Hi @Sagar Shimpi
This is what I got from
Hadoop: The Definitive Guide, 4th Edition
All the YARN schedulers try to honor locality requests. On a busy cluster, if an application requests a particular node, there is a good chance that other containers are running on it at the time of the request. The obvious course of action is to immediately loosen the locality requirement and allocate a container on the same rack. However, it has been observed in practice that waiting a short time (no more than a few seconds) can dramatically increase the chances of being allocated a container on the requested node, and therefore increase the efficiency of the cluster. This feature is called delay scheduling, and it is supported by both the Capacity Scheduler and the Fair Scheduler.
Every node manager in a YARN cluster periodically sends a heartbeat request to the resource manager—by default, one per second. Heartbeats carry information about the node manager’s running containers and the resources available for new containers, so each heartbeat is a potential scheduling opportunity for an application to run a container.
When using delay scheduling, the scheduler doesn’t simply use the first scheduling opportunity it receives, but waits for up to a given maximum number of scheduling opportunities to occur before loosening the locality constraint and taking the next scheduling opportunity.
For the Capacity Scheduler, delay scheduling is configured by setting yarn.scheduler.capacity.node-locality-delay to a positive integer representing the number of scheduling opportunities that it is prepared to miss before loosening the node constraint to match any node in the same rack.
