From c07d60bc906f3db97d79fffa6bc49c4b3a24970c Mon Sep 17 00:00:00 2001
From: Sergii Golovatiuk <sgolovatiuk@mirantis.com>
Date: Tue, 7 Feb 2017 15:01:02 +0100
Subject: [PATCH] Kubernetes Reliability Improvements
- Exclude kubelet CPU/RAM (kube-reserved) from cgroup. It decreases a
chance of overcommitment
- Add a possibility to modify Kubelet node-status-update-frequency
- Add a posibility to configure node-monitor-grace-period,
node-monitor-period, pod-eviction-timeout for Kubernetes controller
manager
- Add Kubernetes Relaibility Documentation with recomendations for
various scenarios.
Signed-off-by: Sergii Golovatiuk <sgolovatiuk@mirantis.com>
---
docs/kubernetes-reliability.md | 103 ++++++++++++++++++
docs/large-deployments.md | 17 ++-
roles/kubernetes/master/defaults/main.yml | 3 +
.../kube-controller-manager.manifest.j2 | 3 +
roles/kubernetes/node/defaults/main.yml | 1 +
roles/kubernetes/node/templates/kubelet.j2 | 5 +-
6 files changed, 126 insertions(+), 6 deletions(-)
create mode 100644 docs/kubernetes-reliability.md
diff --git a/docs/kubernetes-reliability.md b/docs/kubernetes-reliability.md
new file mode 100644
index 000000000..82ec65f2e
--- /dev/null
+++ b/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.
diff --git a/docs/large-deployments.md b/docs/large-deployments.md
index 721064942..18e3b7877 100644
--- a/docs/large-deployments.md
+++ b/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
- end up with the 'm' skipped for docker as well. This is required as docker does not
- understand k8s units well.
+ will be submitted as 'M', if applied for ``docker run``, and cpu K8s units
+ will end up with the 'm' skipped for docker as well. This is required as
+ 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.
diff --git a/roles/kubernetes/master/defaults/main.yml b/roles/kubernetes/master/defaults/main.yml
index 874925adf..510968b13 100644
--- a/roles/kubernetes/master/defaults/main.yml
+++ b/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
diff --git a/roles/kubernetes/master/templates/manifests/kube-controller-manager.manifest.j2 b/roles/kubernetes/master/templates/manifests/kube-controller-manager.manifest.j2
index 49dd05ba8..7bcd51cc4 100644
--- a/roles/kubernetes/master/templates/manifests/kube-controller-manager.manifest.j2
+++ b/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}}
diff --git a/roles/kubernetes/node/defaults/main.yml b/roles/kubernetes/node/defaults/main.yml
index d60b76208..46cc10c47 100644
--- a/roles/kubernetes/node/defaults/main.yml
+++ b/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
diff --git a/roles/kubernetes/node/templates/kubelet.j2 b/roles/kubernetes/node/templates/kubelet.j2
index 65f98d432..8ec348a05 100644
--- a/roles/kubernetes/node/templates/kubelet.j2
+++ b/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' %}
--
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