[ https://issues.apache.org/jira/browse/YARN-371?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Sandy Ryza reassigned YARN-371:
-------------------------------
Assignee: Sandy Ryza
> Consolidate resource requests in AM-RM heartbeat
> ------------------------------------------------
>
> Key: YARN-371
> URL: https://issues.apache.org/jira/browse/YARN-371
> Project: Hadoop YARN
> Issue Type: Improvement
> Components: api, resourcemanager, scheduler
> Affects Versions: 2.0.2-alpha
> Reporter: Sandy Ryza
> Assignee: Sandy Ryza
>
> Each AMRM heartbeat consists of a list of resource requests. Currently, each resource
request consists of a container count, a resource vector, and a location, which may be a node,
a rack, or "*". When an application wishes to request a task run in multiple localtions, it
must issue a request for each location. This means that for a node-local task, it must issue
three requests, one at the node-level, one at the rack-level, and one with * (any). These
requests are not linked with each other, so when a container is allocated for one of them,
the RM has no way of knowing which others to get rid of. When a node-local container is allocated,
this is handled by decrementing the number of requests on that node's rack and in *. But when
the scheduler allocates a task with a node-local request on its rack, the request on the node
is left there. This can cause delay-scheduling to try to assign a container on a node that
nobody cares about anymore.
> Additionally, unless I am missing something, the current model does not allow requests
for containers only on a specific node or specific rack. While this is not a use case for
MapReduce currently, it is conceivable that it might be something useful to support in the
future, for example to schedule long-running services that persist state in a particular location,
or for applications that generally care less about latency than data-locality.
> Lastly, the ability to understand which requests are for the same task will possibly
allow future schedulers to make more intelligent scheduling decisions, as well as permit a
more exact understanding of request load.
> I would propose the tweak of allowing a single ResourceRequest to encapsulate all the
location information for a task. So instead of just a single location, a ResourceRequest
would contain an array of locations, including nodes that it would be happy with, racks that
it would be happy with, and possibly *. Side effects of this change would be a reduction
in the amount of data that needs to be transferred in a heartbeat, as well in as the RM's
memory footprint, becaused what used to be different requests for the same task are now able
to share some common data.
> While this change breaks compatibility, if it is going to happen, it makes sense to do
it now, before YARN becomes beta.
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