Scheduling

In Kubernetes, scheduling refers to the process of assigning pods to nodes in a cluster based on various factors such as resource requirements, node capacity, and other constraints. Kubernetes scheduler is responsible for determining where and how to run pods within the cluster.

Prerequisites

This section builds on skills from both the Quickstart and the tutorial on Basic Kubernetes.

Learning Objectives

You will learn how to query the cluster to see its current status and the status of its nodes in real-time.
You will be able to select a specific cluster node and examine its features (e.g. type of GPU).
You will understand how to enforce specific node types and resource requirements within a yaml file.
You will be able to set a preference for a node type or resource, but not require that type.

Explore the system

Let's start with looking what's available in the the system. You have already had the chance to see the list of all the nodes:

kubectl get nodes

This is a very long list...and growing. It may be too much information for you to parse, but in the next steps, we'll learn how to sort and filter these results.

For example, you can see which nodes have which GPU type:

kubectl get nodes -L nvidia.com/gpu.product

Now, pick one node, and see what other resources it has:

kubectl get nodes -o yaml <nodename>

If you picked a node with a GPU, look for the "nvidia.com/gpu.product" in the output.

You might not find it right away, as this output is rather long. But we can try a simpler command:

kubectl describe node <node_name>

This will return a shorter, but less descriptive result. The describe command can be used to get the description of many kinds of resources or processes in Kubernetes. What other things can you think of that you could use this command for?

Validating requirements

In the next section we will play with adding requirements to pod yaml files. But first let's make sure those resources are actually available.

As a simple example, let's pick a specific GPU type:

kubectl get node -l 'nvidia.com/gpu.product=NVIDIA-GeForce-GTX-1080'

Did you get any hits?

Here we look for nodes with 100Gbps NICs:

kubectl get node -l 'nautilus.io/network=100000'

Did you get any hits?

How about a negative selector? And let's see what do we get:

kubectl get node -l 'nvidia.com/gpu.product!=NVIDIA-GeForce-GTX-1080, nvidia.com/gpu.product!=NVIDIA-GeForce-GTX-1070' -L nvidia.com/gpu.product

By the way, many of these queries are exposed as part of the Nautilus portal. You can visit the Resources page to see a table with all the current nodes and their features.

Requirements in pods

You have already used requirements in the pods. Here is something very similar to the pod in our first tutorial:

apiVersion: v1
kind: Pod
metadata:
  name: test-pod
spec:
  containers:
  - name: mypod
    image: rocker/cuda
    resources:
      limits:
        memory: 100Mi
        cpu: 100m
      requests:
        memory: 100Mi
        cpu: 100m
    command: ["sh", "-c", "sleep infinity"]

But we set the resource requests and limits to be really low, so it was virtually guaranteed that the pod would start.

Now, let's add one more requirement. Let's ask for a GPU. We also change the container, so that we get the proper drivers in place.

Note: While you can ask for a fraction of a CPU, you cannot ask for a fraction of a GPU in our current setup. You should also keep the same number for requirements and limits.

apiVersion: v1
kind: Pod
metadata:
  name: test-gpupod
spec:
  containers:
  - name: mypod
    image: rocker/cuda
    resources:
      limits:
        memory: 100Mi
        cpu: 100m
        nvidia.com/gpu: 1
      requests:
        memory: 100Mi
        cpu: 100m
        nvidia.com/gpu: 1
    command: ["sh", "-c", "sleep infinity"]

Once the pod started, login using kubectl exec and check what kind of GPU you got:

nvidia-smi

Remember to destroy the old pod.

Let's now ask for a specific GPU type by creating a new yaml file, called 'test-gpupod'

apiVersion: v1
kind: Pod
metadata:
  name: test-gpupod
spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
        - matchExpressions:
          - key: nvidia.com/gpu.product
            operator: In
            values:
            - NVIDIA-GeForce-GTX-1080-Ti
  containers:
  - name: mypod
    image: rocker/cuda
    resources:
      limits:
        memory: 100Mi
        cpu: 100m
        nvidia.com/gpu: 1
      requests:
        memory: 100Mi
        cpu: 100m
        nvidia.com/gpu: 1
    command: ["sh", "-c", "sleep infinity"]

Did the pod start? It might take a minute for the system to locate the resources, and spin up the pod. You can use kubetcl get pods -o wide to check on the status.

Once the pod is up and running, let's log in and check if you indeed got the desired GPU type. Do you remember the command?

kubectl exec test-gpupod -it -- /bin/bash

You should now be inside of the software container running in the pod. The image we used (rocker/cuda) comes ready for CUDA, so you should be able to use the command nvidia-smi to see a readout of the GPU type and its details.

Preferences in pods

Sometimes you would prefer something, but can live with less or a substitution. In this example, let's ask for the fastest GPU in out pool, but not as a hard requirement. Here you can either write over your original test-gpupod.yaml or you can create a different file using the following:

apiVersion: v1
kind: Pod
metadata:
  name: test-gpupod
spec:
  affinity:
    nodeAffinity:
      preferredDuringSchedulingIgnoredDuringExecution:
      - weight: 1
        preference:
          matchExpressions:
          - key: nvidia.com/gpu.product
            operator: In
            values:
            - "NVIDIA-GeForce-RTX-2080-Ti"
            - "Tesla-V100-SXM2-32GB"
  containers:
  - name: mypod
    image: rocker/cuda
    resources:
      limits:
        memory: 100Mi
        cpu: 100m
        nvidia.com/gpu: 1
      requests:
        memory: 100Mi
        cpu: 100m
        nvidia.com/gpu: 1
    command: ["sh", "-c", "sleep infinity"]

Now check the pod. How long did it take for it to start? What GPU type did you get.

Using tolerations

There are several parts of the PRP Kubernetes pool that are off-limits to regular users. One of them is Science-DMZ nodes that don't have access to the regular internet.

Optional Section: reserved nodes and taints

Here is a Pod yaml that will try to run on one:

apiVersion: v1
kind: Pod
metadata:
  name: test-suncavepod
spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
        - matchExpressions:
          - key: kubernetes.io/hostname
            operator: In
            values:
            - igrok-la.cenic.net
  containers:
  - name: mypod
    image: alpine
    resources:
      limits:
        memory: 100Mi
        cpu: 100m
      requests:
        memory: 100Mi
        cpu: 100m
    command: ["sh", "-c", "sleep infinity"]

Go ahead, request a pod like that. Did the pod start?

It should not have. There are essentially no nodes that will match the above requirements. Check for yourself:

kubectl get events --sort-by=.metadata.creationTimestamp

Now, look up the list of nodes that were supposed to match:

kubectl get node igrok-la.cenic.net

Look for the node details. kubectl get nodes igrok-la.cenic.net -o yaml. Search for any taints. You should see something along the lines of:

...
spec:
  taints:
  - effect: NoSchedule
    key: nautilus.io/noceph
    value: "true"
...

You have been granted permission to run on those nodes, so let's now add the toleration that will allow you to run there (remember to remove the old Pod):

apiVersion: v1
kind: Pod
metadata:
  name: test-scidmzpod
spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
        - matchExpressions:
          - key: kubernetes.io/hostname
            operator: In
            values:
            - igrok-la.cenic.net
  tolerations:
  - effect: NoSchedule
    key: nautilus.io/noceph
    operator: Exists
  containers:
  - name: mypod
    image: alpine
    resources:
      limits:
        memory: 100Mi
        cpu: 100m
      requests:
        memory: 100Mi
        cpu: 100m
    command: ["sh", "-c", "sleep infinity"]

Try to submit this one.

Did the Pod start?

Please make sure you did not leave any pods behind.