Early versions of Hadoop had a very simple approach to scheduling users’ jobs: they ran in order of submission, using a FIFO scheduler. Typically, each job would use the whole cluster, so jobs had to wait their turn. Although a shared cluster offers great potential for offering large resources to many users, the problem of sharing resources fairly between users requires a better scheduler. Production jobs need to complete in atimely manner, while allowing users who are making smaller ad hoc queries to get results back in a reasonable time.
Later on, the ability to set a job’s priority was added, via the mapred.job.priority property or the setJobPriority() method on JobClient (both of which take one of the values VERY_HIGH, HIGH, NORMAL, LOW, VERY_LOW). When the job scheduler is choosing the next job to run, it selects one with the highest priority. However, with the FIFO scheduler, priorities do not support preemption, so a high-priority job can still beblocked by a long-running low priority job that started before the high-priority job was scheduled.
MapReduce in Hadoop comes with a choice of schedulers. The default is the original FIFO queue-based scheduler, and there are also multiuser schedulers called the Fair Scheduler and the Capacity Scheduler.
The Fair Scheduler
The Fair Scheduler aims to give every user a fair share of the cluster capacity over time. If a single job is running, it gets all of the cluster. As more jobs are submitted, free task slots are given to the jobs in such a way as to give each user a fair share of the cluster.
A short job belonging to one user will complete in a reasonable time even while another user’s long job is running, and the long job will still make progress. Jobs are placed in pools, and by default, each user gets their own pool. A user who submits more jobs than a second user will not get any more cluster resources than the second, on average. It is also possible to define custom pools with guaranteed minimum capacities defined in terms of the number of map and reduce slots, and to set weightings for each pool.
The Fair Scheduler supports preemption, so if a pool has not received its fair share for a certain period of time, then the scheduler will kill tasks in pools running over capacity in order to give the slots to the pool running under capacity.
The Fair Scheduler is a “contrib” module. To enable it, place its JAR file on Hadoop’s classpath, by copying it from Hadoop’s contrib/fairscheduler directory to the lib directory.
Then set the mapred.jobtracker.taskScheduler property to:
The Fair Scheduler will work without further configuration, but to take full advantage of its features and how to configure it (including its web interface), refer to README in the src/contrib/fairscheduler directory of the distribution.
The Capacity Scheduler
The Capacity Scheduler takes a slightly different approach to multiuser scheduling. A cluster is made up of a number of queues (like the Fair Scheduler’s pools), which may be hierarchical (so a queue may be the child of another queue), and each queue has an allocated capacity. This is like the Fair Scheduler, except that within each queue, jobs are scheduled using FIFO scheduling (with priorities). In effect, the Capacity Schedulerallows users or organizations (defined using queues) to simulate a separate MapReduce cluster with FIFO scheduling for each user or organization. The Fair Scheduler, by contrast, (which actually also supports FIFO job scheduling within pools as an option, making it like the Capacity Scheduler) enforces fair sharing within each pool, so running jobs share the pool’s resources.
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