Merge pull request #29060 from ehashman/swap-blog

1.22 feature blog for alpha swap support

Author: k8s-ci-robot

content/en/blog/_posts/2021-08-09-alpha-swap-support.md

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+---
+layout: blog
+title: 'New in Kubernetes v1.22: alpha support for using swap memory'
+date: 2021-08-09
+slug: run-nodes-with-swap-alpha
+---
+
+**Author:** Elana Hashman (Red Hat)
+
+The 1.22 release introduced alpha support for configuring swap memory usage for
+Kubernetes workloads on a per-node basis.
+
+In prior releases, Kubernetes did not support the use of swap memory on Linux,
+as it is difficult to provide guarantees and account for pod memory utilization
+when swap is involved. As part of Kubernetes' earlier design, swap support was
+considered out of scope, and a kubelet would by default fail to start if swap
+was detected on a node.
+
+However, there are a number of [use cases](https://github.com/kubernetes/enhancements/blob/9d127347773ad19894ca488ee04f1cd3af5774fc/keps/sig-node/2400-node-swap/README.md#user-stories)
+that would benefit from Kubernetes nodes supporting swap, including improved
+node stability, better support for applications with high memory overhead but
+smaller working sets, the use of memory-constrained devices, and memory
+flexibility.
+
+Hence, over the past two releases, [SIG Node](https://github.com/kubernetes/community/tree/master/sig-node#readme) has
+been working to gather appropriate use cases and feedback, and propose a design
+for adding swap support to nodes in a controlled, predictable manner so that
+Kubernetes users can perform testing and provide data to continue building
+cluster capabilities on top of swap. The alpha graduation of swap memory
+support for nodes is our first milestone towards this goal!
+
+## How does it work?
+
+There are a number of possible ways that one could envision swap use on a node.
+To keep the scope manageable for this initial implementation, when swap is
+already provisioned and available on a node, [we have proposed](https://github.com/kubernetes/enhancements/blob/9d127347773ad19894ca488ee04f1cd3af5774fc/keps/sig-node/2400-node-swap/README.md#proposal)
+the kubelet should be able to be configured such that:
+
+- It can start with swap on.
+- It will direct the Container Runtime Interface to allocate zero swap memory
+  to Kubernetes workloads by default.
+- You can configure the kubelet to specify swap utilization for the entire
+  node.
+
+Swap configuration on a node is exposed to a cluster admin via the
+[`memorySwap` in the KubeletConfiguration](/docs/reference/config-api/kubelet-config.v1beta1/).
+As a cluster administrator, you can specify the node's behaviour in the
+presence of swap memory by setting `memorySwap.swapBehavior`.
+
+This is possible through the addition of a `memory_swap_limit_in_bytes` field
+to the container runtime interface (CRI). The kubelet's config will control how
+much swap memory the kubelet instructs the container runtime to allocate to
+each container via the CRI. The container runtime will then write the swap
+settings to the container level cgroup.
+
+## How do I use it?
+
+On a node where swap memory is already provisioned, Kubernetes use of swap on a
+node can be enabled by enabling the `NodeSwap` feature gate on the kubelet, and
+disabling the `failSwapOn` [configuration setting](/docs/reference/config-api/kubelet-config.v1beta1/#kubelet-config-k8s-io-v1beta1-KubeletConfiguration)
+or the `--fail-swap-on` command line flag.
+
+You can also optionally configure `memorySwap.swapBehavior` in order to
+specify how a node will use swap memory. For example,
+
+```yaml
+memorySwap:
+  swapBehavior: LimitedSwap
+```
+
+The available configuration options for `swapBehavior` are:
+
+- `LimitedSwap` (default): Kubernetes workloads are limited in how much swap
+  they can use. Workloads on the node not managed by Kubernetes can still swap.
+- `UnlimitedSwap`: Kubernetes workloads can use as much swap memory as they
+  request, up to the system limit.
+
+If configuration for `memorySwap` is not specified and the feature gate is
+enabled, by default the kubelet will apply the same behaviour as the
+`LimitedSwap` setting.
+
+The behaviour of the `LimitedSwap` setting depends if the node is running with
+v1 or v2 of control groups (also known as "cgroups"):
+
+- **cgroups v1:** Kubernetes workloads can use any combination of memory and
+  swap, up to the pod's memory limit, if set.
+- **cgroups v2:** Kubernetes workloads cannot use swap memory.
+
+### Caveats
+
+Having swap available on a system reduces predictability. Swap's performance is
+worse than regular memory, sometimes by many orders of magnitude, which can
+cause unexpected performance regressions. Furthermore, swap changes a system's
+behaviour under memory pressure, and applications cannot directly control what
+portions of their memory usage are swapped out. Since enabling swap permits
+greater memory usage for workloads in Kubernetes that cannot be predictably
+accounted for, it also increases the risk of noisy neighbours and unexpected
+packing configurations, as the scheduler cannot account for swap memory usage.
+
+The performance of a node with swap memory enabled depends on the underlying
+physical storage. When swap memory is in use, performance will be significantly
+worse in an I/O operations per second (IOPS) constrained environment, such as a
+cloud VM with I/O throttling, when compared to faster storage mediums like
+solid-state drives or NVMe.
+
+Hence, we do not recommend the use of swap for certain performance-constrained
+workloads or environments. Cluster administrators and developers should
+benchmark their nodes and applications before using swap in production
+scenarios, and [we need your help](#how-do-i-get-involved) with that!
+
+## Looking ahead
+
+The Kubernetes 1.22 release introduces alpha support for swap memory on nodes,
+and we will continue to work towards beta graduation in the 1.23 release. This
+will include:
+
+* Adding support for controlling swap consumption at the Pod level via cgroups.
+   * This will include the ability to set a system-reserved quantity of swap
+     from what kubelet detects on the host.
+* Determining a set of metrics for node QoS in order to evaluate the
+  performance and stability of nodes with and without swap enabled.
+* Collecting feedback from test user cases.
+  * We will consider introducing new configuration modes for swap, such as a
+    node-wide swap limit for workloads.
+
+## How can I learn more?
+
+You can review the current [documentation](https://kubernetes.io/docs/concepts/architecture/nodes/#swap-memory)
+on the Kubernetes website.
+
+For more information, and to assist with testing and provide feedback, please
+see [KEP-2400](https://github.com/kubernetes/enhancements/issues/2400) and its
+[design proposal](https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2400-node-swap/README.md).
+
+## How do I get involved?
+
+Your feedback is always welcome! SIG Node [meets regularly](https://github.com/kubernetes/community/tree/master/sig-node#meetings)
+and [can be reached](https://github.com/kubernetes/community/tree/master/sig-node#contact)
+via [Slack](https://slack.k8s.io/) (channel **#sig-node**), or the SIG's
+[mailing list](https://groups.google.com/forum/#!forum/kubernetes-sig-node).
+Feel free to reach out to me, Elana Hashman (**@ehashman** on Slack and GitHub)
+if you'd like to help.