add workload placement docs

2017-08-15 11:14:01 -05:00
parent 35dc007365
commit 5b4ef96268
2 changed files with 146 additions and 0 deletions
@@ -140,6 +140,7 @@ toc:
  - docs/tasks/administer-cluster/cpu-memory-limit.md
  - docs/tasks/administer-cluster/out-of-resource.md
  - docs/tasks/administer-cluster/reserve-compute-resources.md
+  - docs/tasks/administer-cluster/cpu-management-policies.md
  - docs/tasks/administer-cluster/guaranteed-scheduling-critical-addon-pods.md
  - docs/tasks/administer-cluster/declare-network-policy.md
  - title: Install Network Policy Provider
@@ -0,0 +1,145 @@
+---
+title: Control CPU Management Policies on the Node
+---
+
+* TOC
+{:toc}
+
+Kubernetes keeps many aspects of how pods execute on nodes abstracted
+from the user. This is by design.  However, some workloads require
+stronger guarantees in terms of latency and/or performance in order to operate
+acceptably.  The kubelet provides methods to enable more complex workload
+placement policies while keeping the abstraction free from explicit placement
+directives.
+
+## CPU Management Policies
+
+By default, the kubelet uses [CFS quota](https://en.wikipedia.org/wiki/Completely_Fair_Scheduler)
+to enforce pod CPU limits.  When the node runs many CPU bound pods,
+the workload can move to different CPU cores depending on
+whether the pod is throttled and which CPU cores are available at
+scheduling time.  Many workloads are not sensitive to this migration and thus
+work fine without any intervention.
+
+However, in workloads where CPU cache affinity and scheduling latency
+significantly affect workload performance, the kubelet allows alternative CPU
+management policies to determine some placement preferences on the node.
+
+Enable these management policies with the `--cpu-manager-policy` kubelet
+option.  There are two supported policies:
+
+* `none`: the default, which represents the existing scheduling behavior
+* `static`: allows pods with certain resource characteristics to be
+  granted increased CPU affinity and exclusivity on the node.
+
+### None policy
+
+The `none` policy explicitly enables the existing default CPU
+affinity scheme, providing no affinity beyond what the OS scheduler does
+automatically.  Limits on CPU usage for
+[Guaranteed pods](/docs/tasks/configure-pod-container/quality-service-pod)
+are enforced using CFS quota.
+
+### Static policy
+
+The `static` policy allows containers in `Guaranteed` pods with integer CPU
+`requests` access to exclusive CPUs on the node.  This exclusivity is enforced
+using the [cpuset cgroup controller](https://www.kernel.org/doc/Documentation/cgroup-v1/cpusets.txt).
+
+**Note:** System services such as the container runtime and the kubelet itself can continue to run on these exclusive CPUs.  The exclusivity only extends to other pods.
+{: .note}
+
+This policy manages a shared pool of CPUs that initially contains all CPUs in the
+node minus any reservations by the kubelet `--kube-reserved` or
+`--system-reserved` options. CPUs reserved by these options are taken, in
+integer quantity, from the initial shared pool in ascending order by physical
+core ID.  This shared pool is the set of CPUs on which any containers in
+`BestEffort` and `Burstable` pods run. Containers in `Guaranteed` pods with fractional
+cpu `requests` also run on CPUs in the shared pool. Only containers that are
+both part of a `Guaranteed` pod and have integer CPU `requests` are assigned
+exclusive CPUs.
+
+**Note:** When reserving CPU with `--kube-reserved` or `--system-reserved` options, it is advised to use *integer* CPU quantities.
+{: .note}
+
+As `Guaranteed` pods whose containers fit the requirements for being statically
+assigned are scheduled to the node, CPUs are removed from the shared pool and
+placed in the cpuset for the container.  CFS quota is not used to bound
+the CPU usage of these containers as their usage is bound by the scheduling domain
+itself. In others words, the number of CPUs in the container cpuset is equal to the integer
+CPU `limit` specified in the pod spec.  This static assignment increases CPU
+affinity and decreases context switches due to throttling for the CPU bound
+workload.
+
+In the event that the shared pool is depleted the kubelet takes two actions:
+
+* Evict all pods that include a container that does not specify a `cpu`
+  quantity in `requests` as those pods now have no CPUs on which to run.
+* Set a `NodeCPUPressure` node condition to `true` in the node status. When
+  this condition is true, the scheduler will not assign any pod to the node
+  that has a container which lacks a `cpu` quantity in `requests`.
+
+Consider the containers in the following pod specs:
+
+```yaml
+spec:
+  containers:
+  - name: nginx
+    image: nginx
+```
+
+This pod runs in the `BestEffort` QoS class because no resource `requests` or
+`limits` are specified. It is evicted if shared pool is depleted.  It runs
+in the shared pool.
+
+```yaml
+spec:
+  containers:
+  - name: nginx
+    image: nginx
+    resources:
+      limits:
+        memory: "200Mi"
+      requests:
+        memory: "100Mi"
+```
+
+This pod runs in the `Burstable` QoS class because resource `requests` do not
+equal `limits` and the `cpu` quantity is not specified. It is 
+evicted if shared pool is depleted.  It runs in the shared pool.
+
+
+```yaml
+spec:
+  containers:
+  - name: nginx
+    image: nginx
+    resources:
+      limits:
+        memory: "200Mi"
+        cpu: "2"
+      requests:
+        memory: "100Mi"
+        cpu: "1"
+```
+
+This pod runs in the `Burstable` QoS class because resource `requests` do not
+equal `limits`. The non-zero `cpu` quantity in `requests` prevents the
+shared pool from depleting. It runs in the shared pool.
+
+
+```yaml
+spec:
+  containers:
+  - name: nginx
+    image: nginx
+    resources:
+      limits:
+        memory: "200Mi"
+        cpu: "2"
+```
+
+This pod runs in the `Guaranteed` QoS class because only `limits` are specified
+and `requests` are set equal to `limits` when not explicitly specified. The
+`nginx` container is granted 2 exclusive CPUs.
+