Device Plugins

Kubernetes provides a device plugin framework that you can use to advertise system hardware resources to the Kubelet.

Instead of customizing the code for Kubernetes itself, vendors can implement a device plugin that you deploy either manually or as a DaemonSet. The targeted devices include GPUs, high-performance NICs, FPGAs, InfiniBand adapters, and other similar computing resources that may require vendor specific initialization and setup.

Device plugin registration

The kubelet exports a Registration gRPC service:

service Registration {
	rpc Register(RegisterRequest) returns (Empty) {}
}

A device plugin can register itself with the kubelet through this gRPC service. During the registration, the device plugin needs to send:

• The name of its Unix socket.
• The Device Plugin API version against which it was built.
• The ResourceName it wants to advertise. Here ResourceName needs to follow the extended resource naming scheme as vendor-domain/resourcetype. (For example, an NVIDIA GPU is advertised as nvidia.com/gpu.)

Following a successful registration, the device plugin sends the kubelet the list of devices it manages, and the kubelet is then in charge of advertising those resources to the API server as part of the kubelet node status update. For example, after a device plugin registers hardware-vendor.example/foo with the kubelet and reports two healthy devices on a node, the node status is updated to advertise that the node has 2 "Foo" devices installed and available.

Then, users can request devices as part of a Pod specification (see container). Requesting extended resources is similar to how you manage requests and limits for other resources, with the following differences:

• Extended resources are only supported as integer resources and cannot be overcommitted.
• Devices cannot be shared between containers.

Example

Suppose a Kubernetes cluster is running a device plugin that advertises resource hardware-vendor.example/foo on certain nodes. Here is an example of a pod requesting this resource to run a demo workload:

---
apiVersion: v1
kind: Pod
metadata:
  name: demo-pod
spec:
  containers:
    - name: demo-container-1
      image: registry.k8s.io/pause:2.0
      resources:
        limits:
          hardware-vendor.example/foo: 2
#
# This Pod needs 2 of the hardware-vendor.example/foo devices
# and can only schedule onto a Node that's able to satisfy
# that need.
#
# If the Node has more than 2 of those devices available, the
# remainder would be available for other Pods to use.

Device plugin implementation

The general workflow of a device plugin includes the following steps:

• Initialization. During this phase, the device plugin performs vendor specific initialization and setup to make sure the devices are in a ready state.

• The plugin starts a gRPC service, with a Unix socket under host path /var/lib/kubelet/device-plugins/, that implements the following interfaces:

  service DevicePlugin {
        // GetDevicePluginOptions returns options to be communicated with Device Manager.
        rpc GetDevicePluginOptions(Empty) returns (DevicePluginOptions) {}
  
        // ListAndWatch returns a stream of List of Devices
        // Whenever a Device state change or a Device disappears, ListAndWatch
        // returns the new list
        rpc ListAndWatch(Empty) returns (stream ListAndWatchResponse) {}
  
        // Allocate is called during container creation so that the Device
        // Plugin can run device specific operations and instruct Kubelet
        // of the steps to make the Device available in the container
        rpc Allocate(AllocateRequest) returns (AllocateResponse) {}
  
        // GetPreferredAllocation returns a preferred set of devices to allocate
        // from a list of available ones. The resulting preferred allocation is not
        // guaranteed to be the allocation ultimately performed by the
        // devicemanager. It is only designed to help the devicemanager make a more
        // informed allocation decision when possible.
        rpc GetPreferredAllocation(PreferredAllocationRequest) returns (PreferredAllocationResponse) {}
  
        // PreStartContainer is called, if indicated by Device Plugin during registeration phase,
        // before each container start. Device plugin can run device specific operations
        // such as resetting the device before making devices available to the container.
        rpc PreStartContainer(PreStartContainerRequest) returns (PreStartContainerResponse) {}
  }
  

  Note: Plugins are not required to provide useful implementations for GetPreferredAllocation() or PreStartContainer(). Flags indicating which (if any) of these calls are available should be set in the DevicePluginOptions message sent back by a call to GetDevicePluginOptions(). The kubelet will always call GetDevicePluginOptions() to see which optional functions are available, before calling any of them directly.

• The plugin registers itself with the kubelet through the Unix socket at host path /var/lib/kubelet/device-plugins/kubelet.sock.

• After successfully registering itself, the device plugin runs in serving mode, during which it keeps monitoring device health and reports back to the kubelet upon any device state changes. It is also responsible for serving Allocate gRPC requests. During Allocate, the device plugin may do device-specific preparation; for example, GPU cleanup or QRNG initialization. If the operations succeed, the device plugin returns an AllocateResponse that contains container runtime configurations for accessing the allocated devices. The kubelet passes this information to the container runtime.

Handling kubelet restarts

A device plugin is expected to detect kubelet restarts and re-register itself with the new kubelet instance. A new kubelet instance deletes all the existing Unix sockets under /var/lib/kubelet/device-plugins when it starts. A device plugin can monitor the deletion of its Unix socket and re-register itself upon such an event.

Device plugin deployment

You can deploy a device plugin as a DaemonSet, as a package for your node's operating system, or manually.

The canonical directory /var/lib/kubelet/device-plugins requires privileged access, so a device plugin must run in a privileged security context. If you're deploying a device plugin as a DaemonSet, /var/lib/kubelet/device-plugins must be mounted as a Volume in the plugin's PodSpec.

If you choose the DaemonSet approach you can rely on Kubernetes to: place the device plugin's Pod onto Nodes, to restart the daemon Pod after failure, and to help automate upgrades.

API compatibility

Previously, the versioning scheme required the Device Plugin's API version to match exactly the Kubelet's version. Since the graduation of this feature to Beta in v1.12 this is no longer a hard requirement. The API is versioned and has been stable since Beta graduation of this feature. Because of this, kubelet upgrades should be seamless but there still may be changes in the API before stabilization making upgrades not guaranteed to be non-breaking.

// PodResourcesLister is a service provided by the kubelet that provides information about the
// node resources consumed by pods and containers on the node
service PodResourcesLister {
    rpc List(ListPodResourcesRequest) returns (ListPodResourcesResponse) {}
    rpc GetAllocatableResources(AllocatableResourcesRequest) returns (AllocatableResourcesResponse) {}
}

// ListPodResourcesResponse is the response returned by List function
message ListPodResourcesResponse {
    repeated PodResources pod_resources = 1;
}
// PodResources contains information about the node resources assigned to a pod
message PodResources {
string name = 1;
string namespace = 2;
repeated ContainerResources containers = 3;
}
// ContainerResources contains information about the resources assigned to a container
message ContainerResources {
string name = 1;
repeated ContainerDevices devices = 2;
repeated int64 cpu_ids = 3;
repeated ContainerMemory memory = 4;
}
// ContainerMemory contains information about memory and hugepages assigned to a container
message ContainerMemory {
string memory_type = 1;
uint64 size = 2;
TopologyInfo topology = 3;
}
// Topology describes hardware topology of the resource
message TopologyInfo {
repeated NUMANode nodes = 1;
}
// NUMA representation of NUMA node
message NUMANode {
int64 ID = 1;
}
// ContainerDevices contains information about the devices assigned to a container
message ContainerDevices {
string resource_name = 1;
repeated string device_ids = 2;
TopologyInfo topology = 3;
}

// AllocatableResourcesResponses contains informations about all the devices known by the kubelet
message AllocatableResourcesResponse {
    repeated ContainerDevices devices = 1;
    repeated int64 cpu_ids = 2;
    repeated ContainerMemory memory = 3;
}

--feature-gates=KubeletPodResourcesGetAllocatable=true

message TopologyInfo {
    repeated NUMANode nodes = 1;
}
message NUMANode {
int64 ID = 1;
}

pluginapi.Device{ID: "25102017", Health: pluginapi.Healthy, Topology:&pluginapi.TopologyInfo{Nodes: []*pluginapi.NUMANode{&pluginapi.NUMANode{ID: 0,},}}}