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<!DOCTYPE document PUBLIC "-//APACHE//DTD Documentation V2.0//EN" "http://forrest.apache.org/dtd/document-v20.dtd">
<document>
  <header>
    <title>Fair Scheduler</title>
  </header>
  <body>

    <section>
      <title>Purpose</title>

      <p>This document describes the Fair Scheduler, a pluggable MapReduce
        scheduler that provides a way to share large clusters.</p>
    </section>

    <section>
      <title>Introduction</title>
      <p>Fair scheduling is a method of assigning resources to jobs
        such that all jobs get, on average, an equal share of resources
        over time. When there is a single job running, that job uses the
        entire cluster. When other jobs are submitted, tasks slots that
        free up are assigned to the new jobs, so that each job gets
        roughly the same amount of CPU time. Unlike the default Hadoop
        scheduler, which forms a queue of jobs, this lets short jobs finish
        in reasonable time while not starving long jobs. It is also an easy
        way to share a cluster between multiple of users.
        Fair sharing can also work with job priorities - the priorities are
        used as weights to determine the fraction of total compute time that
        each job gets.
      </p>
      <p>
        The fair scheduler organizes jobs into <em>pools</em>, and 
        divides resources fairly between these pools. By default, there is a 
        separate pool for each user, so that each user gets an equal share 
        of the cluster. It is also possible to set a job's pool based on the
        user's Unix group or any jobconf property. 
        Within each pool, jobs can be scheduled using either fair sharing or 
        first-in-first-out (FIFO) scheduling.
      </p>
      <p>
        In addition to providing fair sharing, the Fair Scheduler allows
        assigning guaranteed <em>minimum shares</em> to pools, which is useful
        for ensuring that certain users, groups or production applications
        always get sufficient resources. When a pool contains jobs, it gets
        at least its minimum share, but when the pool does not need its full
        guaranteed share, the excess is split between other pools.
      </p>
      <p>
        If a pool's minimum share is not met for some period of time, the
        scheduler optionally supports <em>preemption</em> of jobs in other
        pools. The pool will be allowed to kill tasks from other pools to make
        room to run. Preemption can be used to guarantee
        that "production" jobs are not starved while also allowing
        the Hadoop cluster to also be used for experimental and research jobs.
        In addition, a pool can also be allowed to preempt tasks if it is
        below half of its fair share for a configurable timeout (generally
        set larger than the minimum share preemption timeout).
        When choosing tasks to kill, the fair scheduler picks the
        most-recently-launched tasks from over-allocated jobs, 
        to minimize wasted computation.
        Preemption does not cause the preempted jobs to fail, because Hadoop
        jobs tolerate losing tasks; it only makes them take longer to finish.
      </p>
      <p>
        The Fair Scheduler can limit the number of concurrent
        running jobs per user and per pool. This can be useful when a 
        user must submit hundreds of jobs at once, or for ensuring that
        intermediate data does not fill up disk space on a cluster when too many
        concurrent jobs are running.
        Setting job limits causes jobs submitted beyond the limit to wait
        until some of the user/pool's earlier jobs finish.
        Jobs to run from each user/pool are chosen in order of priority and then
        submit time.
      </p>
      <p>
        Finally, the Fair Scheduler can limit the number of concurrent
        running tasks per pool. This can be useful when jobs have a
        dependency on an external service like a database or web
        service that could be overloaded if too many map or reduce
        tasks are run at once.
      </p>
    </section>

    <section>
      <title>Installation</title>
      <p>
        To run the fair scheduler in your Hadoop installation, you need to put
        it on the CLASSPATH. The easiest way is to copy the 
        <em>hadoop-*-fairscheduler.jar</em> from
        <em>HADOOP_HOME/build/contrib/fairscheduler</em> to <em>HADOOP_HOME/lib</em>.
        Alternatively you can modify <em>HADOOP_CLASSPATH</em> to include this jar, in
        <em>HADOOP_CONF_DIR/hadoop-env.sh</em>
      </p>
      <p>
       You will also need to set the following property in the Hadoop config 
       file  <em>HADOOP_CONF_DIR/mapred-site.xml</em> to have Hadoop use 
       the fair scheduler:
      </p>
<source>
&lt;property&gt;
  &lt;name&gt;mapred.jobtracker.taskScheduler&lt;/name&gt;
  &lt;value&gt;org.apache.hadoop.mapred.FairScheduler&lt;/value&gt;
&lt;/property&gt;
</source>
      <p>
        Once you restart the cluster, you can check that the fair scheduler 
        is running by going to <em>http://&lt;jobtracker URL&gt;/scheduler</em> 
        on the JobTracker's web UI. A &quot;job scheduler administration&quot; page should 
        be visible there. This page is described in the Administration section.
      </p>
      <p>
        If you wish to compile the fair scheduler from source, run <em> ant 
        package</em> in your HADOOP_HOME directory. This will build
        <em>build/contrib/fair-scheduler/hadoop-*-fairscheduler.jar</em>.
      </p>
    </section>
    
    <section>
      <title>Configuration</title>
      <p>
        The Fair Scheduler contains configuration in two places -- algorithm
        parameters are set in <em>HADOOP_CONF_DIR/mapred-site.xml</em>, while 
        a separate XML file called the <em>allocation file</em>, 
        located by default in
        <em>HADOOP_CONF_DIR/fair-scheduler.xml</em>, is used to configure
        pools, minimum shares, running job limits and preemption timeouts.
        The allocation file is reloaded periodically at runtime, 
        allowing you to change pool settings without restarting 
        your Hadoop cluster.
      </p>
      <p>
        For a minimal installation, to just get equal sharing between users,
        you will not need to edit the allocation file.
      </p>
      <section>
      <title>Scheduler Parameters in mapred-site.xml</title>
        <p>
          The following parameters can be set in <em>mapred-site.xml</em>
          to affect the behavior of the fair scheduler:
        </p>
        <p><strong>Basic Parameters</strong></p>
        <table>
          <tr>
          <th>Name</th><th>Description</th>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.preemption
          </td>
          <td>
            Boolean property for enabling preemption. Default: false.
          </td>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.pool
          </td>
          <td>
            Specify the pool that a job belongs in.  
            If this is specified then mapred.fairscheduler.poolnameproperty is ignored.
          </td>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.poolnameproperty
          </td>
          <td>
            Specify which jobconf property is used to determine the pool that a
            job belongs in. String, default: <em>user.name</em>
            (i.e. one pool for each user). 
            Another useful value is <em>mapred.job.queue.name</em> to use MapReduce's "queue"
            system for access control lists (see below).
            mapred.fairscheduler.poolnameproperty is used only for jobs in which 
            mapred.fairscheduler.pool is not explicitly set.
          </td>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.allow.undeclared.pools
          </td>
          <td>
            Boolean property for enabling job submission to pools not declared
            in the allocation file. Default: true.
          </td>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.allocation.file
          </td>
          <td>
            Can be used to have the scheduler use a different allocation file
            than the default one (<em>HADOOP_CONF_DIR/fair-scheduler.xml</em>).
            Must be an absolute path to the allocation file.
          </td>
          </tr>
        </table>
        <p> <br></br></p>
        <p><strong>Advanced Parameters</strong> </p>
        <table>
          <tr>
          <th>Name</th><th>Description</th>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.sizebasedweight
          </td>
          <td>
            Take into account job sizes in calculating their weights for fair 
            sharing. By default, weights are only based on job priorities. 
            Setting this flag to true will make them based on the size of the 
            job (number of tasks needed) as well,though not linearly 
            (the weight will be proportional to the log of the number of tasks 
            needed). This lets larger jobs get larger fair shares while still 
            providing enough of a share to small jobs to let them finish fast. 
            Boolean value, default: false.
          </td>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.preemption.only.log
          </td>
          <td>
            This flag will cause the scheduler to run through the preemption
            calculations but simply log when it wishes to preempt a task,
            without actually preempting the task. 
            Boolean property, default: false.
            This property can be useful for
            doing a "dry run" of preemption before enabling it to make sure
            that you have not set timeouts too aggressively.
            You will see preemption log messages in your JobTracker's output
            log (<em>HADOOP_LOG_DIR/hadoop-jobtracker-*.log</em>).
            The messages look as follows:<br/>
            <code>Should preempt 2 tasks for job_20090101337_0001: tasksDueToMinShare = 2, tasksDueToFairShare = 0</code>
          </td>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.update.interval
          </td>
          <td>
            Interval at which to update fair share calculations. The default
            of 500ms works well for clusters with fewer than 500 nodes, 
            but larger values reduce load on the JobTracker for larger clusters.
            Integer value in milliseconds, default: 500.
          </td>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.preemption.interval
          </td>
          <td>
            Interval at which to check for tasks to preempt. The default
            of 15s works well for timeouts on the order of minutes.
            It is not recommended to set timeouts much smaller than this
            amount, but you can use this value to make preemption computations
            run more often if you do set such timeouts. A value of less than
            5s will probably be too small, however, as it becomes less than
            the inter-heartbeat interval.
            Integer value in milliseconds, default: 15000.
          </td>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.weightadjuster
          </td>
          <td>
          An extension point that lets you specify a class to adjust the 
          weights of running jobs. This class should implement the 
          <em>WeightAdjuster</em> interface. There is currently one example 
          implementation - <em>NewJobWeightBooster</em>, which increases the 
          weight of jobs for the first 5 minutes of their lifetime to let 
          short jobs finish faster. To use it, set the weightadjuster 
          property to the full class name, 
          <code>org.apache.hadoop.mapred.NewJobWeightBooster</code>.
          NewJobWeightBooster itself provides two parameters for setting the 
          duration and boost factor.
          <ul>
          <li><em>mapred.newjobweightbooster.factor</em>
            Factor by which new jobs weight should be boosted. 
            Default is 3.</li>
          <li><em>mapred.newjobweightbooster.duration</em>
            Boost duration in milliseconds. Default is 300000 for 5 minutes.</li>
          </ul>
          </td>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.loadmanager
          </td>
          <td>
            An extension point that lets you specify a class that determines 
            how many maps and reduces can run on a given TaskTracker. This class 
            should implement the LoadManager interface. By default the task caps 
            in the Hadoop config file are used, but this option could be used to 
            make the load based on available memory and CPU utilization for example.
          </td>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.taskselector
          </td>
          <td>
          An extension point that lets you specify a class that determines 
          which task from within a job to launch on a given tracker. This can be 
          used to change either the locality policy (e.g. keep some jobs within 
          a particular rack) or the speculative execution algorithm (select 
          when to launch speculative tasks). The default implementation uses 
          Hadoop's default algorithms from JobInProgress.
          </td>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.eventlog.enabled
          </td>
          <td>
            Enable a detailed log of fair scheduler events, useful for
            debugging.
            This log is stored in <em>HADOOP_LOG_DIR/fairscheduler</em>.
	    <strong>NOTICE: This setting is for experts only.</strong>
            Boolean value, default: false.
          </td>
          </tr>
          <tr>
          <td>
            mapred.fairscheduler.dump.interval
          </td>
          <td>
            If using the event log, this is the interval at which to dump
            complete scheduler state (list of pools and jobs) to the log.
	    <strong>NOTICE: This setting is for experts only.</strong>
            Integer value in milliseconds, default: 10000.
          </td>
          </tr>
        </table>
      </section>  
      <section>
        <title>Allocation File (fair-scheduler.xml)</title>
        <p>
        The allocation file configures minimum shares, running job
        limits, weights and preemption timeouts for each pool.
        Only users/pools whose values differ from the defaults need to be
        explicitly configured in this file.
        The allocation file is located in
        <em>HADOOP_HOME/conf/fair-scheduler.xml</em>.
        It can contain the following types of elements:
        </p>
        <ul>
        <li><em>pool</em> elements, which configure each pool.
        These may contain the following sub-elements:
          <ul>
          <li><em>minMaps</em> and <em>minReduces</em>,
            to set the pool's minimum share of task slots.</li>
          <li><em>maxMaps</em> and <em>maxReduces</em>, to set the
            pool's maximum concurrent task slots.</li>
          <li><em>schedulingMode</em>, the pool's internal scheduling mode,
          which can be <em>fair</em> for fair sharing or <em>fifo</em> for
          first-in-first-out.</li>
          <li><em>maxRunningJobs</em>, 
          to limit the number of jobs from the 
          pool to run at once (defaults to infinite).</li>
          <li><em>weight</em>, to share the cluster 
          non-proportionally with other pools. For example, a pool with weight 2.0 will get a 2x higher share than other pools. The default weight is 1.0.</li>
          <li><em>minSharePreemptionTimeout</em>, the
            number of seconds the pool will wait before
            killing other pools' tasks if it is below its minimum share
            (defaults to infinite).</li>
          </ul>
        </li>
        <li><em>user</em> elements, which may contain a 
        <em>maxRunningJobs</em> element to limit 
        jobs. Note that by default, there is a pool for each 
        user, so per-user limits are not necessary.</li>
        <li><em>poolMaxJobsDefault</em>, which sets the default running 
        job limit for any pools whose limit is not specified.</li>
        <li><em>userMaxJobsDefault</em>, which sets the default running 
        job limit for any users whose limit is not specified.</li>
        <li><em>defaultMinSharePreemptionTimeout</em>, 
        which sets the default minimum share preemption timeout 
        for any pools where it is not specified.</li>
        <li><em>fairSharePreemptionTimeout</em>, 
        which sets the preemption timeout used when jobs are below half
        their fair share.</li>
        <li><em>defaultPoolSchedulingMode</em>, which sets the default scheduling 
        mode (<em>fair</em> or <em>fifo</em>) for pools whose mode is
        not specified.</li>
        </ul>
        <p>
        Pool and user elements only required if you are setting
        non-default values for the pool/user. That is, you do not need to
        declare all users and all pools in your config file before running
        the fair scheduler. If a user or pool is not listed in the config file,
        the default values for limits, preemption timeouts, etc will be used.
        </p>
        <p>
        An example allocation file is given below : </p>
<source>
&lt;?xml version="1.0"?&gt;  
&lt;allocations&gt;  
  &lt;pool name="sample_pool"&gt;
    &lt;minMaps&gt;5&lt;/minMaps&gt;
    &lt;minReduces&gt;5&lt;/minReduces&gt;
    &lt;maxMaps&gt;25&lt;/maxMaps&gt;
    &lt;maxReduces&gt;25&lt;/maxReduces&gt;
    &lt;minSharePreemptionTimeout&gt;300&lt;/minSharePreemptionTimeout&gt;
  &lt;/pool&gt;
  &lt;user name="sample_user"&gt;
    &lt;maxRunningJobs&gt;6&lt;/maxRunningJobs&gt;
  &lt;/user&gt;
  &lt;userMaxJobsDefault&gt;3&lt;/userMaxJobsDefault&gt;
  &lt;fairSharePreemptionTimeout&gt;600&lt;/fairSharePreemptionTimeout&gt;
&lt;/allocations&gt;
</source>
        <p>
        This example creates a pool sample_pool with a guarantee of 5 map 
        slots and 5 reduce slots. The pool also has a minimum share preemption
        timeout of 300 seconds (5 minutes), meaning that if it does not get its
        guaranteed share within this time, it is allowed to kill tasks from
        other pools to achieve its share. The pool has a cap of 25 map and 25
        reduce slots, which means that once 25 tasks are running, no more will
        be scheduled even if the pool's fair share is higher.
        The example also limits the number of running jobs 
        per user to 3, except for sample_user, who can run 6 jobs concurrently. 
        Finally, the example sets a fair share preemption timeout of 600 seconds
        (10 minutes). If a job is below half its fair share for 10 minutes, it
        will be allowed to kill tasks from other jobs to achieve its share.
        Note that the preemption settings require preemption to be
        enabled in <em>mapred-site.xml</em> as described earlier.
        </p>
        <p>
        Any pool not defined in the allocation file will have no guaranteed 
        capacity and no preemption timeout. Also, any pool or user with no max 
        running jobs set in the file will be allowed to run an unlimited 
        number of jobs.
        </p>
      </section>
      <section>
        <title>Access Control Lists (ACLs)</title>
        <p>
          The fair scheduler can be used in tandem with the "queue" based access
          control system in MapReduce to restrict which pools each user can access.
          To do this, first enable ACLs and set up some queues as described in the
          <a href="mapred_tutorial.html#Job+Authorization">MapReduce usage guide</a>,
          then set the fair scheduler to use one pool per queue by adding
          the following property in <em>HADOOP_CONF_DIR/mapred-site.xml</em>:
        </p>
<source>
&lt;property&gt;
  &lt;name&gt;mapred.fairscheduler.poolnameproperty&lt;/name&gt;
  &lt;value&gt;mapred.job.queue.name&lt;/value&gt;
&lt;/property&gt;
</source>
        <p>
          You can then set the minimum share, weight, and internal scheduling mode
          for each pool as described earlier.
          In addition, make sure that users submit jobs to the right queue by setting
          the <em>mapred.job.queue.name</em> property in their jobs.
        </p>
      </section>
    </section>
    <section>
    <title> Administration</title>
    <p>
      The fair scheduler provides support for administration at runtime 
      through two mechanisms:
    </p> 
    <ol>
    <li>
      It is possible to modify minimum shares, limits, weights, preemption
      timeouts and pool scheduling modes at runtime by editing the allocation
      file. The scheduler will reload this file 10-15 seconds after it 
      sees that it was modified.
     </li>
     <li>
     Current jobs, pools, and fair shares  can be examined through the 
     JobTracker's web interface, at
     <em>http://&lt;JobTracker URL&gt;/scheduler</em>. 
     On this interface, it is also possible to modify jobs' priorities or 
     move jobs from one pool to another and see the effects on the fair 
     shares (this requires JavaScript).
     </li>
    </ol>
    <p>
      The following fields can be seen for each job on the web interface:
     </p>
     <ul>
     <li><em>Submitted</em> - Date and time job was submitted.</li>
     <li><em>JobID, User, Name</em> - Job identifiers as on the standard 
     web UI.</li>
     <li><em>Pool</em> - Current pool of job. Select another value to move job to 
     another pool.</li>
     <li><em>Priority</em> - Current priority. Select another value to change the 
     job's priority</li>
     <li><em>Maps/Reduces Finished</em>: Number of tasks finished / total tasks.</li>
     <li><em>Maps/Reduces Running</em>: Tasks currently running.</li>
     <li><em>Map/Reduce Fair Share</em>: The average number of task slots that this 
     job should have at any given time according to fair sharing. The actual
     number of tasks will go up and down depending on how much compute time
     the job has had, but on average it will get its fair share amount.</li>
     </ul>
     <p>
     In addition, it is possible to view an "advanced" version of the web 
     UI by going to <em>http://&lt;JobTracker URL&gt;/scheduler?advanced</em>. 
     This view shows two more columns:
     </p>
     <ul>
     <li><em>Maps/Reduce Weight</em>: Weight of the job in the fair sharing 
     calculations. This depends on priority and potentially also on 
     job size and job age if the <em>sizebasedweight</em> and 
     <em>NewJobWeightBooster</em> are enabled.</li>
     </ul>
    </section>
    <section>
      <title>Metrics</title>
      <p>
        The fair scheduler can export metrics using the Hadoop metrics interface.
        This can be enabled by adding an entry to <code>hadoop-metrics.properties</code>
        to enable the <code>fairscheduler</code> metrics context. For example, to
        simply retain the metrics in memory so they may be viewed in the <code>/metrics</code>
        servlet:
      </p>
      <p>
        <code>fairscheduler.class=org.apache.hadoop.metrics.spi.NoEmitMetricsContext</code>
      </p>
      <p>
        Metrics are generated for each pool and job, and contain the same information that
        is visible on the <code>/scheduler</code> web page.
      </p>
    </section>

    <!--
    <section>
    <title>Implementation</title>
    <p>There are two aspects to implementing fair scheduling: Calculating 
    each job's fair share, and choosing which job to run when a task slot 
    becomes available.</p>
    <p>To select jobs to run, the scheduler then keeps track of a 
    &quot;deficit&quot; for each job - the difference between the amount of
     compute time it should have gotten on an ideal scheduler, and the amount 
     of compute time it actually got. This is a measure of how 
     &quot;unfair&quot; we've been to the job. Every few hundred 
     milliseconds, the scheduler updates the deficit of each job by looking
     at how many tasks each job had running during this interval vs. its 
     fair share. Whenever a task slot becomes available, it is assigned to 
     the job with the highest deficit. There is one exception - if there 
     were one or more jobs who were not meeting their pool capacity 
     guarantees, we only choose among these &quot;needy&quot; jobs (based 
     again on their deficit), to ensure that the scheduler meets pool 
     guarantees as soon as possible.</p>
     <p>
     The fair shares are calculated by dividing the capacity of the cluster 
     among runnable jobs according to a &quot;weight&quot; for each job. By 
     default the weight is based on priority, with each level of priority 
     having 2x higher weight than the next (for example, VERY_HIGH has 4x the 
     weight of NORMAL). However, weights can also be based on job sizes and ages, 
     as described in the Configuring section. For jobs that are in a pool, 
     fair shares also take into account the minimum guarantee for that pool. 
     This capacity is divided among the jobs in that pool according again to 
     their weights.
     </p>
     <p>When limits on a user's running jobs or a pool's running jobs 
     are in place, we choose which jobs get to run by sorting all jobs in order 
     of priority and then submit time, as in the standard Hadoop scheduler. Any 
     jobs that fall after the user/pool's limit in this ordering are queued up 
     and wait idle until they can be run. During this time, they are ignored 
     from the fair sharing calculations and do not gain or lose deficit (their 
     fair share is set to zero).</p>
     <p>
     Preemption is implemented by periodically checking whether jobs are
     below their minimum share or below half their fair share. If a job has
     been below its share for sufficiently long, it is allowed to kill
     other jobs' tasks. The tasks chosen are the most-recently-launched
     tasks from over-allocated jobs, to minimize the amount of wasted
     computation.
     </p>
     <p>
     Finally, the fair scheduler provides several extension points where
     the basic functionality can be extended. For example, the weight
     calculation can be modified to give a priority boost to new jobs,
     implementing a "shortest job first" policy which reduces response
     times for interactive jobs even further.
     These extension points are listed in
     <a href="#Scheduler+Parameters+in+mapred-site.xml">Advanced Parameters</a>.
     </p>
    </section>
    -->
  </body>  
</document>
