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https://issues.apache.org/jira/browse/YARN-371?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13569277#comment-13569277
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Arun C Murthy commented on YARN-371:
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{quote}
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.
{quote}
-1
The main advantage of the proposed model is that it is extremely compact in
terms of the amount of state necessary, per application, on the ResourceManager
for scheduling and the amount of information passed around between the
ApplicationMaster & ResourceManager. This is crucial for scaling the
ResourceManager. The amount of information, per application, in this model is
always O(cluster size), whereas in the current Hadoop Map-Reduce JobTracker it
is O(number of tasks) which could run into hundreds of thousands of tasks. For
large jobs it is sufficient to ask for containers only on racks and not
specific hosts since the ApplicationMaster can use them appropriately since
each rack has many appropriate resources (i.e. input splits for MapReduce
applications).
To be clear, we should avoid *task* specific view and stay 'resource-specific'
view.
> 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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