3 Schedulers in YARN

The scheduler class is set in yarn-default.xml
Information about the currently running scheduler can be found by opening the ResourceManager web UI and selecting the Scheduler option under the Cluster menu on the left (e.g., http://your_cluster:8088/cluster/scheduler)

FIFO
The FIFO scheduler is basically a simple “first come, first served” scheduler in which the JobTracker pulls jobs from a work queue, oldest job first. Typically, FIFO schedules have no sense of job priority or scope. The FIFO schedule is practical for small workloads, but is feature-poor and can cause issues when large shared clusters are used.


Capacity
The Capacity scheduler permits sharing a cluster while giving each user or group certain minimum capacity guarantees.
The Capacity scheduler works best when the workloads are well known, which helps in assigning the minimum capacity. For this scheduler to work most effectively, each queue should be assigned a minimal capacity that is less than the maximal expected workload. Within each queue, multiple applications are scheduled using hierarchical FIFO queues

Fair
Fair scheduling is a method of assigning resources to applications such that all applications get, on average, an equal share of resources over time.
Under the Fair scheduler, when a single application is running, that application may request the entire cluster (if needed). If additional applications are submitted, resources that are free are assigned “fairly” to the new applications so that each application gets roughly the same amount of resources.

The Fair scheduler works best when there is a lot of variability between queues. Unlike with the Capacity scheduler, all jobs make progress rather than proceeding in a FIFO fashion in their respective queues.