Given an application id the YarnClient can return an
ApplicationResourceUsageReport that gives information about resources that
are 'used', 'reserved', and 'needed'. I'm trying to get a better
understanding of what determines whether a resource is 'used', 'reserved',
or 'needed'. If I request a container with some resource, is that 'used'
once the resource manager has given the container to the application master
even if the application master has not started the container yet? In what
situation is a resource 'reserved'? Is 'needed' the sum of the two, or
does it have some other meaning?