jaehoon ko created YARN-3856:

             Summary: YARN shoud allocate container that is closest to the data
                 Key: YARN-3856
                 URL: https://issues.apache.org/jira/browse/YARN-3856
             Project: Hadoop YARN
          Issue Type: Improvement
          Components: resourcemanager
    Affects Versions: 2.7.0
         Environment: Hadoop cluster with multi-level network hierarchy
            Reporter: jaehoon ko

Currently, given a Container request for a host, ResourceManager allocates a 
Container with following priorities (RMContainerAllocator.java):
 - Requested host
 - a host in the same rack as the requested host
 - any host

This can lead to a sub-optimal allocation if Hadoop cluster is deployed on 
multi-level networked hosts (which is typical). For example, let's suppose a 
network architecture with one core switches, two aggregate switches, four ToR 
switches, and 8 hosts. Each switch has two downlinks. Rack IDs of hosts are as 
h1, h2: /c/a1/t1
h3, h4: /c/a1/t2
h5, h6: /c/a2/t3
h7, h8: /c/a2/t4

To allocate a container for data in h1, Hadoop first tries h1 itself, then h2, 
then any of h3 ~ h8. Clearly, h3 or h4 are better than h5~h8 in terms of 
network distance and bandwidth. However, current implementation choose one from 
h3~h8 with equal probabilities.

This limitation is more obvious when considering hadoop clusters deployed on VM 
or containers. In this case, only the VMs or containers running in the same 
physical host are considered rack local, and actual rack-local hosts are chosen 
with same probabilities as far hosts.

The root cause of this limitation is that RMContainerAllocator.java performs 
exact matching on rack id to find a rack local host. Alternatively, we can 
perform longest-prefix matching to find a closest host. Using the same network 
architecture as above, with longest-prefix matching, hosts are selected with 
the following priorities:
 h3 or h4
 h5 or h6 or h7 or h8

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