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From "Sandy Ryza (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (YARN-371) Consolidate resource requests in AM-RM heartbeat
Date Mon, 04 Feb 2013 20:26:14 GMT

    [ https://issues.apache.org/jira/browse/YARN-371?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13570595#comment-13570595
] 

Sandy Ryza commented on YARN-371:
---------------------------------

Arun,

No offense taken!  I realize this discussion isn't likely to reach a quick resolution, so
I'd be happy to move it to yarn-dev if that would be a better forum.  I'll also change the
name of the JIRA to reflect the more general issue and not a specific solution.

You make very good point.  However, as Bobby points out, I don't think that changing the protocol
requires the shift that you describe inside the ResourceManager. The scheduler is perfectly
able to translate requests as described above into the current model.  The change would merely
allow the model to be determined by the scheduler, which is both pluggable and can change
in future versions, instead of the protocol, which must remain stable.  The change would allow
future flexibility on the tradeoff between the amount of state that needs to be stored and
the exactness of the scheduling.  For this JIRA I am only suggesting changing the protocol,
not making any structural changes to the schedulers.

To be clear, I am not suggesting that a separate ResourceRequest is required for each task.
 If a number of tasks are requested with only a rack-level locality, by all means they should
be bundled into the same request.  That said, it is true that in many cases, the amount of
data in a request will increase.  Assuming a compact representation that uses 14 bytes per
task, an application requesting a hundred thousand tasks, each on a unique set of three nodes,
would be sending 1.5 MB over the wire.  If this is a prohibitively large amount, then we could
possibly allow for different request formats?  All schedulers would be required to accept
requests both in the "resource-centric" and "task-centric" formats, and, for schedulers that
are internally resource-centric, a utility would be provided for converting task-centric requests
to resource-centric ones.  I realize that this makes things hairier, but I believe that, in
addition to the advantages it can provide to present-day MR, such as more accurate delay scheduling,
down the line, not supporting "lossless" requests will severely limit the types of applications
that can be handled by YARN.
                
> 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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