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https://issues.apache.org/jira/browse/YARN-371?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13571734#comment-13571734
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Karthik Kambatla commented on YARN-371:
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IIUC, changing the protocol doesn't necessarily require changing how the AM
uses it. We should still be able to send requests the way we do today in the
new protocol that being proposed here.
The AM should be able to make that decision. A custom MR-AM (or any other AM)
might decide a task-centric resource model is better. Also, the same AM might
want to adopt different request models for different requests? Answering
Bobby's questions should help us figure out the threshold if one really were to
adopt this hybrid model. By no means, I am proposing we do this in MR at this
moment.
I believe Sandy's proposal to change the protocol to ensure we don't break
compatibility later (thinking Hadoop-2,3, etc) makes sense, even if we don't
want to change the way we use it.
Please correct me if I am missing something here.
> Resource-centric compression in AM-RM protocol limits scheduling
> ----------------------------------------------------------------
>
> 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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