Bikas Saha commented on YARN-752:

YARN's default and most efficient scheduling behavior comes when locality can 
relaxed from node to rack etc. The AMRMClient should simply do the right thing 
in this context and add racks for the nodes provided by the user, if the user 
has already not done so. If this is not done then its the burden of the user to 
do it and if the user forgets to do it then YARN will probably not give it any 
containers and the job will be stuck. Hence, by default the AMRMClient should 
always fill in missing racks. If the user has already done the right thing then 
this functionality will be a no-op. This cannot be optional. The only time this 
should not be done is when the user has asked for specific nodes/racks. Even 
there the AMRMClient will have to add missing racks but with a special flag or 
else the schedulers will not assign containers to the app. Support for that 
feature is yet to be added in AMRMClient and is tracked by another jira. 
If the user has specified a mix of nodes and racks, even then we need to ensure 
that racks are added for nodes whose racks are missing.
> In AMRMClient, automatically add corresponding rack requests for requested 
> nodes
> --------------------------------------------------------------------------------
>                 Key: YARN-752
>                 URL: https://issues.apache.org/jira/browse/YARN-752
>             Project: Hadoop YARN
>          Issue Type: Improvement
>          Components: api, applications
>    Affects Versions: 2.0.4-alpha
>            Reporter: Sandy Ryza
>            Assignee: Sandy Ryza
>         Attachments: YARN-752-1.patch, YARN-752-1.patch, YARN-752.patch
> A ContainerRequest that includes node-level requests must also include 
> matching rack-level requests for the racks that those nodes are on.  When a 
> node is present without its rack, it makes sense for the client to 
> automatically add the node's rack.

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