hadoop/hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-site/src/site/apt/CapacityScheduler.apt.vm

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  Hadoop Map Reduce Next Generation-${project.version} - Capacity Scheduler
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Hadoop MapReduce Next Generation - Capacity Scheduler

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* {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 no 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 rouge 
  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 dispropotionate amount of resources in the cluster. Also, the 
  <<<CapacityScheduler>>> provides limits on initialized/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 <heirarchical queues> to ensure 
  resources are shared among the sub-queues of an organization before other 
  queues are allowed to use free resources, there-by providing <affinity> 
  for sharing free resources among applications of a given organization.
   
* {Features}

  The <<<CapacityScheduler>>> supports the following features:
  
  * Heirarchical Queues - Heirarchy 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, there-by 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. Adminstrators 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 it's 
    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. This ensures that resources are available in a predictable and 
    elastic manner to queues, thus preventing artifical 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 monpolizing 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 also 
      <add additional queues> 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 queueus>. 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, there-by accomodating applications with differing resource
    requirements. Currently, <memory> is the the resource requirement supported.
  
  []
  
* {Configuration}

  * Setting up <<<ResourceManager>>> to use <<<CapacityScheduler>>>
  
    To configure the <<<ResourceManager>>> to use the <<<CapacityScheduler>>>, set
    the following property in the <<conf/yarn-site.xml>>:
  
*--------------------------------------+--------------------------------------+
|| Property                            || Value                                |
*--------------------------------------+--------------------------------------+
| <<<yarn.resourcemanager.scheduler.class>>> | |
| | <<<org.apache.hadoop.yarn.server.resourcemanager.scheduler.fifo.CapacityScheduler>>> |
*--------------------------------------------+--------------------------------------------+

  * Setting up <queues>
   
    <<conf/capacity-scheduler.xml>> is the configuration file for the
    <<<CapacityScheduler>>>.  
  
    The <<<CapacityScheduler>>> has a pre-defined queue called <root>. All 
    queueus in the system are children of the root queue.

    Further queues can be setup by configuring 
    <<<yarn.scheduler.capacity.root.queues>>> with a list of comma-separated
    child queues.
    
    The configuration for <<<CapacityScheduler>>> uses a concept called
    <queue path> to configure the heirarchy of queues. The <queue path> is the
    full path of the queue's heirarcy, starting at <root>, with . (dot) as the 
    delimiter.
    
    A given queue's children can be defined with the configuration knob:
    <<<yarn.scheduler.capacity.<queue-path>.queues>>>
     
    Here is an example with three top-level child-queues <<<a>>>, <<<b>>> and 
    <<<c>>> and some sub-queues for <<<a>>> and <<<b>>>:
     
----    
<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>
----    

  * Queue Properties
  
    * Resource Allocation
  
*--------------------------------------+--------------------------------------+
|| Property                            || Description                         |
*--------------------------------------+--------------------------------------+
| <<<yarn.scheduler.capacity.<queue-path>.capacity>>> | |
| | Queue <capacity> in percentage (%). | 
| | The sum of capacities for all queues, at each level, should be less than | 
| | or equal to 100. | 
| | Applications in the queue may consume more resources than the queue's | 
| | capacity if there are free resources, providing elasticity. |
*--------------------------------------+--------------------------------------+
| <<<yarn.scheduler.capacity.<queue-path>.maximum-capacity>>> |   | 
| | Maximum queue capacity in percentage (%). |
| | This limits the <elasticity> for applications in the queue. |
*--------------------------------------+--------------------------------------+
| <<<yarn.scheduler.capacity.<queue-path>.minimum-user-limit-percent>>> |   | 
| | Each queue enforces a limit on the percentage of resources allocated to a | 
| | user at any given time, if there is demand for resources. The user limit | 
| | can vary between a minimum and maximum value. The former depends on the | 
| | number of users who have submitted applications, and the latter is set to | 
| | this property value. For e.g., suppose the value of this property is 25. | 
| | If two users have submitted applications to a queue, no single user can |
| | use more than 50% of the queue resources. If a third user submits an | 
| | application, no single user can use more than 33% of the queue resources. |
| | With 4 or more users, no user can use more than 25% of the queues |
| | resources. A value of 100 implies no user limits are imposed. |
*--------------------------------------+--------------------------------------+
| <<<yarn.scheduler.capacity.<queue-path>.user-limit-factor>>> |   | 
| | The multiple of the queue capacity which can be configured to allow a | 
| | single user to acquire more resources. By default this is set to 1 which | 
| | ensures that a single user can never take more than the queue's configured | 
| | capacity irrespective of how idle th cluster is. |
*--------------------------------------+--------------------------------------+

    * Running and Pending Application Limits
    
    
    The <<<CapacityScheduler>>> supports the following parameters to control 
    the runnign and pending applications:
    

*--------------------------------------+--------------------------------------+
|| Property                            || Description                         |
*--------------------------------------+--------------------------------------+
| <<<yarn.scheduler.capacity.maximum-applications>>> | |
| | Maximum number of jobs in the system which can be concurently active |
| | both running and pending. Limits on each queue are directly proportional |
| | to their queue capacities. |
*--------------------------------------+--------------------------------------+
| yarn.scheduler.capacity.maximum-am-resource-percent | |
| | Maximum percent of resources in the cluster which can be used to run |
| | application masters - controls number of concurrent running applications. |
*--------------------------------------+--------------------------------------+

    * Queue Administration & Permissions
    
    The <<<CapacityScheduler>>> supports the following parameters to  
    the administer the queues:
    
    
*--------------------------------------+--------------------------------------+
|| Property                            || Description                         |
*--------------------------------------+--------------------------------------+
| <<<yarn.scheduler.capacity.<queue-path>.state>>> | |
| | The <state> of the queue. Can be one of <<<RUNNING>>> or <<<STOPPED>>>. |
| | If a queue is in <<<STOPPED>>> state, new applications cannot be |
| | submitted to <itself> or <any of its child queueus>. | 
| | Thus, if the <root> queue is <<<STOPPED>>> no applications can be | 
| | submitted to the entire cluster. |
| | Existing applications continue to completion, thus the queue can be 
| | <drained> gracefully. | 
*--------------------------------------+--------------------------------------+
| <<<yarn.scheduler.capacity.root.<queue-path>.acl_submit_jobs>>> | |
| | The <ACL> which controls who can <submit> jobs to the given queue. |
| | If the given user/group has necessary ACLs on the given queue or |
| | <one of the parent queues in the hierarchy> they can submit jobs. | 
*--------------------------------------+--------------------------------------+
| <<<yarn.scheduler.capacity.root.<queue-path>.acl_administer_jobs>>> | |
| | The <ACL> which controls who can <administer> jobs on the given queue. |
| | If the given user/group has necessary ACLs on the given queue or |
| | <one of the parent queues in the hierarchy> they can administer jobs. | 
*--------------------------------------+--------------------------------------+
    
    <Note:> An <ACL> is of the form <user1>, <user2><space><group1>, <group2>.
    The special value of <<*>> implies <anyone>. The special value of <space>
    implies <no one>.
     
    * Reviewing the configuration of the CapacityScheduler

      Once the installation and configuration is completed, you can review it 
      after starting the YARN cluster from the web-ui.

      * Start the YARN cluster in the normal manner.

      * Open the <<<ResourceManager>>> web UI.

      * The </scheduler> web-page should show the resource usages of individual 
        queues.
      
      []
      
* {Changing Queue Configuration}

  Changing queue properties and adding new queues is very simple. You need to
  edit <<conf/capacity-scheduler.xml>> and run <rmadmin -refreshQueues>.
  
----
$ vi $HADOOP_CONF_DIR/capacity-scheduler.xml
$ $YARN_HOME/bin/rmadmin -refreshQueues
----  

  <Note:> Queues cannot be <deleted>, only addition of new queues is supported -
  the updated queue configuration should be a valid one i.e. queue-capacity at
  each <level> should be equal to 100%.