Merge pull request #989 from holser/kubelet_remedy

Kubernetes Reliability Improvements

docs/kubernetes-reliability.md

@@ -0,0 +1,103 @@
+# Overview
+
+Distributed system such as Kubernetes are designed to be resilient to the
+failures.  More details about Kubernetes High-Availability (HA) may be found at
+[Building High-Availability Clusters](https://kubernetes.io/docs/admin/high-availability/)
+
+To have a simple view the most of parts of HA will be skipped to describe
+Kubelet<->Controller Manager communication only.
+
+By default the normal behavior looks like:
+
+1. Kubelet updates it status to apiserver periodically, as specified by
+   `--node-status-update-frequency`. The default value is **10s**.
+
+2. Kubernetes controller manager checks the statuses of Kubelets every
+   `–-node-monitor-period`. The default value is **5s**.
+
+3. In case the status is updated  within `--node-monitor-grace-period` of time,
+   Kubernetes controller manager considers healthy status of Kubelet. The
+   default value is **40s**.
+
+> Kubernetes controller manager and Kubelets work asynchronously. It means that
+> the delay may include any network latency, API Server latency, etcd latency,
+> latency caused by load on one's master nodes and so on. So if
+> `--node-status-update-frequency` is set to 5s in reality it may appear in
+> etcd in 6-7 seconds or even longer when etcd cannot commit data to quorum
+> nodes.
+
+# Failure
+
+Kubelet will try to make `nodeStatusUpdateRetry` post attempts. Currently
+`nodeStatusUpdateRetry` is constantly set to 5 in
+[kubelet.go](https://github.com/kubernetes/kubernetes/blob/release-1.5/pkg/kubelet/kubelet.go#L102).
+
+Kubelet will try to update the status in
+[tryUpdateNodeStatus](https://github.com/kubernetes/kubernetes/blob/release-1.5/pkg/kubelet/kubelet_node_status.go#L345)
+function. Kubelet uses `http.Client()` Golang method, but has no specified
+timeout. Thus there may be some glitches when API Server is overloaded while
+TCP connection is established.
+
+So, there will be `nodeStatusUpdateRetry` * `--node-status-update-frequency`
+attempts to set a status of node.
+
+At the same time Kubernetes controller manager will try to check
+`nodeStatusUpdateRetry` times every `--node-monitor-period` of time. After
+`--node-monitor-grace-period` it will consider node unhealthy. It will remove
+its pods based on `--pod-eviction-timeout`
+
+Kube proxy has a watcher over API. Once pods are evicted, Kube proxy will
+notice and will update iptables of the node. It will remove endpoints from
+services so pods from failed node won't be accessible anymore.
+
+# Recommendations for different cases
+
+## Fast Update and Fast Reaction
+
+If `-–node-status-update-frequency` is set to **4s** (10s is default).
+`--node-monitor-period` to **2s** (5s is default).
+`--node-monitor-grace-period` to **20s** (40s is default).
+`--pod-eviction-timeout` is set to **30s** (5m is default)
+
+In such scenario, pods will be evicted in **50s** because the node will be
+considered as down after **20s**, and `--pod-eviction-timeout` occurs after
+**30s** more.  However, this scenario creates an overhead on etcd as every node
+will try to update its status every 2 seconds.
+
+If the environment has 1000 nodes, there will be 15000 node updates per
+minute which may require large etcd containers or even dedicated nodes for etcd.
+
+> If we calculate the number of tries, the division will give 5, but in reality
+> it will be from 3 to 5 with `nodeStatusUpdateRetry` attempts of each try. The
+> total number of attemtps will vary from 15 to 25 due to latency of all
+> components.
+
+## Medium Update and Average Reaction
+
+Let's set `-–node-status-update-frequency` to **20s**
+`--node-monitor-grace-period` to **2m** and `--pod-eviction-timeout` to **1m**.
+In that case, Kubelet will try to update status every 20s. So, it will be 6 * 5
+= 30 attempts before Kubernetes controller manager will consider unhealthy
+status of node. After 1m it will evict all pods. The total time will be 3m
+before eviction process.
+
+Such scenario is good for medium environments as 1000 nodes will require 3000
+etcd updates per minute.
+
+> In reality, there will be from 4 to 6 node update tries. The total number of
+> of attempts will vary from 20 to 30.
+
+## Low Update and Slow reaction
+
+Let's set `-–node-status-update-frequency` to **1m**.
+`--node-monitor-grace-period` will set to **5m** and `--pod-eviction-timeout`
+to **1m**. In this scenario, every kubelet will try to update the status every
+minute. There will be 5 * 5 = 25 attempts before unhealty status. After 5m,
+Kubernetes controller manager will set unhealthy status. This means that pods
+will be evicted after 1m after being marked unhealthy. (6m in total).
+
+> In reality, there will be from 3 to 5 tries. The total number of attempt will
+> vary from 15 to 25.
+
+There can be different combinations such as Fast Update with Slow reaction to
+satisfy specific cases.

docs/large-deployments.md

@@ -3,7 +3,8 @@ Large deployments of K8s
 
 For a large scaled deployments, consider the following configuration changes:
 
-* Tune [ansible settings](http://docs.ansible.com/ansible/intro_configuration.html)
+* Tune [ansible settings]
+  (http://docs.ansible.com/ansible/intro_configuration.html)
   for `forks` and `timeout` vars to fit large numbers of nodes being deployed.
 
 * Override containers' `foo_image_repo` vars to point to intranet registry.
@@ -23,9 +24,15 @@ For a large scaled deployments, consider the following configuration changes:
 * Tune CPU/memory limits and requests. Those are located in roles' defaults
   and named like ``foo_memory_limit``, ``foo_memory_requests`` and
   ``foo_cpu_limit``, ``foo_cpu_requests``. Note that 'Mi' memory units for K8s
-  will be submitted as 'M', if applied for ``docker run``, and cpu K8s units will
+  will be submitted as 'M', if applied for ``docker run``, and cpu K8s units
-  end up with the 'm' skipped for docker as well. This is required as docker does not
+  will end up with the 'm' skipped for docker as well. This is required as
-  understand k8s units well.
+  docker does not understand k8s units well.
+
+* Tune ``kubelet_status_update_frequency`` to increase reliability of kubelet.
+  ``kube_controller_node_monitor_grace_period``,
+  ``kube_controller_node_monitor_period``,
+  ``kube_controller_pod_eviction_timeout`` for better Kubernetes reliability.
+  Check out [Kubernetes Reliability](kubernetes-reliability.md)
 
 * Add calico-rr nodes if you are deploying with Calico or Canal. Nodes recover
   from host/network interruption much quicker with calico-rr. Note that
@@ -33,7 +40,7 @@ For a large scaled deployments, consider the following configuration changes:
   etcd role is okay).
 
 * Check out the
-  [Inventory](https://github.com/kubernetes-incubator/kargo/blob/master/docs/getting-started.md#building-your-own-inventory)
+  [Inventory](getting-started.md#building-your-own-inventory)
   section of the Getting started guide for tips on creating a large scale
   Ansible inventory.
 

roles/kubernetes/master/defaults/main.yml

@@ -18,6 +18,9 @@ kube_controller_memory_limit: 512M
 kube_controller_cpu_limit: 250m
 kube_controller_memory_requests: 170M
 kube_controller_cpu_requests: 100m
+kube_controller_node_monitor_grace_period: 40s
+kube_controller_node_monitor_period: 5s
+kube_controller_pod_eviction_timeout: 5m0s
 kube_scheduler_memory_limit: 512M
 kube_scheduler_cpu_limit: 250m
 kube_scheduler_memory_requests: 170M

roles/kubernetes/master/templates/manifests/kube-controller-manager.manifest.j2

@@ -28,6 +28,9 @@ spec:
     - --cluster-signing-cert-file={{ kube_cert_dir }}/ca.pem
     - --cluster-signing-key-file={{ kube_cert_dir }}/ca-key.pem
     - --enable-hostpath-provisioner={{ kube_hostpath_dynamic_provisioner }}
+    - --node-monitor-grace-period={{ kube_controller_node_monitor_grace_period }}
+    - --node-monitor-period={{ kube_controller_node_monitor_period }}
+    - --pod-eviction-timeout={{ kube_controller_pod_eviction_timeout }}
     - --v={{ kube_log_level }}
 {% if cloud_provider is defined and cloud_provider in ["openstack", "azure"] %}
     - --cloud-provider={{cloud_provider}}

roles/kubernetes/node/defaults/main.yml

@@ -12,6 +12,7 @@ kube_proxy_masquerade_all: true
 # Limits for kube components and nginx load balancer app
 kubelet_memory_limit: 512M
 kubelet_cpu_limit: 100m
+kubelet_status_update_frequency: 10s
 kube_proxy_memory_limit: 2000M
 kube_proxy_cpu_limit: 500m
 kube_proxy_memory_requests: 256M

roles/kubernetes/node/templates/kubelet.j2

@@ -9,7 +9,10 @@ KUBELET_ADDRESS="--address={{ ip | default("0.0.0.0") }}"
 KUBELET_HOSTNAME="--hostname-override={{ ansible_hostname }}"
 
 {# Base kubelet args #}
-{% set kubelet_args_base %}--pod-manifest-path={{ kube_manifest_dir }} --pod-infra-container-image={{ pod_infra_image_repo }}:{{ pod_infra_image_tag }}{% endset %}
+{% set kubelet_args_base %}--pod-manifest-path={{ kube_manifest_dir }} \
+--pod-infra-container-image={{ pod_infra_image_repo }}:{{ pod_infra_image_tag }} \
+--kube-reserved cpu={{ kubelet_cpu_limit }},memory={{ kubelet_memory_limit|regex_replace('Mi', 'M') }} \
+--node-status-update-frequency={{ kubelet_status_update_frequency }}{% endset %}
 
 {# DNS settings for kubelet #}
 {% if dns_mode == 'kubedns' %}