Kubernetes 1.27: Quality-of-Service for Memory Resources (alpha)

Kubernetes v1.27, released in April 2023, introduced changes to Memory QoS (alpha) to improve memory management capabilites in Linux nodes.

Support for Memory QoS was initially added in Kubernetes v1.22, and later some limitations around the formula for calculating memory.high were identified. These limitations are addressed in Kubernetes v1.27.

Background

Kubernetes allows you to optionally specify how much of each resources a container needs in the Pod specification. The most common resources to specify are CPU and Memory.

For example, a Pod manifest that defines container resource requirements could look like:

apiVersion: v1
kind: Pod
metadata:
  name: example
spec:
  containers:
  - name: nginx
    resources:
      requests:
        memory: "64Mi"
        cpu: "250m"
      limits:
        memory: "64Mi"
        cpu: "500m"

• spec.containers[].resources.requests

  When you specify the resource request for containers in a Pod, the Kubernetes scheduler uses this information to decide which node to place the Pod on. The scheduler ensures that for each resource type, the sum of the resource requests of the scheduled containers is less than the total allocatable resources on the node.

• spec.containers[].resources.limits

  When you specify the resource limit for containers in a Pod, the kubelet enforces those limits so that the running containers are not allowed to use more of those resources than the limits you set.

When the kubelet starts a container as a part of a Pod, kubelet passes the container's requests and limits for CPU and memory to the container runtime. The container runtime assigns both CPU request and CPU limit to a container. Provided the system has free CPU time, the containers are guaranteed to be allocated as much CPU as they request. Containers cannot use more CPU than the configured limit i.e. containers CPU usage will be throttled if they use more CPU than the specified limit within a given time slice.

Prior to Memory QoS feature, the container runtime only used the memory limit and discarded the memory request (requests were, and still are, also used to influence scheduling). If a container uses more memory than the configured limit, the Linux Out Of Memory (OOM) killer will be invoked.

Let's compare how the container runtime on Linux typically configures memory request and limit in cgroups, with and without Memory QoS feature:

• Memory request

  The memory request is mainly used by kube-scheduler during (Kubernetes) Pod scheduling. In cgroups v1, there are no controls to specify the minimum amount of memory the cgroups must always retain. Hence, the container runtime did not use the value of requested memory set in the Pod spec.

  cgroups v2 introduced a memory.min setting, used to specify the minimum amount of memory that should remain available to the processes within a given cgroup. If the memory usage of a cgroup is within its effective min boundary, the cgroup’s memory won’t be reclaimed under any conditions. If the kernel cannot maintain at least memory.min bytes of memory for the processes within the cgroup, the kernel invokes its OOM killer. In other words, the kernel guarantees at least this much memory is available or terminates processes (which may be outside the cgroup) in order to make memory more available. Memory QoS maps memory.min to spec.containers[].resources.requests.memory to ensure the availability of memory for containers in Kubernetes Pods.

• Memory limit

  The memory.limit specifies the memory limit, beyond which if the container tries to allocate more memory, Linux kernel will terminate a process with an OOM (Out of Memory) kill. If the terminated process was the main (or only) process inside the container, the container may exit.

  In cgroups v1, memory.limit_in_bytes interface is used to set the memory usage limit. However, unlike CPU, it was not possible to apply memory throttling: as soon as a container crossed the memory limit, it would be OOM killed.

  In cgroups v2, memory.max is analogous to memory.limit_in_bytes in cgroupv1. Memory QoS maps memory.max to spec.containers[].resources.limits.memory to specify the hard limit for memory usage. If the memory consumption goes above this level, the kernel invokes its OOM Killer.

  cgroups v2 also added memory.high configuration. Memory QoS uses memory.high to set memory usage throttle limit. If the memory.high limit is breached, the offending cgroups are throttled, and the kernel tries to reclaim memory which may avoid an OOM kill.

How it works

Cgroups v2 memory controller interfaces & Kubernetes container resources mapping

Memory QoS uses the memory controller of cgroups v2 to guarantee memory resources in Kubernetes. cgroupv2 interfaces that this feature uses are:

• memory.max
• memory.min
• memory.high.

memory.max is mapped to limits.memory specified in the Pod spec. The kubelet and the container runtime configure the limit in the respective cgroup. The kernel enforces the limit to prevent the container from using more than the configured resource limit. If a process in a container tries to consume more than the specified limit, kernel terminates a process(es) with an Out of Memory (OOM) error.

memory.min is mapped to requests.memory, which results in reservation of memory resources that should never be reclaimed by the kernel. This is how Memory QoS ensures the availability of memory for Kubernetes pods. If there's no unprotected reclaimable memory available, the OOM killer is invoked to make more memory available.

For memory protection, in addition to the original way of limiting memory usage, Memory QoS throttles workload approaching its memory limit, ensuring that the system is not overwhelmed by sporadic increases in memory usage. A new field, memoryThrottlingFactor, is available in the KubeletConfiguration when you enable MemoryQoS feature. It is set to 0.9 by default. memory.high is mapped to throttling limit calculated by using memoryThrottlingFactor, requests.memory and limits.memory as in the formula below, and rounding down the value to the nearest page size:

Summary:

memory.min calculations for cgroups heirarchy

When container memory requests are made, kubelet passes memory.min to the back-end CRI runtime (such as containerd or CRI-O) via the Unified field in CRI during container creation. For every i^th container in a pod, the memory.min in container level cgroups will be set to:

memory.min =  pod.spec.containers[i].resources.requests[memory]

Since the memory.min interface requires that the ancestor cgroups directories are all set, the pod and node cgroups directories need to be set correctly.

For every i^th container in a pod, memory.min in pod level cgroup:

memory.min = \sum_{i=0}^{no. of pods}pod.spec.containers[i].resources.requests[memory]

For every j^th container in every i^th pod on a node, memory.min in node level cgroup:

memory.min = \sum_{i}^{no. of nodes}\sum_{j}^{no. of pods}pod[i].spec.containers[j].resources.requests[memory]

Kubelet will manage the cgroups hierarchy of the pod level and node level cgroups directly using the libcontainer library (from the runc project), while container cgroups limits are managed by the container runtime.

Support for Pod QoS classes

Based on user feedback for the Alpha feature in Kubernetes v1.22, some users would like to opt out of MemoryQoS on a per-pod basis to ensure there is no early memory throttling. Therefore, in Kubernetes v1.27 Memory QOS also supports memory.high to be set as per Quality of Service(QoS) for Pod classes. Following are the different cases for memory.high as per QOS classes:

1. Guaranteed pods by their QoS definition require memory requests=memory limits and are not overcommitted. Hence MemoryQoS feature is disabled on those pods by not setting memory.high. This ensures that Guaranteed pods can fully use their memory requests up to their set limit, and not hit any throttling.

2. Burstable pods by their QoS definition require at least one container in the Pod with CPU or memory request or limit set.

   • When requests.memory and limits.memory are set, the formula is used as-is:

     

   • When requests.memory is set and limits.memory is not set, limits.memory is substituted for node allocatable memory in the formula:

     

3. BestEffort by their QoS definition do not require any memory or CPU limits or requests. For this case, kubernetes sets requests.memory = 0 and substitute limits.memory for node allocatable memory in the formula:

   

Summary: Only Pods in Burstable and BestEffort QoS classes will set memory.high. Guaranteed QoS pods do not set memory.high as their memory is guaranteed.

How do I use it?

The prerequisites for enabling Memory QoS feature on your Linux node are:

1. Verify the requirements related to Kubernetes support for cgroups v2 are met.
2. Ensure CRI Runtime supports Memory QoS. At the time of writing, only containerd and CRI-O provide support compatible with Memory QoS (alpha). This was implemented in the following PRs:
   • Containerd: Feature: containerd-cri support LinuxContainerResources.Unified #5627.
   • CRI-O: implement kube alpha features for 1.22 #5207.

Memory QoS remains an alpha feature for Kubernetes v1.27. You can enable the feature by setting MemoryQoS=true in the kubelet configuration file:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
featureGates:
  MemoryQoS: true

How do I get involved?

Huge thank you to all the contributors who helped with the design, implementation, and review of this feature:

• Dixita Narang (ndixita)
• Tim Xu (xiaoxubeii)
• Paco Xu (pacoxu)
• David Porter(bobbypage)
• Mrunal Patel(mrunalp)

For those interested in getting involved in future discussions on Memory QoS feature, you can reach out SIG Node by several means:

• Slack: #sig-node
• Mailing list
• Open Community Issues/PRs

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