What additional benefit does Yarn bring to the existing map reduce?
Yarn differs in its infrastructure layer from the original map reduce architecture in the following way:
In YARN, the job tracker is split into two different daemons called Resource Manager and Node Manager (node specific). The resource manager only manages the allocation of resources to the different jobs apart from comprising a scheduler which just takes care of the scheduling jobs without worrying about any monitoring or status updates. Different resources such as memory, cpu time, network bandwidth etc. are put into one unit called the Resource Container. There are different AppMasters running on different nodes which talk to a number of these resource containers and accordingly update the Node Manager with the monitoring/status details.
I want to know that how does using this kind of an approach increase the performance from the map-reduce perspective? Also, if there is any definitive content on the motivation behind Yarn and its benefits over the existing implementation of Map-reduce, please point me to the same.
Answer
Here are some of the articles (1, 2, 3) about YARN. These talk about the benefits of using YARN.
YARN is more general than MR and it should be possible to run other computing models like BSP besides MR. Prior to YARN, it required a separate cluster for MR, BSP and others. Now they they can coexist in a single cluster, which leads to higher usage of the cluster. Here are some of the applications ported to YARN.
From a MapReduce perspective in legacy MR there are separate slots for Map and Reduce tasks, but in YARN their is no fixed purpose of a container. The same container can be used for a Map task, Reduce task, Hama BSP Task or something else. This leads to better utilization.
Also, it makes it possible to run different versions of Hadoop in the same cluster which is not possible with legacy MR, which makes is easy from a maintenance point.
Here are some of the additional links for YARN. Also, Hadoop: The Definitive Guide, 3rd Edition has an entire section dedicated to YARN.
FYI, it had been a bit controversial to develop YARN instead of using some of frameworks which had been doing something similar and had been running for ages successfully with bugs ironed out.