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Kubernetes Spark Scheduler Extender

k8s-spark-scheduler-extender is a Kubernetes Scheduler Extender that is designed to provide gang scheduling capabilities for running Apache Spark on Kubernetes.

Running Spark applications at scale on Kubernetes with the default kube-scheduler is prone to resource starvation and oversubscription. Naively scheduling driver pods can occupy space that should be reserved for their executors. Using k8s-spark-scheduler-extender guarantees that a driver will only be scheduled if there is space in the cluster for all of its executors. It can also guarantee scheduling order for drivers, with respect to their creation timestamp.

Requirements:

• Kubernetes: 1.11.0
• Spark: Any snapshot build that includes commit f6cc354d83. This is expected to be in Spark 3.x

Spark scheduler extender is a Witchcraft server, and uses Godel for testing and building. It is meant to be deployed with a new kube-scheduler instance, running alongside the default scheduler. This way, non-spark pods can continue to be scheduled by the default scheduler, and opt-in pods are scheduled using the spark-sdcheduler.

Usage

To set up the scheduler extender as a new scheduler named spark-scheduler, run:

kubectl apply -f examples/extender.yml

This will create a new service account, a cluster binding for permissions, a config map and a deployment, all under namespace spark. It is worth noting that this example sets up the new scheduler with a super user. k8s-spark-scheduler-extender groups nodes in the cluster with a label specified in its configuration. Nodes that this scheduler will consider should have this label set. FIFO order is preserved for pods that have a node affinity or a node selector set for the same instance-group label. The given example configuration sets this label as instance-group.

Refer to Spark's website for documentation on running Spark with Kubernetes. To schedule a spark application using spark-scheduler, you must apply the following metadata to driver and executor pods.

driver:

apiVersion: v1
kind: Pod
metadata:
  labels:
    spark-app-id: my-custom-id
  annotations:
    spark-driver-cpu: 1
    spark-driver-mem: 1Gi
    spark-executor-cpu: 2
    spark-executor-mem: 4Gi
    spark-executor-count: 8
spec:
  schedulerName: spark-scheduler

executor:

apiVersion: v1
kind: Pod
metadata:
  labels:
    spark-app-id: my-custom-id
spec:
  schedulerName: spark-scheduler

As of f6cc354d83, spark supports specifying pod templates for driver and executors. Although spark configuration can also be used to apply label and annotations, the pod template feature in spark is the only way of setting schedulerName. To apply the above overrides, you should save them as files and set these configuration overrides:

"spark.kubernetes.driver.podTemplateFile": "/path/to/driver.template",
"spark.kubernetes.executor.podTemplateFile": "/path/to/executor.template"

Dynamic Allocation

k8s-spark-scheduler-extender also supports running Spark applications in dynamic allocation mode. You can find more information about how to configure Spark to make use of dynamic allocation in the Spark documentation.
To inform k8s-spark-scheduler-extender that you are running an application with dynamic allocation enabled, you should omit setting the spark-executor-count annotation on the driver pod, and instead set the following three annotations:

• spark-dynamic-allocation-enabled: "true"
• spark-dynamic-allocation-min-executor-count: minimum number of executors to always reserve resources for. Should be equal to the spark.dynamicAllocation.minExecutors value you set in the Spark configuration
• spark-dynamic-allocation-max-executor-count: maximum number of executors to allow your application to request at a given time. Should be equal to the spark.dynamicAllocation.maxExecutors value you set in the Spark configuration

If dynamic allocation is enabled, k8s-spark-scheduler-extender will guarantee that your application will only get scheduled if the driver and executors until the minimum executor count fit to the cluster. Executors over the minimum are not reserved for, and are only scheduled if there is capacity to do so when they are requested by the application.

Configuration

k8s-spark-scheduler-extender is a witchcraft service, and supports configuration options detailed in the github documentation. Additional configuration options are:

• fifo: a boolean flag to turn on FIFO processing of spark drivers. With this turned on, younger spark drivers will be blocked from scheduling until the cluster has space for the oldest spark driver. Executor scheduling is unaffected from this.
• kube-config: path to a kube-config file
• binpack: the algorithm to binpack pods in a spark application over the free space in the cluster. Currently available options are distribute-evenly and tightly-pack, the former being the default. They differ on how they distribute the executors, distribute-evenly round-robin's available nodes, whereas tightly-pack fills one node before moving to the next.
• qps and burst: These are parameters for rate limiting kubernetes clients, used directly in client construction.

Development

Use ./godelw docker build to build an image using the Dockerfile template. Built image will use the default configuration. Deployment created by kubectl apply -f examples/extender.yml can be used to iterate locally.

Use ./examples/submit-test-spark-app.sh <id> <executor-count> <driver-cpu> <driver-mem> <driver-nvidia-gpus> <executor-cpu> <executor-mem> <executor-nvidia-gpus> to mock a spark application launch. Created pods will have a node selector for instance-group: main, so desired nodes in the cluster should be modified to have this label set.

Use ./godelw verify to run tests and style checks

Contributing

The team welcomes contributions! To make changes:

• Fork the repo and make a branch
• Write your code (ideally with tests) and make sure the CircleCI build passes
• Open a PR (optionally linking to a github issue)

License

This project is made available under the Apache 2.0 License.