The gang scheduling feature provided by Container Service for Kubernetes (ACK) is developed on top of the new kube-scheduler framework. This feature provides a solution to job scheduling in all-or-nothing scenarios. This topic describes how to enable gang scheduling.

Prerequisites

  • An ACK Pro cluster is created. For more information, see Create an ACK Pro cluster.
    Important Gang scheduling is available for only ACK Pro clusters. To enable gang scheduling for ACK dedicated clusters, Submit a ticket to apply to be added to a whitelist.
  • The following table describes the versions of the system components that are required.
    Component Required version
    Kubernetes 1.16 and later
    Helm 3.0 and later
    Docker 19.03.5
    Operating system CentOS 7.6, CentOS 7.7, Ubuntu 16.04, Ubuntu 18.04, and Alibaba Cloud Linux 2.

Background information

Gang scheduling is a scheduling algorithm that schedules multiple correlated processes to different processors in a parallel system and simultaneously starts these processes. Gang scheduling aims to start all correlated processes at the same time. This ensures that the process group is not blocked when the system fails to start some processes. For example, if you submit a batch job that contains multiple tasks, either all of the tasks are scheduled or none of them is scheduled. Task scheduling in all-or-nothing scenarios is known as gang scheduling.

Kubernetes is widely used in online service orchestration. ACK wants to use Kubernetes as a platform for the unified management of online services and offline jobs. This improves the resource utilization and performance of clusters. However, kube-scheduler cannot migrate specific offline workloads to Kubernetes clusters. For example, if a job requires all-or-nothing scheduling, all tasks of the job must be scheduled at the same time. If only some of the tasks are started, the started jobs must wait until all the remaining tasks are scheduled. If each submitted job contains unscheduled tasks, all submitted jobs remain in the Pending state and the cluster is deadlocked. To avoid this situation, you must enable gang scheduling for kube-scheduler.

Feature description

In ACK, a pod group is a group of pods that need to be scheduled at the same time. When you submit a job that requires all-or-nothing scheduling, you can add labels to pods. The labels specify the name of the pod group to which the job belongs and the minimum number of tasks that must be scheduled to run the job. kube-scheduler schedules tasks based on the minimum number of tasks that must be scheduled. The tasks are scheduled only when the cluster resources are sufficient to schedule the required number of tasks. Otherwise, the job remains in the Pending state.

How to enable gang scheduling

To enable gang scheduling, set min-available and name by adding labels to the pods. 

labels:
    pod-group.scheduling.sigs.k8s.io/name: tf-smoke-gpu
    pod-group.scheduling.sigs.k8s.io/min-available: "3"
  • name: the name of a pod group.
  • min-available: the minimum number of pods that must be scheduled to run a job. Pods are scheduled only when the computing resources are sufficient to schedule the required number of pods.
Note Pods in the same pod group must be assigned the same priority.

Examples

In this example, a distributed TensorFlow job is used to demonstrate how to enable gang scheduling. The ACK cluster that is used in this example has four GPUs.

  1. Install Arena and deploy an environment in your cluster to run TensorFlow jobs. For more information, see Install Arena.
    Note Arena is a subproject of Kubeflow. Kubeflow is an open source project for Kubernetes-based machine learning. Arena allows you to manage the lifecycle of machine learning jobs by using a CLI or SDK. Lifecycle management includes environment setup, data preparation, model development, model training, and model prediction. This improves the working efficiency of data scientists.
  2. Use the following template to submit a distributed TensorFlow job to the ACK cluster. The job runs on one parameter server (PS) pod and four worker pods. Each worker pod requires two GPUs. 
    apiVersion: "kubeflow.org/v1"
kind: "TFJob"
metadata:
  name: "tf-smoke-gpu"
spec:
  tfReplicaSpecs:
    PS:
      replicas: 1
      template:
        metadata:
          creationTimestamp: null
          labels:
            pod-group.scheduling.sigs.k8s.io/name: tf-smoke-gpu
            pod-group.scheduling.sigs.k8s.io/min-available: "5"
        spec:
          containers:
          - args:
            - python
            - tf_cnn_benchmarks.py
            - --batch_size=32
            - --model=resnet50
            - --variable_update=parameter_server
            - --flush_stdout=true
            - --num_gpus=1
            - --local_parameter_device=cpu
            - --device=cpu
            - --data_format=NHWC
            image: registry.cn-hangzhou.aliyuncs.com/kubeflow-images-public/tf-benchmarks-cpu:v20171202-bdab599-dirty-284af3
            name: tensorflow
            ports:
            - containerPort: 2222
              name: tfjob-port
            resources:
              limits:
                cpu: '1'
            workingDir: /opt/tf-benchmarks/scripts/tf_cnn_benchmarks
          restartPolicy: OnFailure
    Worker:
      replicas: 4
      template:
        metadata:
          creationTimestamp: null
          labels:
            pod-group.scheduling.sigs.k8s.io/name: tf-smoke-gpu
            pod-group.scheduling.sigs.k8s.io/min-available: "5"
        spec:
          containers:
          - args:
            - python
            - tf_cnn_benchmarks.py
            - --batch_size=32
            - --model=resnet50
            - --variable_update=parameter_server
            - --flush_stdout=true
            - --num_gpus=1
            - --local_parameter_device=cpu
            - --device=gpu
            - --data_format=NHWC
            image: registry.cn-hangzhou.aliyuncs.com/kubeflow-images-public/tf-benchmarks-gpu:v20171202-bdab599-dirty-284af3
            name: tensorflow
            ports:
            - containerPort: 2222
              name: tfjob-port
            resources:
              limits:
                nvidia.com/gpu: 2
            workingDir: /opt/tf-benchmarks/scripts/tf_cnn_benchmarks
          restartPolicy: OnFailure
    • Submit the distributed TensorFlow job without enabling gang scheduling
      Run the following command to query the status of the pods that run the TensorFlow job. Only two worker pods are running and the other worker pods are in the Pending state. 
      kubectl get pods
      NAME                    READY   STATUS    RESTARTS   AGE
tf-smoke-gpu-ps-0       1/1     Running   0          6m43s
tf-smoke-gpu-worker-0   1/1     Running   0          6m43s
tf-smoke-gpu-worker-1   1/1     Running   0          6m43s
tf-smoke-gpu-worker-2   0/1     Pending   0          6m43s
tf-smoke-gpu-worker-3   0/1     Pending   0          6m43s
      Run the following command to query the log data of the running worker pods. The returned log data indicates that the running worker pods are waiting for the system to start the pending worker pods. The GPU resources occupied by the running worker pods are not in use. 
      kubectl logs -f tf-smoke-gpu-worker-0
      INFO|2020-05-19T07:02:18|/opt/launcher.py|27| 2020-05-19 07:02:18.199696: I tensorflow/core/distributed_runtime/master.cc:221] CreateSession still waiting for response from worker: /job:worker/replica:0/task:3
INFO|2020-05-19T07:02:28|/opt/launcher.py|27| 2020-05-19 07:02:28.199798: I tensorflow/core/distributed_runtime/master.cc:221] CreateSession still waiting for response from worker: /job:worker/replica:0/task:2
    • Submit the distributed TensorFlow job with gang scheduling enabled
      Run the following command to query the status of the pods that run the TensorFlow job. The computing resources in the cluster are insufficient to schedule the minimum number of pods. Therefore, the pod group cannot be scheduled and all pods are in the Pending state. 
      kubectl get pods
      NAME                    READY   STATUS    RESTARTS   AGE
tf-smoke-gpu-ps-0       0/1     Pending   0          43s
tf-smoke-gpu-worker-0   0/1     Pending   0          43s
tf-smoke-gpu-worker-1   0/1     Pending   0          43s
tf-smoke-gpu-worker-2   0/1     Pending   0          43s
tf-smoke-gpu-worker-3   0/1     Pending   0          43s
      After four GPUs are allocated to the cluster, the computing resources in the cluster are sufficient to schedule the minimum number of pods. After the pod group is scheduled, the four worker pods start to run. Run the following command to query the status of the pods that run the TensorFlow job: 
      kubectl get pods
      NAME                    READY   STATUS    RESTARTS   AGE
tf-smoke-gpu-ps-0       1/1     Running   0          3m16s
tf-smoke-gpu-worker-0   1/1     Running   0          3m16s
tf-smoke-gpu-worker-1   1/1     Running   0          3m16s
tf-smoke-gpu-worker-2   1/1     Running   0          3m16s
tf-smoke-gpu-worker-3   1/1     Running   0          3m16s
      Run the following command to query the log data of a running worker pod. The following output indicates that the tasks have been started. 
      kubectl logs -f tf-smoke-gpu-worker-0
      INFO|2020-05-19T07:15:24|/opt/launcher.py|27| Running warm up
INFO|2020-05-19T07:21:04|/opt/launcher.py|27| Done warm up
INFO|2020-05-19T07:21:04|/opt/launcher.py|27| Step  Img/sec loss
INFO|2020-05-19T07:21:05|/opt/launcher.py|27| 1 images/sec: 31.6 +/- 0.0 (jitter = 0.0) 8.318
INFO|2020-05-19T07:21:15|/opt/launcher.py|27| 10  images/sec: 31.1 +/- 0.4 (jitter = 0.7) 8.343
INFO|2020-05-19T07:21:25|/opt/launcher.py|27| 20  images/sec: 31.5 +/- 0.3 (jitter = 0.7) 8.142