Hadoop: Capacity Scheduler

Purpose

This document describes the CapacityScheduler, a pluggable scheduler for Hadoop which allows for multiple-tenants to securely share a large cluster such that their applications are allocated resources in a timely manner under constraints of allocated capacities.

Overview

The CapacityScheduler is designed to run Hadoop applications as a shared, multi-tenant cluster in an operator-friendly manner while maximizing the throughput and the utilization of the cluster.

Traditionally each organization has it own private set of compute resources that have sufficient capacity to meet the organization’s SLA under peak or near-peak conditions. This generally leads to poor average utilization and overhead of managing multiple independent clusters, one per each organization. Sharing clusters between organizations is a cost-effective manner of running large Hadoop installations since this allows them to reap benefits of economies of scale without creating private clusters. However, organizations are concerned about sharing a cluster because they are worried about others using the resources that are critical for their SLAs.

The CapacityScheduler is designed to allow sharing a large cluster while giving each organization capacity guarantees. The central idea is that the available resources in the Hadoop cluster are shared among multiple organizations who collectively fund the cluster based on their computing needs. There is an added benefit that an organization can access any excess capacity not being used by others. This provides elasticity for the organizations in a cost-effective manner.

Sharing clusters across organizations necessitates strong support for multi-tenancy since each organization must be guaranteed capacity and safe-guards to ensure the shared cluster is impervious to single rogue application or user or sets thereof. The CapacityScheduler provides a stringent set of limits to ensure that a single application or user or queue cannot consume disproportionate amount of resources in the cluster. Also, the CapacityScheduler provides limits on initialized and pending applications from a single user and queue to ensure fairness and stability of the cluster.

The primary abstraction provided by the CapacityScheduler is the concept of queues. These queues are typically setup by administrators to reflect the economics of the shared cluster.

To provide further control and predictability on sharing of resources, the CapacityScheduler supports hierarchical queues to ensure resources are shared among the sub-queues of an organization before other queues are allowed to use free resources, thereby providing affinity for sharing free resources among applications of a given organization.

Features

The CapacityScheduler supports the following features:

• Hierarchical Queues - Hierarchy of queues is supported to ensure resources are shared among the sub-queues of an organization before other queues are allowed to use free resources, thereby providing more control and predictability.

• Capacity Guarantees - Queues are allocated a fraction of the capacity of the grid in the sense that a certain capacity of resources will be at their disposal. All applications submitted to a queue will have access to the capacity allocated to the queue. Administrators can configure soft limits and optional hard limits on the capacity allocated to each queue.

• Security - Each queue has strict ACLs which controls which users can submit applications to individual queues. Also, there are safe-guards to ensure that users cannot view and/or modify applications from other users. Also, per-queue and system administrator roles are supported.

• Elasticity - Free resources can be allocated to any queue beyond its capacity. When there is demand for these resources from queues running below capacity at a future point in time, as tasks scheduled on these resources complete, they will be assigned to applications on queues running below the capacity (preemption is also supported). This ensures that resources are available in a predictable and elastic manner to queues, thus preventing artificial silos of resources in the cluster which helps utilization.

• Multi-tenancy - Comprehensive set of limits are provided to prevent a single application, user and queue from monopolizing resources of the queue or the cluster as a whole to ensure that the cluster isn’t overwhelmed.

• Operability

  • Runtime Configuration - The queue definitions and properties such as capacity, ACLs can be changed, at runtime, by administrators in a secure manner to minimize disruption to users. Also, a console is provided for users and administrators to view current allocation of resources to various queues in the system. Administrators can add additional queues at runtime, but queues cannot be deleted at runtime.
  • Drain applications - Administrators can stop queues at runtime to ensure that while existing applications run to completion, no new applications can be submitted. If a queue is in STOPPED state, new applications cannot be submitted to itself or any of its child queues. Existing applications continue to completion, thus the queue can be drained gracefully. Administrators can also start the stopped queues.

• Resource-based Scheduling - Support for resource-intensive applications, where-in a application can optionally specify higher resource-requirements than the default, thereby accommodating applications with differing resource requirements. Currently, memory is the resource requirement supported.

• Queue Mapping based on User or Group - This feature allows users to map a job to a specific queue based on the user or group.

• Priority Scheduling - This feature allows applications to be submitted and scheduled with different priorities. Higher integer value indicates higher priority for an application. Currently Application priority is supported only for FIFO ordering policy.

Configuration

<property>
  <name>yarn.scheduler.capacity.root.queues</name>
  <value>a,b,c</value>
  <description>The queues at the this level (root is the root queue).
  </description>
</property>

<property>
  <name>yarn.scheduler.capacity.root.a.queues</name>
  <value>a1,a2</value>
  <description>The queues at the this level (root is the root queue).
  </description>
</property>

<property>
  <name>yarn.scheduler.capacity.root.b.queues</name>
  <value>b1,b2,b3</value>
  <description>The queues at the this level (root is the root queue).
  </description>
</property>

 <property>
   <name>yarn.scheduler.capacity.queue-mappings</name>
   <value>u:user1:queue1,g:group1:queue2,u:%user:%user,u:user2:%primary_group</value>
   <description>
     Here, <user1> is mapped to <queue1>, <group1> is mapped to <queue2>, 
     maps users to queues with the same name as user, <user2> is mapped 
     to queue name same as <primary group> respectively. The mappings will be 
     evaluated from left to right, and the first valid mapping will be used.
   </description>
 </property>

Changing Queue Configuration

Changing queue/scheduler properties and adding/removing queues can be done in two ways, via file or via API. This behavior can be changed via yarn.scheduler.configuration.store.class in yarn-site.xml. Possible values are file, which allows modifying properties via file; memory, which allows modifying properties via API, but does not persist changes across restart; leveldb, which allows modifying properties via API and stores changes in leveldb backing store; and zk, which allows modifying properties via API and stores changes in zookeeper backing store. The default value is file.

$ vi $HADOOP_CONF_DIR/capacity-scheduler.xml
$ $HADOOP_YARN_HOME/bin/yarn rmadmin -refreshQueues

Updating a Container (Experimental - API may change in the future)

Once an Application Master has received a Container from the Resource Manager, it may request the Resource Manager to update certain attributes of the container.

Currently only two types of container updates are supported:

• Resource Update : Where the AM can request the RM to update the resource size of the container. For eg: Change the container from a 2GB, 2 vcore container to a 4GB, 2 vcore container.
• ExecutionType Update : Where the AM can request the RM to update the ExecutionType of the container. For eg: Change the execution type from GUARANTEED to OPPORTUNISTIC or vice versa.

This is facilitated by the AM populating the updated_containers field, which is a list of type UpdateContainerRequestProto, in AllocateRequestProto. The AM can make multiple container update requests in the same allocate call.

The schema of the UpdateContainerRequestProto is as follows:

message UpdateContainerRequestProto {
  required int32 container_version = 1;
  required ContainerIdProto container_id = 2;
  required ContainerUpdateTypeProto update_type = 3;
  optional ResourceProto capability = 4;
  optional ExecutionTypeProto execution_type = 5;
}

The ContainerUpdateTypeProto is an enum:

enum ContainerUpdateTypeProto {
  INCREASE_RESOURCE = 0;
  DECREASE_RESOURCE = 1;
  PROMOTE_EXECUTION_TYPE = 2;
  DEMOTE_EXECUTION_TYPE = 3;
}

As constrained by the above enum, the scheduler currently supports changing either the resource update OR executionType of a container in one update request.

The AM must also provide the latest ContainerProto it received from the RM. This is the container which the RM will attempt to update.

If the RM is able to update the requested container, the updated container will be returned, in the updated_containers list field of type UpdatedContainerProto in the AllocateResponseProto return value of either the same allocate call or in one of the subsequent calls.

The schema of the UpdatedContainerProto is as follows:

message UpdatedContainerProto {
  required ContainerUpdateTypeProto update_type = 1;
  required ContainerProto container = 2;
}

It specifies the type of container update that was performed on the Container and the updated Container object which container an updated token.

The container token can then be used by the AM to ask the corresponding NM to either start the container, if the container has not already been started or update the container using the updated token.

The DECREASE_RESOURCE and DEMOTE_EXECUTION_TYPE container updates are automatic - the AM does not explicitly have to ask the NM to decrease the resources of the container. The other update types require the AM to explicitly ask the NM to update the container.

If the yarn.resourcemanager.auto-update.containers configuration parameter is set to true (false by default), The RM will ensure that all container updates are automatic.