods even when there is resource pressure on a node.

Eviction signals and thresholds

The kubelet uses various parameters to make eviction decisions, like the following:

• Eviction signals
• Eviction thresholds
• Monitoring intervals

Eviction signals

Eviction signals are the current state of a particular resource at a specific point in time. The kubelet uses eviction signals to make eviction decisions by comparing the signals to eviction thresholds, which are the minimum amount of the resource that should be available on the node.

The kubelet uses the following eviction signals:

Eviction Signal	Description	Linux Only
memory.available	memory.available := node.status.capacity[memory] - node.stats.memory.workingSet
nodefs.available	nodefs.available := node.stats.fs.available
nodefs.inodesFree	nodefs.inodesFree := node.stats.fs.inodesFree	•
imagefs.available	imagefs.available := node.stats.runtime.imagefs.available
imagefs.inodesFree	imagefs.inodesFree := node.stats.runtime.imagefs.inodesFree	•
containerfs.available	containerfs.available := node.stats.runtime.containerfs.available
containerfs.inodesFree	containerfs.inodesFree := node.stats.runtime.containerfs.inodesFree	•
pid.available	pid.available := node.stats.rlimit.maxpid - node.stats.rlimit.curproc	•

In this table, the Description column shows how kubelet gets the value of the signal. Each signal supports either a percentage or a literal value. The kubelet calculates the percentage value relative to the total capacity associated with the signal.

Memory signals

On Linux nodes, the value for memory.available is derived from the cgroupfs instead of tools like free -m. This is important because free -m does not work in a container, and if users use the node allocatable feature, out of resource decisions are made local to the end user Pod part of the cgroup hierarchy as well as the root node. This script or cgroupv2 script reproduces the same set of steps that the kubelet performs to calculate memory.available. The kubelet excludes inactive_file (the number of bytes of file-backed memory on the inactive LRU list) from its calculation, as it assumes that memory is reclaimable under pressure.

Note:

        <div class="feature-state-notice feature-beta" title="Feature Gate: HugepageAwareEviction">
          <span class="feature-state-name">Feature state:</span>
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             <span class="feature-state-stage">Beta</span> since Kubernetes v1.37; enabled by default
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On nodes with hugepages configured, the kubelet subtracts the node's total hugepage capacity from memory.available used by eviction manager. Without this adjustment, hugepage-reserved RAM inflates AvailableBytes because the memory cgroup controller does not track hugetlb allocations in its working set. This can delay eviction and lead to OOM kills. To restore the previous behavior, disable the HugepageAwareEviction feature gate.

On Windows nodes, the value for memory.available is derived from the node's global memory commit levels (queried through the GetPerformanceInfo() system call) by subtracting the node's global CommitTotal from the node's CommitLimit. Please note that CommitLimit can change if the node's page-file size changes!

Filesystem signals

The kubelet recognizes three specific filesystem identifiers that can be used with eviction signals (<identifier>.inodesFree or <identifier>.available):

1. nodefs: The node's main filesystem, used for local disk volumes, emptyDir volumes not backed by memory, log storage, ephemeral storage, and more. For example, nodefs contains /var/lib/kubelet.

2. imagefs: An optional filesystem that container runtimes can use to store container images (which are the read-only layers). If there is no separate containerfs, the image filesystem also stores container writable layers.

3. containerfs: An optional filesystem that container runtimes can use to store container writable layers. When containerfs is used, the imagefs filesystem can be split to only store images (read-only layers) and nothing else.

These identifiers describe the filesystems as the kubelet observes them. They do not always mean three different mount points: in common layouts, two or all three identifiers can refer to the same underlying filesystem.

Note:

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          <span class="feature-state-name">Feature state:</span>
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             <span class="feature-state-stage">Beta</span> since Kubernetes v1.31; enabled by default
           </span>
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The split image filesystem feature adds new eviction signals, thresholds, and metrics for containerfs. To use containerfs, the Kubernetes release v1.37 requires the KubeletSeparateDiskGC feature gate to be enabled. For Kubernetes v1.37, only CRI-O (v1.29 or higher) offers containerfs filesystem support.

The kubelet supports three common layouts for container filesystems:

• Everything is on the single nodefs, also referred to as "rootfs" or simply "root". In this layout, nodefs, imagefs, and containerfs all refer to the same underlying filesystem.

• Container runtime storage is on a dedicated disk, separate from the root filesystem. In this layout, imagefs and containerfs refer to the same underlying filesystem, which stores both image layers and container writable layers. This is often referred to as "split disk" (or "separate disk") filesystem.

• Container writable layers are on nodefs, and the container images (read-only layers) are stored on a separate imagefs. In this layout, containerfs and nodefs refer to the same underlying filesystem. This is often referred to as a "split image" filesystem.

The kubelet will attempt to auto-discover these filesystems with their current configuration directly from the underlying container runtime and will ignore other local node filesystems.

The kubelet does not support other container filesystems or storage configurations, and it does not currently support multiple filesystems for images and containers.

Deprecated kubelet garbage collection features

Some kubelet garbage collection features are deprecated in favor of eviction:

Existing Flag	Rationale
--maximum-dead-containers	deprecated once old logs are stored outside of container's context
--maximum-dead-containers-per-container	deprecated once old logs are stored outside of container's context
--minimum-container-ttl-duration	deprecated once old logs are stored outside of container's context

Eviction thresholds

You can specify custom eviction thresholds for the kubelet to use when it makes eviction decisions. You can configure soft and hard eviction thresholds.

Eviction thresholds have the form [eviction-signal][operator][quantity], where:

• eviction-signal is the eviction signal to use.
• operator is the relational operator you want, such as < (less than).
• quantity is the eviction threshold amount, such as 1Gi. The value of quantity must match the quantity representation used by Kubernetes. You can use either literal values or percentages (%).

For example, if a node has 10GiB of total memory and you want trigger eviction if the available memory falls below 1GiB, you can define the eviction threshold as either memory.available<10% or memory.available<1Gi (you cannot use both).

Soft eviction thresholds

A soft eviction threshold pairs an eviction threshold with a required administrator-specified grace period. The kubelet does not evict pods until the grace period is exceeded. The kubelet returns an error on startup if you do not specify a grace period.

You can specify both a soft eviction threshold grace period and a maximum allowed pod termination grace period for kubelet to use during evictions. If you specify a maximum allowed grace period and the soft eviction threshold is met, the kubelet uses the lesser of the two grace periods. If you do not specify a maximum allowed grace period, the kubelet kills evicted pods immediately without graceful termination.

You can use the following flags to configure soft eviction thresholds:

• eviction-soft: A set of eviction thresholds like memory.available<1.5Gi that can trigger pod eviction if held over the specified grace period.
• eviction-soft-grace-period: A set of eviction grace periods like memory.available=1m30s that define how long a soft eviction threshold must hold before triggering a Pod eviction.
• eviction-max-pod-grace-period: The maximum allowed grace period (in seconds) to use when terminating pods in response to a soft eviction threshold being met.

Hard eviction thresholds

A hard eviction threshold has no grace period. When a hard eviction threshold is met, the kubelet kills pods immediately without graceful termination to reclaim the starved resource.

You can use the eviction-hard flag to configure a set of hard eviction thresholds like memory.available<1Gi.

The kubelet has the following default hard eviction thresholds:

• memory.available<100Mi (Linux nodes)
• memory.available<500Mi (Windows nodes)
• nodefs.available<10%
• imagefs.available<15%
• nodefs.inodesFree<5% (Linux nodes)
• imagefs.inodesFree<5% (Linux nodes)

These default values of hard eviction thresholds will only be set if none of the parameters is changed. If you change the value of any parameter, then the values of other parameters will not be inherited as the default values and will be set to zero. In order to provide custom values, you should provide all the thresholds respectively. You can also set the kubelet config MergeDefaultEvictionSettings to true in the kubelet configuration file. If set to true and any parameter is changed, then the other parameters will inherit their default values instead of 0.

The containerfs.available and containerfs.inodesFree (Linux nodes) default eviction thresholds will be set as follows:

• If containerfs and nodefs refer to the same underlying filesystem, then containerfs thresholds are set the same as nodefs.

• If containerfs and imagefs refer to the same underlying filesystem, then containerfs thresholds are set the same as imagefs.

Setting custom overrides for thresholds related to containerfs is not supported, and a warning will be issued if an attempt to do so is made; any provided custom values will be ignored.

Eviction monitoring interval

The kubelet evaluates eviction thresholds based on its configured housekeeping-interval, which defaults to 10s.

Node conditions

The kubelet reports node conditions to reflect that the node is under pressure because hard or soft eviction threshold is met, independent of configured grace periods.

The kubelet maps eviction signals to node conditions as follows:

Node Condition	Eviction Signal	Description
MemoryPressure	memory.available	Available memory on the node has satisfied an eviction threshold
DiskPressure	nodefs.available, nodefs.inodesFree, imagefs.available, imagefs.inodesFree, containerfs.available, or containerfs.inodesFree	Available disk space and inodes on either the node's root filesystem, image filesystem, or container filesystem has satisfied an eviction threshold
PIDPressure	pid.available	Available processes identifiers on the (Linux) node has fallen below an eviction threshold

The control plane also maps these node conditions to taints.

The kubelet updates the node conditions based on the configured --node-status-update-frequency, which defaults to 10s.

Node condition oscillation

In some cases, nodes oscillate above and below soft eviction thresholds without holding for the defined grace periods. This causes the reported node condition to constantly switch between true and false, leading to bad eviction decisions.

To protect against oscillation, you can use the eviction-pressure-transition-period flag, which controls how long the kubelet must wait before transitioning a node condition to a different state. The transition period has a default value of 5m.

Reclaiming node level resources

The kubelet tries to reclaim node-level resources before it evicts end-user pods.

When a DiskPressure node condition is reported, the kubelet reclaims node-level resources based on the filesystems on the node.

Without imagefs or containerfs

If the node only has a nodefs filesystem that meets eviction thresholds, the kubelet frees up disk space in the following order:

1. Garbage collect dead pods and containers.
2. Delete unused images.

With imagefs

If the node has a dedicated imagefs filesystem for container runtimes to use, the kubelet does the following:

• If the nodefs filesystem meets the eviction thresholds, the kubelet garbage collects dead pods and containers.

• If the imagefs filesystem meets the eviction thresholds, the kubelet deletes all unused images.

With imagefs and containerfs

If the node has a dedicated containerfs alongside the imagefs filesystem configured for the container runtimes to use, then kubelet will attempt to reclaim resources as follows:

• If the containerfs filesystem meets the eviction thresholds, the kubelet garbage collects dead pods and containers.

• If the imagefs filesystem meets the eviction thresholds, the kubelet deletes all unused images.

Pod selection for kubelet eviction

If the kubelet's attempts to reclaim node-level resources don't bring the eviction signal below the threshold, the kubelet begins to evict end-user pods.

The kubelet uses the following parameters to determine the pod eviction order:

1. Whether the pod's resource usage exceeds requests
2. Pod Priority
3. The pod's resource usage relative to requests

As a result, kubelet ranks and evicts pods in the following order:

1. BestEffort or Burstable pods where the usage exceeds requests. These pods are evicted based on their Priority and then by how much their usage level exceeds the request.

2. Guaranteed pods and Burstable pods where the usage is less than requests are evicted last, based on their Priority.

Note:

The kubelet does not use the pod's QoS class to determine the eviction order. You can use the QoS class to estimate the most likely pod eviction order when reclaiming resources like memory. QoS classification does not apply to EphemeralStorage requests, so the above scenario will not apply if the node is, for example, under DiskPressure.

Guaranteed pods are guaranteed only when requests and limits are specified for all the containers and they are equal. These pods will never be evicted because of another pod's resource consumption. If a system daemon (such as kubelet and journald) is consuming more resources than were reserved via system-reserved or kube-reserved allocations, and the node only has Guaranteed or Burstable pods using less resources than requests left on it, then the kubelet must choose to evict one of these pods to preserve node stability and to limit the impact of resource starvation on other pods. In this case, it will choose to evict pods of lowest Priority first.

If you are running a static pod and want to avoid having it evicted under resource pressure, set the priority field for that Pod directly. Static pods do not support the priorityClassName field.

When the kubelet evicts pods in response to inode or process ID starvation, it uses the Pods' relative priority to determine the eviction order, because inodes and PIDs have no requests.

The kubelet sorts pods differently based on whether the node has a dedicated imagefs or containerfs filesystem:

Without imagefs or containerfs (nodefs and imagefs use the same filesystem)

• If nodefs triggers evictions, the kubelet sorts pods based on their total disk usage (local volumes + logs and a writable layer of all containers).

With imagefs (nodefs and imagefs filesystems are separate)

• If nodefs triggers evictions, the kubelet sorts pods based on nodefs usage (local volumes + logs of all containers).

• If imagefs triggers evictions, the kubelet sorts pods based on the writable layer usage of all containers.

With imagefs and containerfs (imagefs and containerfs have been split)

• If containerfs triggers evictions, the kubelet sorts pods based on containerfs usage (local volumes + logs and a writable layer of all containers).

• If imagefs triggers evictions, the kubelet sorts pods based on the storage of images rank, which represents the disk usage of a given image.

Minimum eviction reclaim

Note:

As of Kubernetes v1.37, you cannot set a custom value for the containerfs.available metric. The configuration for this specific metric will be set automatically to reflect values set for either the nodefs or imagefs, depending on the configuration.

In some cases, pod eviction only reclaims a small amount of the starved resource. This can lead to the kubelet repeatedly hitting the configured eviction thresholds and triggering multiple evictions.

You can use the --eviction-minimum-reclaim flag or a kubelet config file to configure a minimum reclaim amount for each resource. When the kubelet notices that a resource is starved, it continues to reclaim that resource until it reclaims the quantity you specify.

For example, the following configuration sets minimum reclaim amounts:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
evictionHard:
  memory.available: "500Mi"
  nodefs.available: "1Gi"
  imagefs.available: "100Gi"
evictionMinimumReclaim:
  memory.available: "0Mi"
  nodefs.available: "500Mi"
  imagefs.available: "2Gi"

In this example, if the nodefs.available signal meets the eviction threshold, the kubelet reclaims the resource until the signal reaches the threshold of 1GiB, and then continues to reclaim the minimum amount of 500MiB, until the available nodefs storage value reaches 1.5GiB.

Similarly, the kubelet tries to reclaim the imagefs resource until the imagefs.available value reaches 102Gi, representing 102 GiB of available container image storage. If the amount of storage that the kubelet could reclaim is less than 2GiB, the kubelet doesn't reclaim anything.

The default eviction-minimum-reclaim is 0 for all resources.

Node out of memory behavior

If the node experiences an out of memory (OOM) event prior to the kubelet being able to reclaim memory, the node depends on the oom_killer to respond.

The kubelet sets an oom_score_adj value for each container based on the QoS for the pod.

Quality of Service	oom_score_adj
Guaranteed	-997
BestEffort	1000
Burstable	min(max(2, 1000 - (1000 × memoryRequestBytes) / machineMemoryCapacityBytes), 999)

Note:

The kubelet also sets an oom_score_adj value of -997 for any containers in Pods that have system-node-critical Priority.

If the kubelet can't reclaim memory before a node experiences OOM, the oom_killer calculates an oom_score based on the percentage of memory it's using on the node, and then adds the oom_score_adj to get an effective oom_score for each container. It then kills the container with the highest score.

This means that containers in low QoS pods that consume a large amount of memory relative to their scheduling requests are killed first.

Unlike pod eviction, if a container is OOM killed, the kubelet can restart it based on its restartPolicy.

Good practices

The following sections describe good practice for eviction configuration.

Schedulable resources and eviction policies

When you configure the kubelet with an eviction policy, you should make sure that the scheduler will not schedule pods if they will trigger eviction because they immediately induce memory pressure.

Consider the following scenario:

• Node memory capacity: 10GiB
• Operator wants to reserve 10% of memory capacity for system daemons (kernel, kubelet, etc.)
• Operator wants to evict Pods at 95% memory utilization to reduce incidence of system OOM.

For this to work, the kubelet is launched as follows:

--eviction-hard=memory.available<500Mi
--system-reserved=memory=1.5Gi

In this configuration, the --system-reserved flag reserves 1.5GiB of memory for the system, which is 10% of the total memory + the eviction threshold amount.

The node can reach the eviction threshold if a pod is using more than its request, or if the system is using more than 1GiB of memory, which makes the memory.available signal fall below 500MiB and triggers the threshold.

DaemonSets and node-pressure eviction

Pod priority is a major factor in making eviction decisions. If you do not want the kubelet to evict pods that belong to a DaemonSet, give those pods a high enough priority by specifying a suitable priorityClassName in the pod spec. You can also use a lower priority, or the default, to only allow pods from that DaemonSet to run when there are enough resources.

Known issues

The following sections describe known issues related to out of resource handling.

kubelet may not observe memory pressure right away

By default, the kubelet polls cAdvisor to collect memory usage stats at a regular interval. If memory usage increases within that window rapidly, the kubelet may not observe MemoryPressure fast enough, and the OOM killer will still be invoked.

You can use the --kernel-memcg-notification flag to enable the memcg notification API on the kubelet to get notified immediately when a threshold is crossed.

If you are not trying to achieve extreme utilization, but a sensible measure of overcommit, a viable workaround for this issue is to use the --kube-reserved and --system-reserved flags to allocate memory for the system.

active_file memory is not considered as available memory

On Linux, the kernel tracks the number of bytes of file-backed memory on active least recently used (LRU) list as the active_file statistic. The kubelet treats active_file memory areas as not reclaimable. For workloads that make intensive use of block-backed local storage, including ephemeral local storage, kernel-level caches of file and block data means that many recently accessed cache pages are likely to be counted as active_file. If enough of these kernel block buffers are on the active LRU list, the kubelet is liable to observe this as high resource use and taint the node as experiencing memory pressure - triggering pod eviction.

For more details, see https://github.com/kubernetes/kubernetes/issues/43916

You can work around that behavior by setting the memory limit and memory request the same for containers likely to perform intensive I/O activity. You will need to estimate or measure an optimal memory limit value for that container.