Configure Out Of Resource Handling
- Eviction Policy
- Node OOM Behavior
- Best Practices
- Deprecation of existing feature flags to reclaim disk
- Known issues
This page explains how to configure out of resource handling with kubelet.
The kubelet needs to preserve node stability when available compute resources
are low. This is especially important when dealing with incompressible
compute resources, such as memory or disk space. If such resources are exhausted,
nodes become unstable.
Eviction Policy
The kubelet can proactively monitor for and prevent total starvation of a
compute resource. In those cases, the kubelet can reclaim the starved
resource by proactively failing one or more Pods. When the kubelet fails
a Pod, it terminates all of its containers and transitions its PodPhase to Failed.
Eviction Signals
The kubelet supports eviction decisions based on the signals described in the following
table. The value of each signal is described in the Description column, which is based on
the kubelet summary API.
| Eviction Signal | Description |
|---|---|
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 |
Each of the above signals supports either a literal or percentage based value. The percentage based value is calculated relative to the total capacity associated with each signal.
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
reproduces the same set of steps that the kubelet performs to calculate
memory.available. The kubelet excludes inactive_file (i.e. # of bytes of
file-backed memory on inactive LRU list) from its calculation as it assumes that
memory is reclaimable under pressure.
kubelet supports only two filesystem partitions.
- The
nodefsfilesystem that kubelet uses for volumes, daemon logs, etc. - The
imagefsfilesystem that container runtimes uses for storing images and container writable layers.
imagefs is optional. kubelet auto-discovers these filesystems using
cAdvisor. kubelet does not care about any other filesystems. Any other types
of configurations are not currently supported by the kubelet. For example, it is
not OK to store volumes and logs in a dedicated filesystem.
In future releases, the kubelet will deprecate the existing garbage
collection
support in favor of eviction in response to disk pressure.
Eviction Thresholds
The kubelet supports the ability to specify eviction thresholds that trigger the kubelet to reclaim resources.
Each threshold has the following form:
[eviction-signal][operator][quantity]
where:
eviction-signalis an eviction signal token as defined in the previous table.operatoris the desired relational operator, such as<(less than).quantityis the eviction threshold quantity, such as1Gi. These tokens must match the quantity representation used by Kubernetes. An eviction threshold can also be expressed as a percentage using the%token.
For example, if a node has 10Gi of total memory and you want trigger eviction if
the available memory falls below 1Gi, 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. No action is taken by the kubelet
to reclaim resources associated with the eviction signal until that grace
period has been exceeded. If no grace period is provided, the kubelet
returns an error on startup.
In addition, if a soft eviction threshold has been met, an operator can
specify a maximum allowed Pod termination grace period to use when evicting
pods from the node. If specified, the kubelet uses the lesser value among
the pod.Spec.TerminationGracePeriodSeconds and the max allowed grace period.
If not specified, the kubelet kills Pods immediately with no graceful
termination.
To configure soft eviction thresholds, the following flags are supported:
eviction-softdescribes a set of eviction thresholds (e.g.memory.available<1.5Gi) that if met over a corresponding grace period would trigger a Pod eviction.eviction-soft-grace-perioddescribes a set of eviction grace periods (e.g.memory.available=1m30s) that correspond to how long a soft eviction threshold must hold before triggering a Pod eviction.eviction-max-pod-grace-perioddescribes 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, and if observed, the kubelet
will take immediate action to reclaim the associated starved resource. If a
hard eviction threshold is met, the kubelet kills the Pod immediately
with no graceful termination.
To configure hard eviction thresholds, the following flag is supported:
eviction-harddescribes a set of eviction thresholds (e.g.memory.available<1Gi) that if met would trigger a Pod eviction.
The kubelet has the following default hard eviction threshold:
--eviction-hard=memory.available<100Mi
Eviction Monitoring Interval
The kubelet evaluates eviction thresholds per its configured housekeeping interval.
housekeeping-intervalis the interval between container housekeepings.
Node Conditions
The kubelet maps one or more eviction signals to a corresponding node condition.
If a hard eviction threshold has been met, or a soft eviction threshold has been met
independent of its associated grace period, the kubelet reports a condition that
reflects the node is under pressure.
The following node conditions are defined that correspond to the specified eviction signal.
| 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, or imagefs.inodesFree |
Available disk space and inodes on either the node’s root filesystem or image filesystem has satisfied an eviction threshold |
The kubelet continues to report node status updates at the frequency specified by
--node-status-update-frequency which defaults to 10s.
Oscillation of node conditions
If a node is oscillating above and below a soft eviction threshold, but not exceeding its associated grace period, it would cause the corresponding node condition to constantly oscillate between true and false, and could cause poor scheduling decisions as a consequence.
To protect against this oscillation, the following flag is defined to control how
long the kubelet must wait before transitioning out of a pressure condition.
eviction-pressure-transition-periodis the duration for which thekubelethas to wait before transitioning out of an eviction pressure condition.
The kubelet would ensure that it has not observed an eviction threshold being met
for the specified pressure condition for the period specified before toggling the
condition back to false.
Reclaiming node level resources
If an eviction threshold has been met and the grace period has passed,
the kubelet initiates the process of reclaiming the pressured resource
until it has observed the signal has gone below its defined threshold.
The kubelet attempts to reclaim node level resources prior to evicting end-user Pods. If
disk pressure is observed, the kubelet reclaims node level resources differently if the
machine has a dedicated imagefs configured for the container runtime.
With imagefs
If nodefs filesystem has met eviction thresholds, kubelet frees up disk space by deleting the dead Pods and their containers.
If imagefs filesystem has met eviction thresholds, kubelet frees up disk space by deleting all unused images.
Without imagefs
If nodefs filesystem has met eviction thresholds, kubelet frees up disk space in the following order:
- Delete dead Pods and their containers
- Delete all unused images
Evicting end-user Pods
If the kubelet is unable to reclaim sufficient resource on the node, kubelet begins evicting Pods.
The kubelet ranks Pods for eviction first by whether or not their usage of the starved resource exceeds requests,
then by Priority, and then by the consumption of the starved compute resource relative to the Pods’ scheduling requests.
As a result, kubelet ranks and evicts Pods in the following order:
BestEffortorBurstablePods whose usage of a starved resource exceeds its request. Such pods are ranked by Priority, and then usage above request.Guaranteedpods andBurstablepods whose usage is beneath requests are evicted last.GuaranteedPods are guaranteed only when requests and limits are specified for all the containers and they are equal. Such pods are guaranteed to never be evicted because of another Pod’s resource consumption. If a system daemon (such askubelet,docker, andjournald) is consuming more resources than were reserved viasystem-reservedorkube-reservedallocations, and the node only hasGuaranteedorBurstablePods using less than requests remaining, then the node must choose to evict such a Pod in order to preserve node stability and to limit the impact of the unexpected consumption to other Pods. In this case, it will choose to evict pods of Lowest Priority first.
If necessary, kubelet evicts Pods one at a time to reclaim disk when DiskPressure
is encountered. If the kubelet is responding to inode starvation, it reclaims
inodes by evicting Pods with the lowest quality of service first. If the kubelet
is responding to lack of available disk, it ranks Pods within a quality of service
that consumes the largest amount of disk and kill those first.
With imagefs
If nodefs is triggering evictions, kubelet sorts Pods based on the usage on nodefs
- local volumes + logs of all its containers.
If imagefs is triggering evictions, kubelet sorts Pods based on the writable layer usage of all its containers.
Without imagefs
If nodefs is triggering evictions, kubelet sorts Pods based on their total disk usage
- local volumes + logs & writable layer of all its containers.
Minimum eviction reclaim
In certain scenarios, eviction of Pods could result in reclamation of small amount of resources. This can result in
kubelet hitting eviction thresholds in repeated successions. In addition to that, eviction of resources like disk,
is time consuming.
To mitigate these issues, kubelet can have a per-resource minimum-reclaim. Whenever kubelet observes
resource pressure, kubelet attempts to reclaim at least minimum-reclaim amount of resource below
the configured eviction threshold.
For example, with the following configuration:
--eviction-hard=memory.available<500Mi,nodefs.available<1Gi,imagefs.available<100Gi
--eviction-minimum-reclaim="memory.available=0Mi,nodefs.available=500Mi,imagefs.available=2Gi"`
If an eviction threshold is triggered for memory.available, the kubelet works to ensure
that memory.available is at least 500Mi. For nodefs.available, the kubelet works
to ensure that nodefs.available is at least 1.5Gi, and for imagefs.available it
works to ensure that imagefs.available is at least 102Gi before no longer reporting pressure
on their associated resources.
The default eviction-minimum-reclaim is 0 for all resources.
Scheduler
The node reports a condition when a compute resource is under pressure. The scheduler views that condition as a signal to dissuade placing additional pods on the node.
| Node Condition | Scheduler Behavior |
|---|---|
MemoryPressure |
No new BestEffort Pods are scheduled to the node. |
DiskPressure |
No new Pods are scheduled to the node. |
Node OOM Behavior
If the node experiences a system OOM (out of memory) event prior to the kubelet is able to reclaim memory,
the node depends on the oom_killer to respond.
The kubelet sets a oom_score_adj value for each container based on the quality of service for the Pod.
| Quality of Service | oom_score_adj |
|---|---|
Guaranteed |
-998 |
BestEffort |
1000 |
Burstable |
min(max(2, 1000 - (1000 * memoryRequestBytes) / machineMemoryCapacityBytes), 999) |
If the kubelet is unable to reclaim memory prior to a node experiencing system OOM, the oom_killer calculates
an oom_score based on the percentage of memory it’s using on the node, and then add the oom_score_adj to get an
effective oom_score for the container, and then kills the container with the highest score.
The intended behavior should be that containers with the lowest quality of service that are consuming the largest amount of memory relative to the scheduling request should be killed first in order to reclaim memory.
Unlike Pod eviction, if a Pod container is OOM killed, it may be restarted by the kubelet based on its RestartPolicy.
Best Practices
The following sections describe best practices for out of resource handling.
Schedulable resources and eviction policies
Consider the following scenario:
- Node memory capacity:
10Gi - 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 thrashing and incidence of system OOM.
To facilitate this scenario, the kubelet would be launched as follows:
--eviction-hard=memory.available<500Mi
--system-reserved=memory=1.5Gi
Implicit in this configuration is the understanding that “System reserved” should include the amount of memory covered by the eviction threshold.
To reach that capacity, either some Pod is using more than its request, or the system is using more than 1.5Gi - 500Mi = 1Gi.
This configuration ensures that the scheduler does not place Pods on a node that immediately induce memory pressure and trigger eviction assuming those Pods use less than their configured request.
DaemonSet
It is never desired for kubelet to evict a DaemonSet Pod, since the Pod is
immediately recreated and rescheduled back to the same node.
At the moment, the kubelet has no ability to distinguish a Pod created
from DaemonSet versus any other object. If/when that information is
available, the kubelet could pro-actively filter those Pods from the
candidate set of Pods provided to the eviction strategy.
In general, it is strongly recommended that DaemonSet not
create BestEffort Pods to avoid being identified as a candidate Pod
for eviction. Instead DaemonSet should ideally launch Guaranteed Pods.
Deprecation of existing feature flags to reclaim disk
kubelet has been freeing up disk space on demand to keep the node stable.
As disk based eviction matures, the following kubelet flags are marked for deprecation
in favor of the simpler configuration supported around eviction.
| Existing Flag | New Flag |
|---|---|
--image-gc-high-threshold |
--eviction-hard or eviction-soft |
--image-gc-low-threshold |
--eviction-minimum-reclaim |
--maximum-dead-containers |
deprecated |
--maximum-dead-containers-per-container |
deprecated |
--minimum-container-ttl-duration |
deprecated |
--low-diskspace-threshold-mb |
--eviction-hard or eviction-soft |
--outofdisk-transition-frequency |
--eviction-pressure-transition-period |
Known issues
The following sections describe known issues related to out of resource handling.
kubelet may not observe memory pressure right away
The kubelet currently 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 OOMKiller
will still be invoked. We intend to integrate with the memcg notification API in a future release to reduce this
latency, and instead have the kernel tell us when a threshold has been crossed immediately.
If you are not trying to achieve extreme utilization, but a sensible measure of overcommit, a viable workaround for this issue is to set eviction thresholds at approximately 75% capacity. This increases the ability of this feature to prevent system OOMs, and promote eviction of workloads so cluster state can rebalance.
kubelet may evict more Pods than needed
The Pod eviction may evict more Pods than needed due to stats collection timing gap. This can be mitigated by adding the ability to get root container stats on an on-demand basis (https://github.com/google/cadvisor/issues/1247) in the future.