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Add CPU manager proposal.
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| # CPU Manager | ||
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| _Authors:_ | ||
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| * @ConnorDoyle - Connor Doyle <[email protected]> | ||
| * @flyingcougar - Szymon Scharmach <[email protected]> | ||
| * @sjenning - Seth Jennings <[email protected]> | ||
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| **Contents:** | ||
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| * [Overview](#overview) | ||
| * [Proposed changes](#proposed-changes) | ||
| * [Operations and observability](#operations-and-observability) | ||
| * [Practical challenges](#practical-challenges) | ||
| * [Implementation roadmap](#implementation-roadmap) | ||
| * [Appendix A: cpuset pitfalls](#appendix-a-cpuset-pitfalls) | ||
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| ## Overview | ||
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| _Problems to solve:_ | ||
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| 1. Poor or unpredictable performance observed compared to virtual machine | ||
| based orchestration systems. Application latency and lower CPU | ||
| throughput compared to VMs due to cpu quota being fulfilled across all | ||
| cores, rather than exclusive cores, which results in fewer context | ||
| switches and higher cache affinity. | ||
| 1. Unacceptable latency attributed to the OS process scheduler, especially | ||
| for “fast” virtual network functions (want to approach line rate on | ||
| modern server NICs.) | ||
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| _Solution requirements:_ | ||
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| 1. Provide an API-driven contract from the system to a user: "if you are a | ||
| Guaranteed pod with 1 or more cores of cpu, the system will try to make | ||
| sure that the pod gets its cpu quota primarily from reserved core(s), | ||
| resulting in fewer context switches and higher cache affinity". | ||
| 1. Support the case where in a given pod, one container is latency-critical | ||
| and another is not (e.g. auxillary side-car containers responsible for | ||
| log forwarding, metrics collection and the like.) | ||
| 1. Do not cap CPU quota for guaranteed containers that are granted | ||
| exclusive cores, since that would be antithetical to (1) above. | ||
| 1. Take physical processor topology into account in the CPU affinity policy. | ||
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| ### Related issues | ||
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| * Feature: [Further differentiate performance characteristics associated | ||
| with pod level QoS](https://github.com/kubernetes/features/issues/276) | ||
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| ## Proposed changes | ||
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| ### CPU Manager component | ||
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| The *CPU Manager* is a new software component in Kubelet responsible for | ||
| assigning pod containers to sets of CPUs on the local node. In later | ||
| phases, the scope will expand to include caches, a critical shared | ||
| processor resource. | ||
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| The kuberuntime notifies the CPU manager when containers come and | ||
| go. The first such notification occurs in between the container runtime | ||
| interface calls to create and start the container. The second notification | ||
| occurs after the container is destroyed by the container runtime. The CPU | ||
| Manager writes CPU settings for containers using a new CRI method named | ||
| [`UpdateContainerResources`](https://github.com/kubernetes/kubernetes/pull/46105). | ||
| This new method is invoked from two places in the CPU manager: during each | ||
| call to `RegisterContainer` and also periodically from a separate | ||
| reconciliation loop. | ||
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|  | ||
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| _CPU Manager block diagram. `Policy`, `State`, and `Topology` types are | ||
| factored out of the CPU Manager to promote reuse and to make it easier | ||
| to build and test new policies. The shared state abstraction allows | ||
| other Kubelet components to be agnostic of the CPU manager policy for | ||
| observability and checkpointing extensions._ | ||
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| #### Discovering CPU topology | ||
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| The CPU Manager must understand basic topology. First of all, it must | ||
| determine the number of logical CPUs (hardware threads) available for | ||
| allocation. On architectures that support [hyper-threading][ht], sibling | ||
| threads share a number of hardware resources including the cache | ||
| hierarchy. On multi-socket systems, logical CPUs co-resident on a socket | ||
| share L3 cache. Although there may be some programs that benefit from | ||
| disjoint caches, the policies described in this proposal assume cache | ||
| affinity will yield better application and overall system performance for | ||
| most cases. In all scenarios described below, we prefer to acquire logical | ||
| CPUs topologically. For example, allocating two CPUs on a system that has | ||
| hyper-threading turned on yields both sibling threads on the same | ||
| physical core. Likewise, allocating two CPUs on a non-hyper-threaded | ||
| system yields two cores on the same socket. | ||
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| **Decision:** Initially the CPU Manager will re-use the existing discovery | ||
| mechanism in cAdvisor. | ||
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| Alternate options considered for discovering topology: | ||
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| 1. Read and parse the virtual file [`/proc/cpuinfo`][procfs] and construct a | ||
| convenient data structure. | ||
| 1. Execute a simple program like `lscpu -p` in a subprocess and construct a | ||
| convenient data structure based on the output. Here is an example of | ||
| [data structure to represent CPU topology][topo] in go. The linked package | ||
| contains code to build a ThreadSet from the output of `lscpu -p`. | ||
| 1. Execute a mature external topology program like [`mpi-hwloc`][hwloc] -- | ||
| potentially adding support for the hwloc file format to the Kubelet. | ||
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| #### CPU Manager interfaces (sketch) | ||
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| ```go | ||
| type State interface { | ||
| GetCPUSet(containerID string) (cpuset.CPUSet, bool) | ||
| GetDefaultCPUSet() cpuset.CPUSet | ||
| GetCPUSetOrDefault(containerID string) cpuset.CPUSet | ||
| SetCPUSet(containerID string, cpuset CPUSet) | ||
| SetDefaultCPUSet(cpuset CPUSet) | ||
| Delete(containerID string) | ||
| } | ||
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| type Manager interface { | ||
| Start() | ||
| RegisterContainer(p *Pod, c *Container, containerID string) error | ||
| UnregisterContainer(containerID string) error | ||
| State() state.Reader | ||
| } | ||
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| type Policy interface { | ||
| Name() string | ||
| Start(s state.State) | ||
| RegisterContainer(s State, pod *Pod, container *Container, containerID string) error | ||
| UnregisterContainer(s State, containerID string) error | ||
| } | ||
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| type CPUSet map[int]struct{} // set operations and parsing/formatting helpers | ||
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| type CPUTopology TBD | ||
| ``` | ||
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| #### Configuring the CPU Manager | ||
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| Kubernetes will ship with three CPU manager policies. Only one policy is | ||
| active at a time on a given node, chosen by the operator via Kubelet | ||
| configuration. The three policies are **none**, **static** and **dynamic**. | ||
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| The active CPU manager policy is set through a new Kubelet | ||
| configuration value `--cpu-manager-policy`. The default value is `none`. | ||
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| The number of CPUs that pods may run on can be implicitly controlled using the | ||
| existing node-allocatable configuration settings. See the [node allocatable | ||
| proposal document][node-allocatable] for details. The CPU manager will claim | ||
| `ceiling(node.status.allocatable.cpu)` as the number of CPUs available to | ||
| assign to pods, starting from the highest-numbered physical core and | ||
| descending topologically. It is recommended to configure `kube-reserved` | ||
| and `system-reserved` such that their sum is an integer when the CPU manager | ||
| is enabled. This ensures that `node.status.allocatable.cpu` is also an | ||
| integer. | ||
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| Operator documentation will be updated to explain how to configure the | ||
| system to use the low-numbered physical cores for kube-reserved and | ||
| system-reserved cgroups. | ||
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| Each policy is described below. | ||
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| #### Policy 1: "none" cpuset control [default] | ||
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| This policy preserves the existing Kubelet behavior of doing nothing | ||
| with the cgroup `cpuset.cpus` and `cpuset.mems` controls. This "none" | ||
| policy would become the default CPU Manager policy until the effects of | ||
| the other policies are better understood. | ||
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| #### Policy 2: "static" cpuset control | ||
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| The "static" policy allocates exclusive CPUs for containers if they are | ||
| included in a pod of "Guaranteed" [QoS class][qos] and the container's | ||
| resource limit for the CPU resource is an integer greater than or | ||
| equal to one. All other containers share a set of CPUs. | ||
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| When exclusive CPUs are allocated for a container, those CPUs are | ||
| removed from the allowed CPUs of every other container running on the | ||
| node. Once allocated at pod admission time, an exclusive CPU remains | ||
| assigned to a single container for the lifetime of the pod (until it | ||
| becomes terminal.) | ||
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| Workloads that need to know their own CPU mask, e.g. for managing | ||
| thread-level affinity, can read it from the virtual file `/proc/self/status`: | ||
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| ``` | ||
| $ grep -i cpus /proc/self/status | ||
| Cpus_allowed: 77 | ||
| Cpus_allowed_list: 0-2,4-6 | ||
| ``` | ||
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| Note that containers running in the shared cpuset should not attempt any | ||
| application-level CPU affinity of their own, as those settings may be | ||
| overwritten without notice (whenever exclusive cores are | ||
| allocated or deallocated.) | ||
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| ##### Implementation sketch | ||
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| ```go | ||
| func (p *staticPolicy) Start(s State) { | ||
| fullCpuset := cpuset.NewCPUSet() | ||
| for cpuid := 0; cpuid < p.topology.NumCPUs; cpuid++ { | ||
| fullCpuset.Add(cpuid) | ||
| } | ||
| // Figure out which cores shall not be used in shared pool | ||
| reserved, _ := takeByTopology(p.topology, fullCpuset, p.topology.NumReservedCores) | ||
| s.SetDefaultCPUSet(fullCpuset.Difference(reserved)) | ||
| } | ||
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| func (p *staticPolicy) RegisterContainer(s State, pod *Pod, container *Container, containerID string) error { | ||
| if numCPUs := numGuaranteedCPUs(pod, container); numCPUs != 0 { | ||
| // container should get some exclusively allocated CPUs | ||
| cpuset, err := p.allocateCPUs(s, numCPUs) | ||
| if err != nil { | ||
| return err | ||
| } | ||
| s.SetCPUSet(containerID, cpuset) | ||
| } | ||
| // container belongs in the shared pool (nothing to do; use default cpuset) | ||
| return nil | ||
| } | ||
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| func (p *staticPolicy) UnregisterContainer(s State, containerID string) error { | ||
| if toRelease, ok := s.GetCPUSet(containerID); ok { | ||
| s.Delete(containerID) | ||
| p.releaseCPUs(s, toRelease) | ||
| } | ||
| return nil | ||
| } | ||
| ``` | ||
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| ##### Example pod specs and interpretation | ||
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| | Pod | Interpretation | | ||
| | ------------------------------------------ | ------------------------------ | | ||
| | Pod [Guaranteed]:<br /> A:<br />  cpu: 0.5 | Container **A** is assigned to the shared cpuset. | | ||
| | Pod [Guaranteed]:<br /> A:<br />  cpu: 2.0 | Container **A** is assigned two sibling threads on the same physical core (HT) or two physical cores on the same socket (no HT.)<br /><br /> The shared cpuset is shrunk to make room for the exclusively allocated CPUs. | | ||
| | Pod [Guaranteed]:<br /> A:<br />  cpu: 1.0<br /> B:<br />  cpu: 0.5 | Container **A** is assigned one exclusive CPU and container **B** is assigned to the shared cpuset. | | ||
| | Pod [Guaranteed]:<br /> A:<br />  cpu: 1.5<br /> B:<br />  cpu: 0.5 | Both containers **A** and **B** are assigned to the shared cpuset. | | ||
| | Pod [Burstable] | All containers are assigned to the shared cpuset. | | ||
| | Pod [BestEffort] | All containers are assigned to the shared cpuset. | | ||
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| ##### Example scenarios and interactions | ||
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| 1. _A container arrives that requires exclusive cores._ | ||
| 1. Kuberuntime calls the CRI delegate to create the container. | ||
| 1. Kuberuntime registers the container with the CPU manager. | ||
| 1. CPU manager registers the container to the static policy. | ||
| 1. Static policy acquires CPUs from the default pool, by | ||
| topological-best-fit. | ||
| 1. Static policy updates the state, adding an assignment for the new | ||
| container and removing those CPUs from the default pool. | ||
| 1. CPU manager reads container assignment from the state. | ||
| 1. CPU manager updates the container resources via the CRI. | ||
| 1. Kuberuntime calls the CRI delegate to start the container. | ||
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| 1. _A container that was assigned exclusive cores terminates._ | ||
| 1. Kuberuntime unregisters the container with the CPU manager. | ||
| 1. CPU manager unregisters the container with the static policy. | ||
| 1. Static policy adds the container's assigned CPUs back to the default | ||
| pool. | ||
| 1. Kuberuntime calls the CRI delegate to remove the container. | ||
| 1. Asynchronously, the CPU manager's reconcile loop updates the | ||
| cpuset for all containers running in the shared pool. | ||
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| 1. _The shared pool becomes empty._ | ||
| 1. The CPU manager adds a node condition with effect NoSchedule, | ||
| NoExecute that prevents BestEffort and Burstable QoS class pods from | ||
| running on the node. BestEffort and Burstable QoS class pods are | ||
| evicted from the node. | ||
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| 1. _The shared pool becomes nonempty._ | ||
| 1. The CPU manager removes the node condition with effect NoSchedule, | ||
| NoExecute for BestEffort and Burstable QoS class pods. | ||
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| #### Policy 3: "dynamic" cpuset control | ||
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| _TODO: Describe the policy._ | ||
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| Capturing discussions from resource management meetings and proposal comments: | ||
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| Unlike the static policy, when the dynamic policy allocates exclusive CPUs to | ||
| a container, the cpuset may change during the container's lifetime. If deemed | ||
| necessary, we discussed providing a signal in the following way. We could | ||
| project (a subset of) the CPU manager state into a volume visible to selected | ||
| containers. User workloads could subscribe to update events in a normal Linux | ||
| manner (e.g. inotify.) | ||
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| ##### Implementation sketch | ||
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| ```go | ||
| func (p *dynamicPolicy) Start(s State) { | ||
| // TODO | ||
| } | ||
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| func (p *dynamicPolicy) RegisterContainer(s State, pod *Pod, container *Container, containerID string) error { | ||
| // TODO | ||
| } | ||
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| func (p *dynamicPolicy) UnregisterContainer(s State, containerID string) error { | ||
| // TODO | ||
| } | ||
| ``` | ||
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| ##### Example pod specs and interpretation | ||
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| | Pod | Interpretation | | ||
| | ------------------------------------------ | ------------------------------ | | ||
| | | | | ||
| | | | | ||
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| ## Operations and observability | ||
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| * Checkpointing assignments | ||
| * The CPU Manager must be able to pick up where it left off in case the | ||
| Kubelet restarts for any reason. | ||
| * Read effective CPU assignments at runtime for alerting. This could be | ||
| satisfied by the checkpointing requirement. | ||
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| ## Practical challenges | ||
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| 1. Synchronizing CPU Manager state with the container runtime via the | ||
| CRI. Runc/libcontainer allows container cgroup settings to be updated | ||
| after creation, but neither the Kubelet docker shim nor the CRI | ||
| implement a similar interface. | ||
| 1. Mitigation: [PR 46105](https://github.com/kubernetes/kubernetes/pull/46105) | ||
| 1. Compatibility with the `isolcpus` Linux kernel boot parameter. The operator | ||
| may want to correlate exclusive cores with the isolated CPUs, in which | ||
| case the static policy outlined above, where allocations are taken | ||
| directly from the shared pool, is too simplistic. | ||
| 1. Mitigation: defer supporting this until a new policy tailored for | ||
| use with `isolcpus` can be added. | ||
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| ## Implementation roadmap | ||
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| ### Phase 1: None policy [TARGET: Kubernetes v1.8] | ||
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| * Internal API exists to allocate CPUs to containers | ||
| ([PR 46105](https://github.com/kubernetes/kubernetes/pull/46105)) | ||
| * Kubelet configuration includes a CPU manager policy (initially only none) | ||
| * None policy is implemented. | ||
| * All existing unit and e2e tests pass. | ||
| * Initial unit tests pass. | ||
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| ### Phase 2: Static policy [TARGET: Kubernetes v1.8] | ||
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| * Kubelet can discover "basic" CPU topology (HT-to-physical-core map) | ||
| * Static policy is implemented. | ||
| * Unit tests for static policy pass. | ||
| * e2e tests for static policy pass. | ||
| * Performance metrics for one or more plausible synthetic workloads show | ||
| benefit over none policy. | ||
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| ### Phase 3: Cache allocation | ||
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| * Static policy also manages [cache allocation][cat] on supported platforms. | ||
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| ### Phase 4: Dynamic policy | ||
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| * Dynamic policy is implemented. | ||
| * Unit tests for dynamic policy pass. | ||
| * e2e tests for dynamic policy pass. | ||
| * Performance metrics for one or more plausible synthetic workloads show | ||
| benefit over none policy. | ||
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| ### Phase 5: NUMA | ||
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| * Kubelet can discover "advanced" CPU topology (NUMA). | ||
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| ## Appendix A: cpuset pitfalls | ||
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| 1. [`cpuset.sched_relax_domain_level`][cpuset-files]. "controls the width of | ||
| the range of CPUs over which the kernel scheduler performs immediate | ||
| rebalancing of runnable tasks across CPUs." | ||
| 1. Child cpusets must be subsets of their parents. If B is a child of A, | ||
| then B must be a subset of A. Attempting to shrink A such that B | ||
| would contain allowed CPUs not in A is not allowed (the write will | ||
| fail.) Nested cpusets must be shrunk bottom-up. By the same rationale, | ||
| nested cpusets must be expanded top-down. | ||
| 1. Dynamically changing cpusets by directly writing to the sysfs would | ||
| create inconsistencies with container runtimes. | ||
| 1. The `exclusive` flag. This will not be used. We will achieve | ||
| exclusivity for a CPU by removing it from all other assigned cpusets. | ||
| 1. Tricky semantics when cpusets are combined with CFS shares and quota. | ||
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| [cat]: http://www.intel.com/content/www/us/en/communications/cache-monitoring-cache-allocation-technologies.html | ||
| [cpuset-files]: http://man7.org/linux/man-pages/man7/cpuset.7.html#FILES | ||
| [ht]: http://www.intel.com/content/www/us/en/architecture-and-technology/hyper-threading/hyper-threading-technology.html | ||
| [hwloc]: https://www.open-mpi.org/projects/hwloc | ||
| [node-allocatable]: https://github.com/kubernetes/community/blob/master/contributors/design-proposals/node-allocatable.md#phase-2---enforce-allocatable-on-pods | ||
| [procfs]: http://man7.org/linux/man-pages/man5/proc.5.html | ||
| [qos]: https://github.com/kubernetes/community/blob/master/contributors/design-proposals/resource-qos.md | ||
| [topo]: http://github.com/intelsdi-x/swan/tree/master/pkg/isolation/topo |